Keep only libstore, nix-daemon, and related stuff.

1 files changed, 3351 insertions, 0 deletions

diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
new file mode 100644
index 0000000000..2e2f92fadf
--- /dev/null
+++ b/nix/libstore/build.cc
@@ -0,0 +1,3351 @@
+#include "config.h"
+
+#include "references.hh"
+#include "pathlocks.hh"
+#include "misc.hh"
+#include "globals.hh"
+#include "local-store.hh"
+#include "util.hh"
+#include "archive.hh"
+#include "affinity.hh"
+
+#include <map>
+#include <sstream>
+#include <algorithm>
+
+#include <time.h>
+#include <sys/time.h>
+#include <sys/wait.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/utsname.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <stdio.h>
+#include <cstring>
+
+#include <pwd.h>
+#include <grp.h>
+
+#include <bzlib.h>
+
+/* Includes required for chroot support. */
+#if HAVE_SYS_PARAM_H
+#include <sys/param.h>
+#endif
+#if HAVE_SYS_MOUNT_H
+#include <sys/mount.h>
+#endif
+#if HAVE_SCHED_H
+#include <sched.h>
+#endif
+
+/* In GNU libc 2.11, <sys/mount.h> does not define `MS_PRIVATE', but
+   <linux/fs.h> does.  */
+#if !defined MS_PRIVATE && defined HAVE_LINUX_FS_H
+#include <linux/fs.h>
+#endif
+
+#define CHROOT_ENABLED HAVE_CHROOT && HAVE_UNSHARE && HAVE_SYS_MOUNT_H && defined(MS_BIND) && defined(MS_PRIVATE) && defined(CLONE_NEWNS)
+
+#if CHROOT_ENABLED
+#include <sys/socket.h>
+#include <sys/ioctl.h>
+#include <net/if.h>
+#include <netinet/ip.h>
+#endif
+
+#if HAVE_SYS_PERSONALITY_H
+#include <sys/personality.h>
+#define CAN_DO_LINUX32_BUILDS
+#endif
+
+#if HAVE_STATVFS
+#include <sys/statvfs.h>
+#endif
+
+
+namespace nix {
+
+using std::map;
+
+
+static string pathNullDevice = "/dev/null";
+
+
+/* Forward definition. */
+class Worker;
+struct HookInstance;
+
+
+/* A pointer to a goal. */
+class Goal;
+typedef std::shared_ptr<Goal> GoalPtr;
+typedef std::weak_ptr<Goal> WeakGoalPtr;
+
+/* Set of goals. */
+typedef set<GoalPtr> Goals;
+typedef list<WeakGoalPtr> WeakGoals;
+
+/* A map of paths to goals (and the other way around). */
+typedef map<Path, WeakGoalPtr> WeakGoalMap;
+
+
+
+class Goal : public std::enable_shared_from_this<Goal>
+{
+public:
+    typedef enum {ecBusy, ecSuccess, ecFailed, ecNoSubstituters, ecIncompleteClosure} ExitCode;
+
+protected:
+
+    /* Backlink to the worker. */
+    Worker & worker;
+
+    /* Goals that this goal is waiting for. */
+    Goals waitees;
+
+    /* Goals waiting for this one to finish.  Must use weak pointers
+       here to prevent cycles. */
+    WeakGoals waiters;
+
+    /* Number of goals we are/were waiting for that have failed. */
+    unsigned int nrFailed;
+
+    /* Number of substitution goals we are/were waiting for that
+       failed because there are no substituters. */
+    unsigned int nrNoSubstituters;
+
+    /* Number of substitution goals we are/were waiting for that
+       failed because othey had unsubstitutable references. */
+    unsigned int nrIncompleteClosure;
+
+    /* Name of this goal for debugging purposes. */
+    string name;
+
+    /* Whether the goal is finished. */
+    ExitCode exitCode;
+
+    Goal(Worker & worker) : worker(worker)
+    {
+        nrFailed = nrNoSubstituters = nrIncompleteClosure = 0;
+        exitCode = ecBusy;
+    }
+
+    virtual ~Goal()
+    {
+        trace("goal destroyed");
+    }
+
+public:
+    virtual void work() = 0;
+
+    void addWaitee(GoalPtr waitee);
+
+    virtual void waiteeDone(GoalPtr waitee, ExitCode result);
+
+    virtual void handleChildOutput(int fd, const string & data)
+    {
+        abort();
+    }
+
+    virtual void handleEOF(int fd)
+    {
+        abort();
+    }
+
+    void trace(const format & f);
+
+    string getName()
+    {
+        return name;
+    }
+
+    ExitCode getExitCode()
+    {
+        return exitCode;
+    }
+
+    /* Cancel the goal.  It should wake up its waiters, get rid of any
+       running child processes that are being monitored by the worker
+       (important!), etc. */
+    virtual void cancel(bool timeout) = 0;
+
+protected:
+    void amDone(ExitCode result);
+};
+
+
+/* A mapping used to remember for each child process to what goal it
+   belongs, and file descriptors for receiving log data and output
+   path creation commands. */
+struct Child
+{
+    WeakGoalPtr goal;
+    set<int> fds;
+    bool respectTimeouts;
+    bool inBuildSlot;
+    time_t lastOutput; /* time we last got output on stdout/stderr */
+    time_t timeStarted;
+};
+
+typedef map<pid_t, Child> Children;
+
+
+/* The worker class. */
+class Worker
+{
+private:
+
+    /* Note: the worker should only have strong pointers to the
+       top-level goals. */
+
+    /* The top-level goals of the worker. */
+    Goals topGoals;
+
+    /* Goals that are ready to do some work. */
+    WeakGoals awake;
+
+    /* Goals waiting for a build slot. */
+    WeakGoals wantingToBuild;
+
+    /* Child processes currently running. */
+    Children children;
+
+    /* Number of build slots occupied.  This includes local builds and
+       substitutions but not remote builds via the build hook. */
+    unsigned int nrLocalBuilds;
+
+    /* Maps used to prevent multiple instantiations of a goal for the
+       same derivation / path. */
+    WeakGoalMap derivationGoals;
+    WeakGoalMap substitutionGoals;
+
+    /* Goals waiting for busy paths to be unlocked. */
+    WeakGoals waitingForAnyGoal;
+
+    /* Goals sleeping for a few seconds (polling a lock). */
+    WeakGoals waitingForAWhile;
+
+    /* Last time the goals in `waitingForAWhile' where woken up. */
+    time_t lastWokenUp;
+
+public:
+
+    /* Set if at least one derivation had a BuildError (i.e. permanent
+       failure). */
+    bool permanentFailure;
+
+    LocalStore & store;
+
+    std::shared_ptr<HookInstance> hook;
+
+    Worker(LocalStore & store);
+    ~Worker();
+
+    /* Make a goal (with caching). */
+    GoalPtr makeDerivationGoal(const Path & drvPath, const StringSet & wantedOutputs, BuildMode buildMode = bmNormal);
+    GoalPtr makeSubstitutionGoal(const Path & storePath, bool repair = false);
+
+    /* Remove a dead goal. */
+    void removeGoal(GoalPtr goal);
+
+    /* Wake up a goal (i.e., there is something for it to do). */
+    void wakeUp(GoalPtr goal);
+
+    /* Return the number of local build and substitution processes
+       currently running (but not remote builds via the build
+       hook). */
+    unsigned int getNrLocalBuilds();
+
+    /* Registers a running child process.  `inBuildSlot' means that
+       the process counts towards the jobs limit. */
+    void childStarted(GoalPtr goal, pid_t pid,
+        const set<int> & fds, bool inBuildSlot, bool respectTimeouts);
+
+    /* Unregisters a running child process.  `wakeSleepers' should be
+       false if there is no sense in waking up goals that are sleeping
+       because they can't run yet (e.g., there is no free build slot,
+       or the hook would still say `postpone'). */
+    void childTerminated(pid_t pid, bool wakeSleepers = true);
+
+    /* Put `goal' to sleep until a build slot becomes available (which
+       might be right away). */
+    void waitForBuildSlot(GoalPtr goal);
+
+    /* Wait for any goal to finish.  Pretty indiscriminate way to
+       wait for some resource that some other goal is holding. */
+    void waitForAnyGoal(GoalPtr goal);
+
+    /* Wait for a few seconds and then retry this goal.  Used when
+       waiting for a lock held by another process.  This kind of
+       polling is inefficient, but POSIX doesn't really provide a way
+       to wait for multiple locks in the main select() loop. */
+    void waitForAWhile(GoalPtr goal);
+
+    /* Loop until the specified top-level goals have finished. */
+    void run(const Goals & topGoals);
+
+    /* Wait for input to become available. */
+    void waitForInput();
+
+    unsigned int exitStatus();
+};
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+void addToWeakGoals(WeakGoals & goals, GoalPtr p)
+{
+    // FIXME: necessary?
+    foreach (WeakGoals::iterator, i, goals)
+        if (i->lock() == p) return;
+    goals.push_back(p);
+}
+
+
+void Goal::addWaitee(GoalPtr waitee)
+{
+    waitees.insert(waitee);
+    addToWeakGoals(waitee->waiters, shared_from_this());
+}
+
+
+void Goal::waiteeDone(GoalPtr waitee, ExitCode result)
+{
+    assert(waitees.find(waitee) != waitees.end());
+    waitees.erase(waitee);
+
+    trace(format("waitee `%1%' done; %2% left") %
+        waitee->name % waitees.size());
+
+    if (result == ecFailed || result == ecNoSubstituters || result == ecIncompleteClosure) ++nrFailed;
+
+    if (result == ecNoSubstituters) ++nrNoSubstituters;
+
+    if (result == ecIncompleteClosure) ++nrIncompleteClosure;
+
+    if (waitees.empty() || (result == ecFailed && !settings.keepGoing)) {
+
+        /* If we failed and keepGoing is not set, we remove all
+           remaining waitees. */
+        foreach (Goals::iterator, i, waitees) {
+            GoalPtr goal = *i;
+            WeakGoals waiters2;
+            foreach (WeakGoals::iterator, j, goal->waiters)
+                if (j->lock() != shared_from_this()) waiters2.push_back(*j);
+            goal->waiters = waiters2;
+        }
+        waitees.clear();
+
+        worker.wakeUp(shared_from_this());
+    }
+}
+
+
+void Goal::amDone(ExitCode result)
+{
+    trace("done");
+    assert(exitCode == ecBusy);
+    assert(result == ecSuccess || result == ecFailed || result == ecNoSubstituters || result == ecIncompleteClosure);
+    exitCode = result;
+    foreach (WeakGoals::iterator, i, waiters) {
+        GoalPtr goal = i->lock();
+        if (goal) goal->waiteeDone(shared_from_this(), result);
+    }
+    waiters.clear();
+    worker.removeGoal(shared_from_this());
+}
+
+
+void Goal::trace(const format & f)
+{
+    debug(format("%1%: %2%") % name % f);
+}
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+/* Common initialisation performed in child processes. */
+static void commonChildInit(Pipe & logPipe)
+{
+    restoreAffinity();
+
+    /* Put the child in a separate session (and thus a separate
+       process group) so that it has no controlling terminal (meaning
+       that e.g. ssh cannot open /dev/tty) and it doesn't receive
+       terminal signals. */
+    if (setsid() == -1)
+        throw SysError(format("creating a new session"));
+
+    /* Dup the write side of the logger pipe into stderr. */
+    if (dup2(logPipe.writeSide, STDERR_FILENO) == -1)
+        throw SysError("cannot pipe standard error into log file");
+
+    /* Dup stderr to stdout. */
+    if (dup2(STDERR_FILENO, STDOUT_FILENO) == -1)
+        throw SysError("cannot dup stderr into stdout");
+
+    /* Reroute stdin to /dev/null. */
+    int fdDevNull = open(pathNullDevice.c_str(), O_RDWR);
+    if (fdDevNull == -1)
+        throw SysError(format("cannot open `%1%'") % pathNullDevice);
+    if (dup2(fdDevNull, STDIN_FILENO) == -1)
+        throw SysError("cannot dup null device into stdin");
+    close(fdDevNull);
+}
+
+
+/* Convert a string list to an array of char pointers.  Careful: the
+   string list should outlive the array. */
+const char * * strings2CharPtrs(const Strings & ss)
+{
+    const char * * arr = new const char * [ss.size() + 1];
+    const char * * p = arr;
+    foreach (Strings::const_iterator, i, ss) *p++ = i->c_str();
+    *p = 0;
+    return arr;
+}
+
+
+/* Restore default handling of SIGPIPE, otherwise some programs will
+   randomly say "Broken pipe". */
+static void restoreSIGPIPE()
+{
+    struct sigaction act, oact;
+    act.sa_handler = SIG_DFL;
+    act.sa_flags = 0;
+    sigemptyset(&act.sa_mask);
+    if (sigaction(SIGPIPE, &act, &oact)) throw SysError("resetting SIGPIPE");
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+class UserLock
+{
+private:
+    /* POSIX locks suck.  If we have a lock on a file, and we open and
+       close that file again (without closing the original file
+       descriptor), we lose the lock.  So we have to be *very* careful
+       not to open a lock file on which we are holding a lock. */
+    static PathSet lockedPaths; /* !!! not thread-safe */
+
+    Path fnUserLock;
+    AutoCloseFD fdUserLock;
+
+    string user;
+    uid_t uid;
+    gid_t gid;
+
+public:
+    UserLock();
+    ~UserLock();
+
+    void acquire();
+    void release();
+
+    void kill();
+
+    string getUser() { return user; }
+    uid_t getUID() { return uid; }
+    uid_t getGID() { return gid; }
+
+    bool enabled() { return uid != 0; }
+
+};
+
+
+PathSet UserLock::lockedPaths;
+
+
+UserLock::UserLock()
+{
+    uid = gid = 0;
+}
+
+
+UserLock::~UserLock()
+{
+    release();
+}
+
+
+void UserLock::acquire()
+{
+    assert(uid == 0);
+
+    assert(settings.buildUsersGroup != "");
+
+    /* Get the members of the build-users-group. */
+    struct group * gr = getgrnam(settings.buildUsersGroup.c_str());
+    if (!gr)
+        throw Error(format("the group `%1%' specified in `build-users-group' does not exist")
+            % settings.buildUsersGroup);
+    gid = gr->gr_gid;
+
+    /* Copy the result of getgrnam. */
+    Strings users;
+    for (char * * p = gr->gr_mem; *p; ++p) {
+        debug(format("found build user `%1%'") % *p);
+        users.push_back(*p);
+    }
+
+    if (users.empty())
+        throw Error(format("the build users group `%1%' has no members")
+            % settings.buildUsersGroup);
+
+    /* Find a user account that isn't currently in use for another
+       build. */
+    foreach (Strings::iterator, i, users) {
+        debug(format("trying user `%1%'") % *i);
+
+        struct passwd * pw = getpwnam(i->c_str());
+        if (!pw)
+            throw Error(format("the user `%1%' in the group `%2%' does not exist")
+                % *i % settings.buildUsersGroup);
+
+        createDirs(settings.nixStateDir + "/userpool");
+
+        fnUserLock = (format("%1%/userpool/%2%") % settings.nixStateDir % pw->pw_uid).str();
+
+        if (lockedPaths.find(fnUserLock) != lockedPaths.end())
+            /* We already have a lock on this one. */
+            continue;
+
+        AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT, 0600);
+        if (fd == -1)
+            throw SysError(format("opening user lock `%1%'") % fnUserLock);
+        closeOnExec(fd);
+
+        if (lockFile(fd, ltWrite, false)) {
+            fdUserLock = fd.borrow();
+            lockedPaths.insert(fnUserLock);
+            user = *i;
+            uid = pw->pw_uid;
+
+            /* Sanity check... */
+            if (uid == getuid() || uid == geteuid())
+                throw Error(format("the Nix user should not be a member of `%1%'")
+                    % settings.buildUsersGroup);
+
+            return;
+        }
+    }
+
+    throw Error(format("all build users are currently in use; "
+        "consider creating additional users and adding them to the `%1%' group")
+        % settings.buildUsersGroup);
+}
+
+
+void UserLock::release()
+{
+    if (uid == 0) return;
+    fdUserLock.close(); /* releases lock */
+    assert(lockedPaths.find(fnUserLock) != lockedPaths.end());
+    lockedPaths.erase(fnUserLock);
+    fnUserLock = "";
+    uid = 0;
+}
+
+
+void UserLock::kill()
+{
+    assert(enabled());
+    killUser(uid);
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+struct HookInstance
+{
+    /* Pipes for talking to the build hook. */
+    Pipe toHook;
+
+    /* Pipe for the hook's standard output/error. */
+    Pipe fromHook;
+
+    /* Pipe for the builder's standard output/error. */
+    Pipe builderOut;
+
+    /* The process ID of the hook. */
+    Pid pid;
+
+    HookInstance();
+
+    ~HookInstance();
+};
+
+
+HookInstance::HookInstance()
+{
+    debug("starting build hook");
+
+    Path buildHook = absPath(getEnv("NIX_BUILD_HOOK"));
+
+    /* Create a pipe to get the output of the child. */
+    fromHook.create();
+
+    /* Create the communication pipes. */
+    toHook.create();
+
+    /* Create a pipe to get the output of the builder. */
+    builderOut.create();
+
+    /* Fork the hook. */
+    pid = maybeVfork();
+    switch (pid) {
+
+    case -1:
+        throw SysError("unable to fork");
+
+    case 0:
+        try { /* child */
+
+            commonChildInit(fromHook);
+
+            if (chdir("/") == -1) throw SysError("changing into `/");
+
+            /* Dup the communication pipes. */
+            if (dup2(toHook.readSide, STDIN_FILENO) == -1)
+                throw SysError("dupping to-hook read side");
+
+            /* Use fd 4 for the builder's stdout/stderr. */
+            if (dup2(builderOut.writeSide, 4) == -1)
+                throw SysError("dupping builder's stdout/stderr");
+
+            execl(buildHook.c_str(), buildHook.c_str(), settings.thisSystem.c_str(),
+                (format("%1%") % settings.maxSilentTime).str().c_str(),
+                (format("%1%") % settings.printBuildTrace).str().c_str(),
+                (format("%1%") % settings.buildTimeout).str().c_str(),
+                NULL);
+
+            throw SysError(format("executing `%1%'") % buildHook);
+
+        } catch (std::exception & e) {
+            writeToStderr("build hook error: " + string(e.what()) + "\n");
+        }
+        _exit(1);
+    }
+
+    /* parent */
+    pid.setSeparatePG(true);
+    pid.setKillSignal(SIGTERM);
+    fromHook.writeSide.close();
+    toHook.readSide.close();
+}
+
+
+HookInstance::~HookInstance()
+{
+    try {
+        pid.kill();
+    } catch (...) {
+        ignoreException();
+    }
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+typedef map<string, string> HashRewrites;
+
+
+string rewriteHashes(string s, const HashRewrites & rewrites)
+{
+    foreach (HashRewrites::const_iterator, i, rewrites) {
+        assert(i->first.size() == i->second.size());
+        size_t j = 0;
+        while ((j = s.find(i->first, j)) != string::npos) {
+            debug(format("rewriting @ %1%") % j);
+            s.replace(j, i->second.size(), i->second);
+        }
+    }
+    return s;
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+typedef enum {rpAccept, rpDecline, rpPostpone} HookReply;
+
+class SubstitutionGoal;
+
+class DerivationGoal : public Goal
+{
+private:
+    /* The path of the derivation. */
+    Path drvPath;
+
+    /* The specific outputs that we need to build.  Empty means all of
+       them. */
+    StringSet wantedOutputs;
+
+    /* Whether additional wanted outputs have been added. */
+    bool needRestart;
+
+    /* Whether to retry substituting the outputs after building the
+       inputs. */
+    bool retrySubstitution;
+
+    /* The derivation stored at drvPath. */
+    Derivation drv;
+
+    /* The remainder is state held during the build. */
+
+    /* Locks on the output paths. */
+    PathLocks outputLocks;
+
+    /* All input paths (that is, the union of FS closures of the
+       immediate input paths). */
+    PathSet inputPaths;
+
+    /* Referenceable paths (i.e., input and output paths). */
+    PathSet allPaths;
+
+    /* Outputs that are already valid.  If we're repairing, these are
+       the outputs that are valid *and* not corrupt. */
+    PathSet validPaths;
+
+    /* Outputs that are corrupt or not valid. */
+    PathSet missingPaths;
+
+    /* User selected for running the builder. */
+    UserLock buildUser;
+
+    /* The process ID of the builder. */
+    Pid pid;
+
+    /* The temporary directory. */
+    Path tmpDir;
+
+    /* File descriptor for the log file. */
+    FILE * fLogFile;
+    BZFILE * bzLogFile;
+    AutoCloseFD fdLogFile;
+
+    /* Number of bytes received from the builder's stdout/stderr. */
+    unsigned long logSize;
+
+    /* Pipe for the builder's standard output/error. */
+    Pipe builderOut;
+
+    /* The build hook. */
+    std::shared_ptr<HookInstance> hook;
+
+    /* Whether we're currently doing a chroot build. */
+    bool useChroot;
+
+    Path chrootRootDir;
+
+    /* RAII object to delete the chroot directory. */
+    std::shared_ptr<AutoDelete> autoDelChroot;
+
+    /* All inputs that are regular files. */
+    PathSet regularInputPaths;
+
+    /* Whether this is a fixed-output derivation. */
+    bool fixedOutput;
+
+    typedef void (DerivationGoal::*GoalState)();
+    GoalState state;
+
+    /* Stuff we need to pass to initChild(). */
+    typedef map<Path, Path> DirsInChroot; // maps target path to source path
+    DirsInChroot dirsInChroot;
+    typedef map<string, string> Environment;
+    Environment env;
+
+    /* Hash rewriting. */
+    HashRewrites rewritesToTmp, rewritesFromTmp;
+    typedef map<Path, Path> RedirectedOutputs;
+    RedirectedOutputs redirectedOutputs;
+
+    BuildMode buildMode;
+
+    /* If we're repairing without a chroot, there may be outputs that
+       are valid but corrupt.  So we redirect these outputs to
+       temporary paths. */
+    PathSet redirectedBadOutputs;
+
+    /* Set of inodes seen during calls to canonicalisePathMetaData()
+       for this build's outputs.  This needs to be shared between
+       outputs to allow hard links between outputs. */
+    InodesSeen inodesSeen;
+
+    /* Magic exit code denoting that setting up the child environment
+       failed.  (It's possible that the child actually returns the
+       exit code, but ah well.) */
+    const static int childSetupFailed = 189;
+
+public:
+    DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs, Worker & worker, BuildMode buildMode = bmNormal);
+    ~DerivationGoal();
+
+    void cancel(bool timeout);
+
+    void work();
+
+    Path getDrvPath()
+    {
+        return drvPath;
+    }
+
+    /* Add wanted outputs to an already existing derivation goal. */
+    void addWantedOutputs(const StringSet & outputs);
+
+private:
+    /* The states. */
+    void init();
+    void haveDerivation();
+    void outputsSubstituted();
+    void closureRepaired();
+    void inputsRealised();
+    void tryToBuild();
+    void buildDone();
+
+    /* Is the build hook willing to perform the build? */
+    HookReply tryBuildHook();
+
+    /* Start building a derivation. */
+    void startBuilder();
+
+    /* Initialise the builder's process. */
+    void initChild();
+
+    friend int childEntry(void *);
+
+    /* Check that the derivation outputs all exist and register them
+       as valid. */
+    void registerOutputs();
+
+    /* Open a log file and a pipe to it. */
+    Path openLogFile();
+
+    /* Close the log file. */
+    void closeLogFile();
+
+    /* Delete the temporary directory, if we have one. */
+    void deleteTmpDir(bool force);
+
+    /* Callback used by the worker to write to the log. */
+    void handleChildOutput(int fd, const string & data);
+    void handleEOF(int fd);
+
+    /* Return the set of (in)valid paths. */
+    PathSet checkPathValidity(bool returnValid, bool checkHash);
+
+    /* Abort the goal if `path' failed to build. */
+    bool pathFailed(const Path & path);
+
+    /* Forcibly kill the child process, if any. */
+    void killChild();
+
+    Path addHashRewrite(const Path & path);
+
+    void repairClosure();
+};
+
+
+DerivationGoal::DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs, Worker & worker, BuildMode buildMode)
+    : Goal(worker)
+    , wantedOutputs(wantedOutputs)
+    , needRestart(false)
+    , retrySubstitution(false)
+    , fLogFile(0)
+    , bzLogFile(0)
+    , useChroot(false)
+    , buildMode(buildMode)
+{
+    this->drvPath = drvPath;
+    state = &DerivationGoal::init;
+    name = (format("building of `%1%'") % drvPath).str();
+    trace("created");
+}
+
+
+DerivationGoal::~DerivationGoal()
+{
+    /* Careful: we should never ever throw an exception from a
+       destructor. */
+    try {
+        killChild();
+        deleteTmpDir(false);
+        closeLogFile();
+    } catch (...) {
+        ignoreException();
+    }
+}
+
+
+void DerivationGoal::killChild()
+{
+    if (pid != -1) {
+        worker.childTerminated(pid);
+
+        if (buildUser.enabled()) {
+            /* If we're using a build user, then there is a tricky
+               race condition: if we kill the build user before the
+               child has done its setuid() to the build user uid, then
+               it won't be killed, and we'll potentially lock up in
+               pid.wait().  So also send a conventional kill to the
+               child. */
+            ::kill(-pid, SIGKILL); /* ignore the result */
+            buildUser.kill();
+            pid.wait(true);
+        } else
+            pid.kill();
+
+        assert(pid == -1);
+    }
+
+    hook.reset();
+}
+
+
+void DerivationGoal::cancel(bool timeout)
+{
+    if (settings.printBuildTrace && timeout)
+        printMsg(lvlError, format("@ build-failed %1% - timeout") % drvPath);
+    killChild();
+    amDone(ecFailed);
+}
+
+
+void DerivationGoal::work()
+{
+    (this->*state)();
+}
+
+
+void DerivationGoal::addWantedOutputs(const StringSet & outputs)
+{
+    /* If we already want all outputs, there is nothing to do. */
+    if (wantedOutputs.empty()) return;
+
+    if (outputs.empty()) {
+        wantedOutputs.clear();
+        needRestart = true;
+    } else
+        foreach (StringSet::const_iterator, i, outputs)
+            if (wantedOutputs.find(*i) == wantedOutputs.end()) {
+                wantedOutputs.insert(*i);
+                needRestart = true;
+            }
+}
+
+
+void DerivationGoal::init()
+{
+    trace("init");
+
+    if (settings.readOnlyMode)
+        throw Error(format("cannot build derivation `%1%' - no write access to the Nix store") % drvPath);
+
+    /* The first thing to do is to make sure that the derivation
+       exists.  If it doesn't, it may be created through a
+       substitute. */
+    addWaitee(worker.makeSubstitutionGoal(drvPath));
+
+    state = &DerivationGoal::haveDerivation;
+}
+
+
+void DerivationGoal::haveDerivation()
+{
+    trace("loading derivation");
+
+    if (nrFailed != 0) {
+        printMsg(lvlError, format("cannot build missing derivation `%1%'") % drvPath);
+        amDone(ecFailed);
+        return;
+    }
+
+    /* `drvPath' should already be a root, but let's be on the safe
+       side: if the user forgot to make it a root, we wouldn't want
+       things being garbage collected while we're busy. */
+    worker.store.addTempRoot(drvPath);
+
+    assert(worker.store.isValidPath(drvPath));
+
+    /* Get the derivation. */
+    drv = derivationFromPath(worker.store, drvPath);
+
+    foreach (DerivationOutputs::iterator, i, drv.outputs)
+        worker.store.addTempRoot(i->second.path);
+
+    /* Check what outputs paths are not already valid. */
+    PathSet invalidOutputs = checkPathValidity(false, buildMode == bmRepair);
+
+    /* If they are all valid, then we're done. */
+    if (invalidOutputs.size() == 0 && buildMode == bmNormal) {
+        amDone(ecSuccess);
+        return;
+    }
+
+    /* Check whether any output previously failed to build.  If so,
+       don't bother. */
+    foreach (PathSet::iterator, i, invalidOutputs)
+        if (pathFailed(*i)) return;
+
+    /* We are first going to try to create the invalid output paths
+       through substitutes.  If that doesn't work, we'll build
+       them. */
+    if (settings.useSubstitutes && !willBuildLocally(drv))
+        foreach (PathSet::iterator, i, invalidOutputs)
+            addWaitee(worker.makeSubstitutionGoal(*i, buildMode == bmRepair));
+
+    if (waitees.empty()) /* to prevent hang (no wake-up event) */
+        outputsSubstituted();
+    else
+        state = &DerivationGoal::outputsSubstituted;
+}
+
+
+void DerivationGoal::outputsSubstituted()
+{
+    trace("all outputs substituted (maybe)");
+
+    if (nrFailed > 0 && nrFailed > nrNoSubstituters + nrIncompleteClosure && !settings.tryFallback)
+        throw Error(format("some substitutes for the outputs of derivation `%1%' failed (usually happens due to networking issues); try `--fallback' to build derivation from source ") % drvPath);
+
+    /*  If the substitutes form an incomplete closure, then we should
+        build the dependencies of this derivation, but after that, we
+        can still use the substitutes for this derivation itself. */
+    if (nrIncompleteClosure > 0 && !retrySubstitution) retrySubstitution = true;
+
+    nrFailed = nrNoSubstituters = nrIncompleteClosure = 0;
+
+    if (needRestart) {
+        needRestart = false;
+        haveDerivation();
+        return;
+    }
+
+    unsigned int nrInvalid = checkPathValidity(false, buildMode == bmRepair).size();
+    if (buildMode == bmNormal && nrInvalid == 0) {
+        amDone(ecSuccess);
+        return;
+    }
+    if (buildMode == bmRepair && nrInvalid == 0) {
+        repairClosure();
+        return;
+    }
+    if (buildMode == bmCheck && nrInvalid > 0)
+        throw Error(format("some outputs of `%1%' are not valid, so checking is not possible") % drvPath);
+
+    /* Otherwise, at least one of the output paths could not be
+       produced using a substitute.  So we have to build instead. */
+
+    /* Make sure checkPathValidity() from now on checks all
+       outputs. */
+    wantedOutputs = PathSet();
+
+    /* The inputs must be built before we can build this goal. */
+    foreach (DerivationInputs::iterator, i, drv.inputDrvs)
+        addWaitee(worker.makeDerivationGoal(i->first, i->second, buildMode == bmRepair ? bmRepair : bmNormal));
+
+    foreach (PathSet::iterator, i, drv.inputSrcs)
+        addWaitee(worker.makeSubstitutionGoal(*i));
+
+    if (waitees.empty()) /* to prevent hang (no wake-up event) */
+        inputsRealised();
+    else
+        state = &DerivationGoal::inputsRealised;
+}
+
+
+void DerivationGoal::repairClosure()
+{
+    /* If we're repairing, we now know that our own outputs are valid.
+       Now check whether the other paths in the outputs closure are
+       good.  If not, then start derivation goals for the derivations
+       that produced those outputs. */
+
+    /* Get the output closure. */
+    PathSet outputClosure;
+    foreach (DerivationOutputs::iterator, i, drv.outputs)
+        computeFSClosure(worker.store, i->second.path, outputClosure);
+
+    /* Filter out our own outputs (which we have already checked). */
+    foreach (DerivationOutputs::iterator, i, drv.outputs)
+        outputClosure.erase(i->second.path);
+
+    /* Get all dependencies of this derivation so that we know which
+       derivation is responsible for which path in the output
+       closure. */
+    PathSet inputClosure;
+    computeFSClosure(worker.store, drvPath, inputClosure);
+    std::map<Path, Path> outputsToDrv;
+    foreach (PathSet::iterator, i, inputClosure)
+        if (isDerivation(*i)) {
+            Derivation drv = derivationFromPath(worker.store, *i);
+            foreach (DerivationOutputs::iterator, j, drv.outputs)
+                outputsToDrv[j->second.path] = *i;
+        }
+
+    /* Check each path (slow!). */
+    PathSet broken;
+    foreach (PathSet::iterator, i, outputClosure) {
+        if (worker.store.pathContentsGood(*i)) continue;
+        printMsg(lvlError, format("found corrupted or missing path `%1%' in the output closure of `%2%'") % *i % drvPath);
+        Path drvPath2 = outputsToDrv[*i];
+        if (drvPath2 == "")
+            addWaitee(worker.makeSubstitutionGoal(*i, true));
+        else
+            addWaitee(worker.makeDerivationGoal(drvPath2, PathSet(), bmRepair));
+    }
+
+    if (waitees.empty()) {
+        amDone(ecSuccess);
+        return;
+    }
+
+    state = &DerivationGoal::closureRepaired;
+}
+
+
+void DerivationGoal::closureRepaired()
+{
+    trace("closure repaired");
+    if (nrFailed > 0)
+        throw Error(format("some paths in the output closure of derivation `%1%' could not be repaired") % drvPath);
+    amDone(ecSuccess);
+}
+
+
+void DerivationGoal::inputsRealised()
+{
+    trace("all inputs realised");
+
+    if (nrFailed != 0) {
+        printMsg(lvlError,
+            format("cannot build derivation `%1%': %2% dependencies couldn't be built")
+            % drvPath % nrFailed);
+        amDone(ecFailed);
+        return;
+    }
+
+    if (retrySubstitution) {
+        haveDerivation();
+        return;
+    }
+
+    /* Gather information necessary for computing the closure and/or
+       running the build hook. */
+
+    /* The outputs are referenceable paths. */
+    foreach (DerivationOutputs::iterator, i, drv.outputs) {
+        debug(format("building path `%1%'") % i->second.path);
+        allPaths.insert(i->second.path);
+    }
+
+    /* Determine the full set of input paths. */
+
+    /* First, the input derivations. */
+    foreach (DerivationInputs::iterator, i, drv.inputDrvs) {
+        /* Add the relevant output closures of the input derivation
+           `*i' as input paths.  Only add the closures of output paths
+           that are specified as inputs. */
+        assert(worker.store.isValidPath(i->first));
+        Derivation inDrv = derivationFromPath(worker.store, i->first);
+        foreach (StringSet::iterator, j, i->second)
+            if (inDrv.outputs.find(*j) != inDrv.outputs.end())
+                computeFSClosure(worker.store, inDrv.outputs[*j].path, inputPaths);
+            else
+                throw Error(
+                    format("derivation `%1%' requires non-existent output `%2%' from input derivation `%3%'")
+                    % drvPath % *j % i->first);
+    }
+
+    /* Second, the input sources. */
+    foreach (PathSet::iterator, i, drv.inputSrcs)
+        computeFSClosure(worker.store, *i, inputPaths);
+
+    debug(format("added input paths %1%") % showPaths(inputPaths));
+
+    allPaths.insert(inputPaths.begin(), inputPaths.end());
+
+    /* Is this a fixed-output derivation? */
+    fixedOutput = true;
+    foreach (DerivationOutputs::iterator, i, drv.outputs)
+        if (i->second.hash == "") fixedOutput = false;
+
+    /* Okay, try to build.  Note that here we don't wait for a build
+       slot to become available, since we don't need one if there is a
+       build hook. */
+    state = &DerivationGoal::tryToBuild;
+    worker.wakeUp(shared_from_this());
+}
+
+
+PathSet outputPaths(const DerivationOutputs & outputs)
+{
+    PathSet paths;
+    foreach (DerivationOutputs::const_iterator, i, outputs)
+        paths.insert(i->second.path);
+    return paths;
+}
+
+
+static string get(const StringPairs & map, const string & key)
+{
+    StringPairs::const_iterator i = map.find(key);
+    return i == map.end() ? (string) "" : i->second;
+}
+
+
+static bool canBuildLocally(const string & platform)
+{
+    return platform == settings.thisSystem
+#ifdef CAN_DO_LINUX32_BUILDS
+        || (platform == "i686-linux" && settings.thisSystem == "x86_64-linux")
+#endif
+        ;
+}
+
+
+bool willBuildLocally(const Derivation & drv)
+{
+    return get(drv.env, "preferLocalBuild") == "1" && canBuildLocally(drv.platform);
+}
+
+
+void DerivationGoal::tryToBuild()
+{
+    trace("trying to build");
+
+    /* Check for the possibility that some other goal in this process
+       has locked the output since we checked in haveDerivation().
+       (It can't happen between here and the lockPaths() call below
+       because we're not allowing multi-threading.)  If so, put this
+       goal to sleep until another goal finishes, then try again. */
+    foreach (DerivationOutputs::iterator, i, drv.outputs)
+        if (pathIsLockedByMe(i->second.path)) {
+            debug(format("putting derivation `%1%' to sleep because `%2%' is locked by another goal")
+                % drvPath % i->second.path);
+            worker.waitForAnyGoal(shared_from_this());
+            return;
+        }
+
+    /* Obtain locks on all output paths.  The locks are automatically
+       released when we exit this function or Nix crashes.  If we
+       can't acquire the lock, then continue; hopefully some other
+       goal can start a build, and if not, the main loop will sleep a
+       few seconds and then retry this goal. */
+    if (!outputLocks.lockPaths(outputPaths(drv.outputs), "", false)) {
+        worker.waitForAWhile(shared_from_this());
+        return;
+    }
+
+    /* Now check again whether the outputs are valid.  This is because
+       another process may have started building in parallel.  After
+       it has finished and released the locks, we can (and should)
+       reuse its results.  (Strictly speaking the first check can be
+       omitted, but that would be less efficient.)  Note that since we
+       now hold the locks on the output paths, no other process can
+       build this derivation, so no further checks are necessary. */
+    validPaths = checkPathValidity(true, buildMode == bmRepair);
+    assert(buildMode != bmCheck || validPaths.size() == drv.outputs.size());
+    if (buildMode != bmCheck && validPaths.size() == drv.outputs.size()) {
+        debug(format("skipping build of derivation `%1%', someone beat us to it") % drvPath);
+        outputLocks.setDeletion(true);
+        amDone(ecSuccess);
+        return;
+    }
+
+    missingPaths = outputPaths(drv.outputs);
+    if (buildMode != bmCheck)
+        foreach (PathSet::iterator, i, validPaths) missingPaths.erase(*i);
+
+    /* If any of the outputs already exist but are not valid, delete
+       them. */
+    foreach (DerivationOutputs::iterator, i, drv.outputs) {
+        Path path = i->second.path;
+        if (worker.store.isValidPath(path)) continue;
+        if (!pathExists(path)) continue;
+        debug(format("removing invalid path `%1%'") % path);
+        deletePath(path);
+    }
+
+    /* Check again whether any output previously failed to build,
+       because some other process may have tried and failed before we
+       acquired the lock. */
+    foreach (DerivationOutputs::iterator, i, drv.outputs)
+        if (pathFailed(i->second.path)) return;
+
+    /* Don't do a remote build if the derivation has the attribute
+       `preferLocalBuild' set.  Also, check and repair modes are only
+       supported for local builds. */
+    bool buildLocally = buildMode != bmNormal || willBuildLocally(drv);
+
+    /* Is the build hook willing to accept this job? */
+    if (!buildLocally) {
+        switch (tryBuildHook()) {
+            case rpAccept:
+                /* Yes, it has started doing so.  Wait until we get
+                   EOF from the hook. */
+                state = &DerivationGoal::buildDone;
+                return;
+            case rpPostpone:
+                /* Not now; wait until at least one child finishes or
+                   the wake-up timeout expires. */
+                worker.waitForAWhile(shared_from_this());
+                outputLocks.unlock();
+                return;
+            case rpDecline:
+                /* We should do it ourselves. */
+                break;
+        }
+    }
+
+    /* Make sure that we are allowed to start a build.  If this
+       derivation prefers to be done locally, do it even if
+       maxBuildJobs is 0. */
+    unsigned int curBuilds = worker.getNrLocalBuilds();
+    if (curBuilds >= settings.maxBuildJobs && !(buildLocally && curBuilds == 0)) {
+        worker.waitForBuildSlot(shared_from_this());
+        outputLocks.unlock();
+        return;
+    }
+
+    try {
+
+        /* Okay, we have to build. */
+        startBuilder();
+
+    } catch (BuildError & e) {
+        printMsg(lvlError, e.msg());
+        outputLocks.unlock();
+        buildUser.release();
+        if (settings.printBuildTrace)
+            printMsg(lvlError, format("@ build-failed %1% - %2% %3%")
+                % drvPath % 0 % e.msg());
+        worker.permanentFailure = true;
+        amDone(ecFailed);
+        return;
+    }
+
+    /* This state will be reached when we get EOF on the child's
+       log pipe. */
+    state = &DerivationGoal::buildDone;
+}
+
+
+void replaceValidPath(const Path & storePath, const Path tmpPath)
+{
+    /* We can't atomically replace storePath (the original) with
+       tmpPath (the replacement), so we have to move it out of the
+       way first.  We'd better not be interrupted here, because if
+       we're repairing (say) Glibc, we end up with a broken system. */
+    Path oldPath = (format("%1%.old-%2%-%3%") % storePath % getpid() % rand()).str();
+    if (pathExists(storePath))
+        rename(storePath.c_str(), oldPath.c_str());
+    if (rename(tmpPath.c_str(), storePath.c_str()) == -1)
+        throw SysError(format("moving `%1%' to `%2%'") % tmpPath % storePath);
+    if (pathExists(oldPath))
+        deletePath(oldPath);
+}
+
+
+void DerivationGoal::buildDone()
+{
+    trace("build done");
+
+    /* Since we got an EOF on the logger pipe, the builder is presumed
+       to have terminated.  In fact, the builder could also have
+       simply have closed its end of the pipe --- just don't do that
+       :-) */
+    int status;
+    pid_t savedPid;
+    if (hook) {
+        savedPid = hook->pid;
+        status = hook->pid.wait(true);
+    } else {
+        /* !!! this could block! security problem! solution: kill the
+           child */
+        savedPid = pid;
+        status = pid.wait(true);
+    }
+
+    debug(format("builder process for `%1%' finished") % drvPath);
+
+    /* So the child is gone now. */
+    worker.childTerminated(savedPid);
+
+    /* Close the read side of the logger pipe. */
+    if (hook) {
+        hook->builderOut.readSide.close();
+        hook->fromHook.readSide.close();
+    }
+    else builderOut.readSide.close();
+
+    /* Close the log file. */
+    closeLogFile();
+
+    /* When running under a build user, make sure that all processes
+       running under that uid are gone.  This is to prevent a
+       malicious user from leaving behind a process that keeps files
+       open and modifies them after they have been chown'ed to
+       root. */
+    if (buildUser.enabled()) buildUser.kill();
+
+    bool diskFull = false;
+
+    try {
+
+        /* Check the exit status. */
+        if (!statusOk(status)) {
+
+            /* Heuristically check whether the build failure may have
+               been caused by a disk full condition.  We have no way
+               of knowing whether the build actually got an ENOSPC.
+               So instead, check if the disk is (nearly) full now.  If
+               so, we don't mark this build as a permanent failure. */
+#if HAVE_STATVFS
+            unsigned long long required = 8ULL * 1024 * 1024; // FIXME: make configurable
+            struct statvfs st;
+            if (statvfs(settings.nixStore.c_str(), &st) == 0 &&
+                (unsigned long long) st.f_bavail * st.f_bsize < required)
+                diskFull = true;
+            if (statvfs(tmpDir.c_str(), &st) == 0 &&
+                (unsigned long long) st.f_bavail * st.f_bsize < required)
+                diskFull = true;
+#endif
+
+            deleteTmpDir(false);
+
+            /* Move paths out of the chroot for easier debugging of
+               build failures. */
+            if (useChroot && buildMode == bmNormal)
+                foreach (PathSet::iterator, i, missingPaths)
+                    if (pathExists(chrootRootDir + *i))
+                        rename((chrootRootDir + *i).c_str(), i->c_str());
+
+            if (WIFEXITED(status) && WEXITSTATUS(status) == childSetupFailed)
+                throw Error(format("failed to set up the build environment for `%1%'") % drvPath);
+
+            if (diskFull)
+                printMsg(lvlError, "note: build failure may have been caused by lack of free disk space");
+
+            throw BuildError(format("builder for `%1%' %2%")
+                % drvPath % statusToString(status));
+        }
+
+        /* Compute the FS closure of the outputs and register them as
+           being valid. */
+        registerOutputs();
+
+        if (buildMode == bmCheck) {
+            amDone(ecSuccess);
+            return;
+        }
+
+        /* Delete unused redirected outputs (when doing hash rewriting). */
+        foreach (RedirectedOutputs::iterator, i, redirectedOutputs)
+            if (pathExists(i->second)) deletePath(i->second);
+
+        /* Delete the chroot (if we were using one). */
+        autoDelChroot.reset(); /* this runs the destructor */
+
+        deleteTmpDir(true);
+
+        /* It is now safe to delete the lock files, since all future
+           lockers will see that the output paths are valid; they will
+           not create new lock files with the same names as the old
+           (unlinked) lock files. */
+        outputLocks.setDeletion(true);
+        outputLocks.unlock();
+
+    } catch (BuildError & e) {
+        printMsg(lvlError, e.msg());
+        outputLocks.unlock();
+        buildUser.release();
+
+        /* When using a build hook, the hook will return a remote
+           build failure using exit code 100.  Anything else is a hook
+           problem. */
+        bool hookError = hook &&
+            (!WIFEXITED(status) || WEXITSTATUS(status) != 100);
+
+        if (settings.printBuildTrace) {
+            if (hook && hookError)
+                printMsg(lvlError, format("@ hook-failed %1% - %2% %3%")
+                    % drvPath % status % e.msg());
+            else
+                printMsg(lvlError, format("@ build-failed %1% - %2% %3%")
+                    % drvPath % 1 % e.msg());
+        }
+
+        /* Register the outputs of this build as "failed" so we won't
+           try to build them again (negative caching).  However, don't
+           do this for fixed-output derivations, since they're likely
+           to fail for transient reasons (e.g., fetchurl not being
+           able to access the network).  Hook errors (like
+           communication problems with the remote machine) shouldn't
+           be cached either. */
+        if (settings.cacheFailure && !hookError && !fixedOutput)
+            foreach (DerivationOutputs::iterator, i, drv.outputs)
+                worker.store.registerFailedPath(i->second.path);
+
+        worker.permanentFailure = !hookError && !fixedOutput && !diskFull;
+        amDone(ecFailed);
+        return;
+    }
+
+    /* Release the build user, if applicable. */
+    buildUser.release();
+
+    if (settings.printBuildTrace)
+        printMsg(lvlError, format("@ build-succeeded %1% -") % drvPath);
+
+    amDone(ecSuccess);
+}
+
+
+HookReply DerivationGoal::tryBuildHook()
+{
+    if (!settings.useBuildHook || getEnv("NIX_BUILD_HOOK") == "") return rpDecline;
+
+    if (!worker.hook)
+        worker.hook = std::shared_ptr<HookInstance>(new HookInstance);
+
+    /* Tell the hook about system features (beyond the system type)
+       required from the build machine.  (The hook could parse the
+       drv file itself, but this is easier.) */
+    Strings features = tokenizeString<Strings>(get(drv.env, "requiredSystemFeatures"));
+    foreach (Strings::iterator, i, features) checkStoreName(*i); /* !!! abuse */
+
+    /* Send the request to the hook. */
+    writeLine(worker.hook->toHook.writeSide, (format("%1% %2% %3% %4%")
+        % (worker.getNrLocalBuilds() < settings.maxBuildJobs ? "1" : "0")
+        % drv.platform % drvPath % concatStringsSep(",", features)).str());
+
+    /* Read the first line of input, which should be a word indicating
+       whether the hook wishes to perform the build. */
+    string reply;
+    while (true) {
+        string s = readLine(worker.hook->fromHook.readSide);
+        if (string(s, 0, 2) == "# ") {
+            reply = string(s, 2);
+            break;
+        }
+        s += "\n";
+        writeToStderr(s);
+    }
+
+    debug(format("hook reply is `%1%'") % reply);
+
+    if (reply == "decline" || reply == "postpone")
+        return reply == "decline" ? rpDecline : rpPostpone;
+    else if (reply != "accept")
+        throw Error(format("bad hook reply `%1%'") % reply);
+
+    printMsg(lvlTalkative, format("using hook to build path(s) %1%") % showPaths(missingPaths));
+
+    hook = worker.hook;
+    worker.hook.reset();
+
+    /* Tell the hook all the inputs that have to be copied to the
+       remote system.  This unfortunately has to contain the entire
+       derivation closure to ensure that the validity invariant holds
+       on the remote system.  (I.e., it's unfortunate that we have to
+       list it since the remote system *probably* already has it.) */
+    PathSet allInputs;
+    allInputs.insert(inputPaths.begin(), inputPaths.end());
+    computeFSClosure(worker.store, drvPath, allInputs);
+
+    string s;
+    foreach (PathSet::iterator, i, allInputs) { s += *i; s += ' '; }
+    writeLine(hook->toHook.writeSide, s);
+
+    /* Tell the hooks the missing outputs that have to be copied back
+       from the remote system. */
+    s = "";
+    foreach (PathSet::iterator, i, missingPaths) { s += *i; s += ' '; }
+    writeLine(hook->toHook.writeSide, s);
+
+    hook->toHook.writeSide.close();
+
+    /* Create the log file and pipe. */
+    Path logFile = openLogFile();
+
+    set<int> fds;
+    fds.insert(hook->fromHook.readSide);
+    fds.insert(hook->builderOut.readSide);
+    worker.childStarted(shared_from_this(), hook->pid, fds, false, false);
+
+    if (settings.printBuildTrace)
+        printMsg(lvlError, format("@ build-started %1% - %2% %3%")
+            % drvPath % drv.platform % logFile);
+
+    return rpAccept;
+}
+
+
+void chmod_(const Path & path, mode_t mode)
+{
+    if (chmod(path.c_str(), mode) == -1)
+        throw SysError(format("setting permissions on `%1%'") % path);
+}
+
+
+int childEntry(void * arg)
+{
+    ((DerivationGoal *) arg)->initChild();
+    return 1;
+}
+
+
+void DerivationGoal::startBuilder()
+{
+    startNest(nest, lvlInfo, format(
+            buildMode == bmRepair ? "repairing path(s) %1%" :
+            buildMode == bmCheck ? "checking path(s) %1%" :
+            "building path(s) %1%") % showPaths(missingPaths));
+
+    /* Right platform? */
+    if (!canBuildLocally(drv.platform)) {
+        if (settings.printBuildTrace)
+            printMsg(lvlError, format("@ unsupported-platform %1% %2%") % drvPath % drv.platform);
+        throw Error(
+            format("a `%1%' is required to build `%3%', but I am a `%2%'")
+            % drv.platform % settings.thisSystem % drvPath);
+    }
+
+    /* Construct the environment passed to the builder. */
+
+    /* Most shells initialise PATH to some default (/bin:/usr/bin:...) when
+       PATH is not set.  We don't want this, so we fill it in with some dummy
+       value. */
+    env["PATH"] = "/path-not-set";
+
+    /* Set HOME to a non-existing path to prevent certain programs from using
+       /etc/passwd (or NIS, or whatever) to locate the home directory (for
+       example, wget looks for ~/.wgetrc).  I.e., these tools use /etc/passwd
+       if HOME is not set, but they will just assume that the settings file
+       they are looking for does not exist if HOME is set but points to some
+       non-existing path. */
+    Path homeDir = "/homeless-shelter";
+    env["HOME"] = homeDir;
+
+    /* Tell the builder where the Nix store is.  Usually they
+       shouldn't care, but this is useful for purity checking (e.g.,
+       the compiler or linker might only want to accept paths to files
+       in the store or in the build directory). */
+    env["NIX_STORE"] = settings.nixStore;
+
+    /* The maximum number of cores to utilize for parallel building. */
+    env["NIX_BUILD_CORES"] = (format("%d") % settings.buildCores).str();
+
+    /* Add all bindings specified in the derivation. */
+    foreach (StringPairs::iterator, i, drv.env)
+        env[i->first] = i->second;
+
+    /* Create a temporary directory where the build will take
+       place. */
+    tmpDir = createTempDir("", "nix-build-" + storePathToName(drvPath), false, false, 0700);
+
+    /* For convenience, set an environment pointing to the top build
+       directory. */
+    env["NIX_BUILD_TOP"] = tmpDir;
+
+    /* Also set TMPDIR and variants to point to this directory. */
+    env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmpDir;
+
+    /* Explicitly set PWD to prevent problems with chroot builds.  In
+       particular, dietlibc cannot figure out the cwd because the
+       inode of the current directory doesn't appear in .. (because
+       getdents returns the inode of the mount point). */
+    env["PWD"] = tmpDir;
+
+    /* Compatibility hack with Nix <= 0.7: if this is a fixed-output
+       derivation, tell the builder, so that for instance `fetchurl'
+       can skip checking the output.  On older Nixes, this environment
+       variable won't be set, so `fetchurl' will do the check. */
+    if (fixedOutput) env["NIX_OUTPUT_CHECKED"] = "1";
+
+    /* *Only* if this is a fixed-output derivation, propagate the
+       values of the environment variables specified in the
+       `impureEnvVars' attribute to the builder.  This allows for
+       instance environment variables for proxy configuration such as
+       `http_proxy' to be easily passed to downloaders like
+       `fetchurl'.  Passing such environment variables from the caller
+       to the builder is generally impure, but the output of
+       fixed-output derivations is by definition pure (since we
+       already know the cryptographic hash of the output). */
+    if (fixedOutput) {
+        Strings varNames = tokenizeString<Strings>(get(drv.env, "impureEnvVars"));
+        foreach (Strings::iterator, i, varNames) env[*i] = getEnv(*i);
+    }
+
+    /* The `exportReferencesGraph' feature allows the references graph
+       to be passed to a builder.  This attribute should be a list of
+       pairs [name1 path1 name2 path2 ...].  The references graph of
+       each `pathN' will be stored in a text file `nameN' in the
+       temporary build directory.  The text files have the format used
+       by `nix-store --register-validity'.  However, the deriver
+       fields are left empty. */
+    string s = get(drv.env, "exportReferencesGraph");
+    Strings ss = tokenizeString<Strings>(s);
+    if (ss.size() % 2 != 0)
+        throw BuildError(format("odd number of tokens in `exportReferencesGraph': `%1%'") % s);
+    for (Strings::iterator i = ss.begin(); i != ss.end(); ) {
+        string fileName = *i++;
+        checkStoreName(fileName); /* !!! abuse of this function */
+
+        /* Check that the store path is valid. */
+        Path storePath = *i++;
+        if (!isInStore(storePath))
+            throw BuildError(format("`exportReferencesGraph' contains a non-store path `%1%'")
+                % storePath);
+        storePath = toStorePath(storePath);
+        if (!worker.store.isValidPath(storePath))
+            throw BuildError(format("`exportReferencesGraph' contains an invalid path `%1%'")
+                % storePath);
+
+        /* If there are derivations in the graph, then include their
+           outputs as well.  This is useful if you want to do things
+           like passing all build-time dependencies of some path to a
+           derivation that builds a NixOS DVD image. */
+        PathSet paths, paths2;
+        computeFSClosure(worker.store, storePath, paths);
+        paths2 = paths;
+
+        foreach (PathSet::iterator, j, paths2) {
+            if (isDerivation(*j)) {
+                Derivation drv = derivationFromPath(worker.store, *j);
+                foreach (DerivationOutputs::iterator, k, drv.outputs)
+                    computeFSClosure(worker.store, k->second.path, paths);
+            }
+        }
+
+        /* Write closure info to `fileName'. */
+        writeFile(tmpDir + "/" + fileName,
+            worker.store.makeValidityRegistration(paths, false, false));
+    }
+
+
+    /* If `build-users-group' is not empty, then we have to build as
+       one of the members of that group. */
+    if (settings.buildUsersGroup != "") {
+        buildUser.acquire();
+        assert(buildUser.getUID() != 0);
+        assert(buildUser.getGID() != 0);
+
+        /* Make sure that no other processes are executing under this
+           uid. */
+        buildUser.kill();
+
+        /* Change ownership of the temporary build directory. */
+        if (chown(tmpDir.c_str(), buildUser.getUID(), buildUser.getGID()) == -1)
+            throw SysError(format("cannot change ownership of `%1%'") % tmpDir);
+
+        /* Check that the Nix store has the appropriate permissions,
+           i.e., owned by root and mode 1775 (sticky bit on so that
+           the builder can create its output but not mess with the
+           outputs of other processes). */
+        struct stat st;
+        if (stat(settings.nixStore.c_str(), &st) == -1)
+            throw SysError(format("cannot stat `%1%'") % settings.nixStore);
+        if (!(st.st_mode & S_ISVTX) ||
+            ((st.st_mode & S_IRWXG) != S_IRWXG) ||
+            (st.st_gid != buildUser.getGID()))
+            throw Error(format(
+                "builder does not have write permission to `%2%'; "
+                "try `chgrp %1% %2%; chmod 1775 %2%'")
+                % buildUser.getGID() % settings.nixStore);
+    }
+
+
+    /* Are we doing a chroot build?  Note that fixed-output
+       derivations are never done in a chroot, mainly so that
+       functions like fetchurl (which needs a proper /etc/resolv.conf)
+       work properly.  Purity checking for fixed-output derivations
+       is somewhat pointless anyway. */
+    useChroot = settings.useChroot;
+
+    if (fixedOutput) useChroot = false;
+
+    /* Hack to allow derivations to disable chroot builds. */
+    if (get(drv.env, "__noChroot") == "1") useChroot = false;
+
+    if (useChroot) {
+#if CHROOT_ENABLED
+        /* Create a temporary directory in which we set up the chroot
+           environment using bind-mounts.  We put it in the Nix store
+           to ensure that we can create hard-links to non-directory
+           inputs in the fake Nix store in the chroot (see below). */
+        chrootRootDir = drvPath + ".chroot";
+        if (pathExists(chrootRootDir)) deletePath(chrootRootDir);
+
+        /* Clean up the chroot directory automatically. */
+        autoDelChroot = std::shared_ptr<AutoDelete>(new AutoDelete(chrootRootDir));
+
+        printMsg(lvlChatty, format("setting up chroot environment in `%1%'") % chrootRootDir);
+
+        /* Create a writable /tmp in the chroot.  Many builders need
+           this.  (Of course they should really respect $TMPDIR
+           instead.) */
+        Path chrootTmpDir = chrootRootDir + "/tmp";
+        createDirs(chrootTmpDir);
+        chmod_(chrootTmpDir, 01777);
+
+        /* Create a /etc/passwd with entries for the build user and the
+           nobody account.  The latter is kind of a hack to support
+           Samba-in-QEMU. */
+        createDirs(chrootRootDir + "/etc");
+
+        writeFile(chrootRootDir + "/etc/passwd",
+            (format(
+                "nixbld:x:%1%:%2%:Nix build user:/:/noshell\n"
+                "nobody:x:65534:65534:Nobody:/:/noshell\n")
+                % (buildUser.enabled() ? buildUser.getUID() : getuid())
+                % (buildUser.enabled() ? buildUser.getGID() : getgid())).str());
+
+        /* Declare the build user's group so that programs get a consistent
+           view of the system (e.g., "id -gn"). */
+        writeFile(chrootRootDir + "/etc/group",
+            (format("nixbld:!:%1%:\n")
+                % (buildUser.enabled() ? buildUser.getGID() : getgid())).str());
+
+        /* Create /etc/hosts with localhost entry. */
+        writeFile(chrootRootDir + "/etc/hosts", "127.0.0.1 localhost\n");
+
+        /* Bind-mount a user-configurable set of directories from the
+           host file system. */
+        foreach (StringSet::iterator, i, settings.dirsInChroot) {
+            size_t p = i->find('=');
+            if (p == string::npos)
+                dirsInChroot[*i] = *i;
+            else
+                dirsInChroot[string(*i, 0, p)] = string(*i, p + 1);
+        }
+        dirsInChroot[tmpDir] = tmpDir;
+
+        /* Make the closure of the inputs available in the chroot,
+           rather than the whole Nix store.  This prevents any access
+           to undeclared dependencies.  Directories are bind-mounted,
+           while other inputs are hard-linked (since only directories
+           can be bind-mounted).  !!! As an extra security
+           precaution, make the fake Nix store only writable by the
+           build user. */
+        createDirs(chrootRootDir + settings.nixStore);
+        chmod_(chrootRootDir + settings.nixStore, 01777);
+
+        foreach (PathSet::iterator, i, inputPaths) {
+            struct stat st;
+            if (lstat(i->c_str(), &st))
+                throw SysError(format("getting attributes of path `%1%'") % *i);
+            if (S_ISDIR(st.st_mode))
+                dirsInChroot[*i] = *i;
+            else {
+                Path p = chrootRootDir + *i;
+                if (link(i->c_str(), p.c_str()) == -1) {
+                    /* Hard-linking fails if we exceed the maximum
+                       link count on a file (e.g. 32000 of ext3),
+                       which is quite possible after a `nix-store
+                       --optimise'. */
+                    if (errno != EMLINK)
+                        throw SysError(format("linking `%1%' to `%2%'") % p % *i);
+                    StringSink sink;
+                    dumpPath(*i, sink);
+                    StringSource source(sink.s);
+                    restorePath(p, source);
+                }
+
+                regularInputPaths.insert(*i);
+            }
+        }
+
+        /* If we're repairing or checking, it's possible that we're
+           rebuilding a path that is in settings.dirsInChroot
+           (typically the dependencies of /bin/sh).  Throw them
+           out. */
+        if (buildMode != bmNormal)
+            foreach (DerivationOutputs::iterator, i, drv.outputs)
+                dirsInChroot.erase(i->second.path);
+
+#else
+        throw Error("chroot builds are not supported on this platform");
+#endif
+    }
+
+    else {
+
+        if (pathExists(homeDir))
+            throw Error(format("directory `%1%' exists; please remove it") % homeDir);
+
+        /* We're not doing a chroot build, but we have some valid
+           output paths.  Since we can't just overwrite or delete
+           them, we have to do hash rewriting: i.e. in the
+           environment/arguments passed to the build, we replace the
+           hashes of the valid outputs with unique dummy strings;
+           after the build, we discard the redirected outputs
+           corresponding to the valid outputs, and rewrite the
+           contents of the new outputs to replace the dummy strings
+           with the actual hashes. */
+        if (validPaths.size() > 0)
+            foreach (PathSet::iterator, i, validPaths)
+                addHashRewrite(*i);
+
+        /* If we're repairing, then we don't want to delete the
+           corrupt outputs in advance.  So rewrite them as well. */
+        if (buildMode == bmRepair)
+            foreach (PathSet::iterator, i, missingPaths)
+                if (worker.store.isValidPath(*i) && pathExists(*i)) {
+                    addHashRewrite(*i);
+                    redirectedBadOutputs.insert(*i);
+                }
+    }
+
+
+    /* Run the builder. */
+    printMsg(lvlChatty, format("executing builder `%1%'") % drv.builder);
+
+    /* Create the log file. */
+    Path logFile = openLogFile();
+
+    /* Create a pipe to get the output of the builder. */
+    builderOut.create();
+
+    /* Fork a child to build the package.  Note that while we
+       currently use forks to run and wait for the children, it
+       shouldn't be hard to use threads for this on systems where
+       fork() is unavailable or inefficient.
+
+       If we're building in a chroot, then also set up private
+       namespaces for the build:
+
+       - The PID namespace causes the build to start as PID 1.
+         Processes outside of the chroot are not visible to those on
+         the inside, but processes inside the chroot are visible from
+         the outside (though with different PIDs).
+
+       - The private mount namespace ensures that all the bind mounts
+         we do will only show up in this process and its children, and
+         will disappear automatically when we're done.
+
+       - The private network namespace ensures that the builder cannot
+         talk to the outside world (or vice versa).  It only has a
+         private loopback interface.
+
+       - The IPC namespace prevents the builder from communicating
+         with outside processes using SysV IPC mechanisms (shared
+         memory, message queues, semaphores).  It also ensures that
+         all IPC objects are destroyed when the builder exits.
+
+       - The UTS namespace ensures that builders see a hostname of
+         localhost rather than the actual hostname.
+    */
+#if CHROOT_ENABLED
+    if (useChroot) {
+        char stack[32 * 1024];
+        pid = clone(childEntry, stack + sizeof(stack) - 8,
+            CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWNET | CLONE_NEWIPC | CLONE_NEWUTS | SIGCHLD, this);
+    } else
+#endif
+    {
+        pid = fork();
+        if (pid == 0) initChild();
+    }
+
+    if (pid == -1) throw SysError("unable to fork");
+
+    /* parent */
+    pid.setSeparatePG(true);
+    builderOut.writeSide.close();
+    worker.childStarted(shared_from_this(), pid,
+        singleton<set<int> >(builderOut.readSide), true, true);
+
+    if (settings.printBuildTrace) {
+        printMsg(lvlError, format("@ build-started %1% - %2% %3%")
+            % drvPath % drv.platform % logFile);
+    }
+}
+
+
+void DerivationGoal::initChild()
+{
+    /* Warning: in the child we should absolutely not make any SQLite
+       calls! */
+
+    bool inSetup = true;
+
+    try { /* child */
+
+#if CHROOT_ENABLED
+        if (useChroot) {
+            /* Initialise the loopback interface. */
+            AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
+            if (fd == -1) throw SysError("cannot open IP socket");
+
+            struct ifreq ifr;
+            strcpy(ifr.ifr_name, "lo");
+            ifr.ifr_flags = IFF_UP | IFF_LOOPBACK | IFF_RUNNING;
+            if (ioctl(fd, SIOCSIFFLAGS, &ifr) == -1)
+                throw SysError("cannot set loopback interface flags");
+
+            fd.close();
+
+            /* Set the hostname etc. to fixed values. */
+            char hostname[] = "localhost";
+            sethostname(hostname, sizeof(hostname));
+            char domainname[] = "(none)"; // kernel default
+            setdomainname(domainname, sizeof(domainname));
+
+            /* Make all filesystems private.  This is necessary
+               because subtrees may have been mounted as "shared"
+               (MS_SHARED).  (Systemd does this, for instance.)  Even
+               though we have a private mount namespace, mounting
+               filesystems on top of a shared subtree still propagates
+               outside of the namespace.  Making a subtree private is
+               local to the namespace, though, so setting MS_PRIVATE
+               does not affect the outside world. */
+            Strings mounts = tokenizeString<Strings>(readFile("/proc/self/mountinfo", true), "\n");
+            foreach (Strings::iterator, i, mounts) {
+                vector<string> fields = tokenizeString<vector<string> >(*i, " ");
+                string fs = decodeOctalEscaped(fields.at(4));
+                if (mount(0, fs.c_str(), 0, MS_PRIVATE, 0) == -1)
+                    throw SysError(format("unable to make filesystem `%1%' private") % fs);
+            }
+
+            /* Set up a nearly empty /dev, unless the user asked to
+               bind-mount the host /dev. */
+            if (dirsInChroot.find("/dev") == dirsInChroot.end()) {
+                createDirs(chrootRootDir + "/dev/shm");
+                createDirs(chrootRootDir + "/dev/pts");
+                Strings ss;
+                ss.push_back("/dev/full");
+#ifdef __linux__
+                if (pathExists("/dev/kvm"))
+                    ss.push_back("/dev/kvm");
+#endif
+                ss.push_back("/dev/null");
+                ss.push_back("/dev/random");
+                ss.push_back("/dev/tty");
+                ss.push_back("/dev/urandom");
+                ss.push_back("/dev/zero");
+                foreach (Strings::iterator, i, ss) dirsInChroot[*i] = *i;
+                createSymlink("/proc/self/fd", chrootRootDir + "/dev/fd");
+                createSymlink("/proc/self/fd/0", chrootRootDir + "/dev/stdin");
+                createSymlink("/proc/self/fd/1", chrootRootDir + "/dev/stdout");
+                createSymlink("/proc/self/fd/2", chrootRootDir + "/dev/stderr");
+            }
+
+            /* Bind-mount all the directories from the "host"
+               filesystem that we want in the chroot
+               environment. */
+            foreach (DirsInChroot::iterator, i, dirsInChroot) {
+                struct stat st;
+                Path source = i->second;
+                Path target = chrootRootDir + i->first;
+                if (source == "/proc") continue; // backwards compatibility
+                debug(format("bind mounting `%1%' to `%2%'") % source % target);
+                if (stat(source.c_str(), &st) == -1)
+                    throw SysError(format("getting attributes of path `%1%'") % source);
+                if (S_ISDIR(st.st_mode))
+                    createDirs(target);
+                else {
+                    createDirs(dirOf(target));
+                    writeFile(target, "");
+                }
+                if (mount(source.c_str(), target.c_str(), "", MS_BIND, 0) == -1)
+                    throw SysError(format("bind mount from `%1%' to `%2%' failed") % source % target);
+            }
+
+            /* Bind a new instance of procfs on /proc to reflect our
+               private PID namespace. */
+            createDirs(chrootRootDir + "/proc");
+            if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0, 0) == -1)
+                throw SysError("mounting /proc");
+
+            /* Mount a new tmpfs on /dev/shm to ensure that whatever
+               the builder puts in /dev/shm is cleaned up automatically. */
+            if (pathExists("/dev/shm") && mount("none", (chrootRootDir + "/dev/shm").c_str(), "tmpfs", 0, 0) == -1)
+                throw SysError("mounting /dev/shm");
+
+            /* Mount a new devpts on /dev/pts.  Note that this
+               requires the kernel to be compiled with
+               CONFIG_DEVPTS_MULTIPLE_INSTANCES=y (which is the case
+               if /dev/ptx/ptmx exists). */
+            if (pathExists("/dev/pts/ptmx") &&
+                !pathExists(chrootRootDir + "/dev/ptmx")
+                && dirsInChroot.find("/dev/pts") == dirsInChroot.end())
+            {
+                if (mount("none", (chrootRootDir + "/dev/pts").c_str(), "devpts", 0, "newinstance,mode=0620") == -1)
+                    throw SysError("mounting /dev/pts");
+                createSymlink("/dev/pts/ptmx", chrootRootDir + "/dev/ptmx");
+
+		/* Make sure /dev/pts/ptmx is world-writable.  With some
+		   Linux versions, it is created with permissions 0.  */
+		chmod_(chrootRootDir + "/dev/pts/ptmx", 0666);
+            }
+
+            /* Do the chroot().  Below we do a chdir() to the
+               temporary build directory to make sure the current
+               directory is in the chroot.  (Actually the order
+               doesn't matter, since due to the bind mount tmpDir and
+               tmpRootDit/tmpDir are the same directories.) */
+            if (chroot(chrootRootDir.c_str()) == -1)
+                throw SysError(format("cannot change root directory to `%1%'") % chrootRootDir);
+        }
+#endif
+
+        commonChildInit(builderOut);
+
+        if (chdir(tmpDir.c_str()) == -1)
+            throw SysError(format("changing into `%1%'") % tmpDir);
+
+        /* Close all other file descriptors. */
+        closeMostFDs(set<int>());
+
+#ifdef CAN_DO_LINUX32_BUILDS
+        /* Change the personality to 32-bit if we're doing an
+           i686-linux build on an x86_64-linux machine. */
+        struct utsname utsbuf;
+        uname(&utsbuf);
+        if (drv.platform == "i686-linux" &&
+            (settings.thisSystem == "x86_64-linux" ||
+             (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "x86_64")))) {
+            if (personality(0x0008 | 0x8000000 /* == PER_LINUX32_3GB */) == -1)
+                throw SysError("cannot set i686-linux personality");
+        }
+
+        /* Impersonate a Linux 2.6 machine to get some determinism in
+           builds that depend on the kernel version. */
+        if ((drv.platform == "i686-linux" || drv.platform == "x86_64-linux") && settings.impersonateLinux26) {
+            int cur = personality(0xffffffff);
+            if (cur != -1) personality(cur | 0x0020000 /* == UNAME26 */);
+        }
+#endif
+
+        /* Fill in the environment. */
+        Strings envStrs;
+        foreach (Environment::const_iterator, i, env)
+            envStrs.push_back(rewriteHashes(i->first + "=" + i->second, rewritesToTmp));
+        const char * * envArr = strings2CharPtrs(envStrs);
+
+        Path program = drv.builder.c_str();
+        std::vector<const char *> args; /* careful with c_str()! */
+        string user; /* must be here for its c_str()! */
+
+        /* If we are running in `build-users' mode, then switch to the
+           user we allocated above.  Make sure that we drop all root
+           privileges.  Note that above we have closed all file
+           descriptors except std*, so that's safe.  Also note that
+           setuid() when run as root sets the real, effective and
+           saved UIDs. */
+        if (buildUser.enabled()) {
+            printMsg(lvlChatty, format("switching to user `%1%'") % buildUser.getUser());
+
+            if (setgroups(0, 0) == -1)
+                throw SysError("cannot clear the set of supplementary groups");
+
+            if (setgid(buildUser.getGID()) == -1 ||
+                getgid() != buildUser.getGID() ||
+                getegid() != buildUser.getGID())
+                throw SysError("setgid failed");
+
+            if (setuid(buildUser.getUID()) == -1 ||
+                getuid() != buildUser.getUID() ||
+                geteuid() != buildUser.getUID())
+                throw SysError("setuid failed");
+        }
+
+        /* Fill in the arguments. */
+        string builderBasename = baseNameOf(drv.builder);
+        args.push_back(builderBasename.c_str());
+        foreach (Strings::iterator, i, drv.args)
+            args.push_back(rewriteHashes(*i, rewritesToTmp).c_str());
+        args.push_back(0);
+
+        restoreSIGPIPE();
+
+        /* Execute the program.  This should not return. */
+        inSetup = false;
+        execve(program.c_str(), (char * *) &args[0], (char * *) envArr);
+
+        throw SysError(format("executing `%1%'") % drv.builder);
+
+    } catch (std::exception & e) {
+        writeToStderr("build error: " + string(e.what()) + "\n");
+        _exit(inSetup ? childSetupFailed : 1);
+    }
+
+    abort(); /* never reached */
+}
+
+
+/* Parse a list of reference specifiers.  Each element must either be
+   a store path, or the symbolic name of the output of the derivation
+   (such as `out'). */
+PathSet parseReferenceSpecifiers(const Derivation & drv, string attr)
+{
+    PathSet result;
+    Paths paths = tokenizeString<Paths>(attr);
+    foreach (Strings::iterator, i, paths) {
+        if (isStorePath(*i))
+            result.insert(*i);
+        else if (drv.outputs.find(*i) != drv.outputs.end())
+            result.insert(drv.outputs.find(*i)->second.path);
+        else throw BuildError(
+            format("derivation contains an illegal reference specifier `%1%'")
+            % *i);
+    }
+    return result;
+}
+
+
+void DerivationGoal::registerOutputs()
+{
+    /* When using a build hook, the build hook can register the output
+       as valid (by doing `nix-store --import').  If so we don't have
+       to do anything here. */
+    if (hook) {
+        bool allValid = true;
+        foreach (DerivationOutputs::iterator, i, drv.outputs)
+            if (!worker.store.isValidPath(i->second.path)) allValid = false;
+        if (allValid) return;
+    }
+
+    ValidPathInfos infos;
+
+    /* Check whether the output paths were created, and grep each
+       output path to determine what other paths it references.  Also make all
+       output paths read-only. */
+    foreach (DerivationOutputs::iterator, i, drv.outputs) {
+        Path path = i->second.path;
+        if (missingPaths.find(path) == missingPaths.end()) continue;
+
+        Path actualPath = path;
+        if (useChroot) {
+            actualPath = chrootRootDir + path;
+            if (pathExists(actualPath)) {
+                /* Move output paths from the chroot to the Nix store. */
+                if (buildMode == bmRepair)
+                    replaceValidPath(path, actualPath);
+                else
+                    if (buildMode != bmCheck && rename(actualPath.c_str(), path.c_str()) == -1)
+                        throw SysError(format("moving build output `%1%' from the chroot to the Nix store") % path);
+            }
+            if (buildMode != bmCheck) actualPath = path;
+        } else {
+            Path redirected = redirectedOutputs[path];
+            if (buildMode == bmRepair
+                && redirectedBadOutputs.find(path) != redirectedBadOutputs.end()
+                && pathExists(redirected))
+                replaceValidPath(path, redirected);
+            if (buildMode == bmCheck)
+                actualPath = redirected;
+        }
+
+        struct stat st;
+        if (lstat(actualPath.c_str(), &st) == -1) {
+            if (errno == ENOENT)
+                throw BuildError(
+                    format("builder for `%1%' failed to produce output path `%2%'")
+                    % drvPath % path);
+            throw SysError(format("getting attributes of path `%1%'") % actualPath);
+        }
+
+#ifndef __CYGWIN__
+        /* Check that the output is not group or world writable, as
+           that means that someone else can have interfered with the
+           build.  Also, the output should be owned by the build
+           user. */
+        if ((!S_ISLNK(st.st_mode) && (st.st_mode & (S_IWGRP | S_IWOTH))) ||
+            (buildUser.enabled() && st.st_uid != buildUser.getUID()))
+            throw BuildError(format("suspicious ownership or permission on `%1%'; rejecting this build output") % path);
+#endif
+
+        /* Apply hash rewriting if necessary. */
+        bool rewritten = false;
+        if (!rewritesFromTmp.empty()) {
+            printMsg(lvlError, format("warning: rewriting hashes in `%1%'; cross fingers") % path);
+
+            /* Canonicalise first.  This ensures that the path we're
+               rewriting doesn't contain a hard link to /etc/shadow or
+               something like that. */
+            canonicalisePathMetaData(actualPath, buildUser.enabled() ? buildUser.getUID() : -1, inodesSeen);
+
+            /* FIXME: this is in-memory. */
+            StringSink sink;
+            dumpPath(actualPath, sink);
+            deletePath(actualPath);
+            sink.s = rewriteHashes(sink.s, rewritesFromTmp);
+            StringSource source(sink.s);
+            restorePath(actualPath, source);
+
+            rewritten = true;
+        }
+
+        startNest(nest, lvlTalkative,
+            format("scanning for references inside `%1%'") % path);
+
+        /* Check that fixed-output derivations produced the right
+           outputs (i.e., the content hash should match the specified
+           hash). */
+        if (i->second.hash != "") {
+
+            bool recursive; HashType ht; Hash h;
+            i->second.parseHashInfo(recursive, ht, h);
+
+            if (!recursive) {
+                /* The output path should be a regular file without
+                   execute permission. */
+                if (!S_ISREG(st.st_mode) || (st.st_mode & S_IXUSR) != 0)
+                    throw BuildError(
+                        format("output path `%1% should be a non-executable regular file") % path);
+            }
+
+            /* Check the hash. */
+            Hash h2 = recursive ? hashPath(ht, actualPath).first : hashFile(ht, actualPath);
+            if (h != h2)
+                throw BuildError(
+                    format("output path `%1%' should have %2% hash `%3%', instead has `%4%'")
+                    % path % i->second.hashAlgo % printHash16or32(h) % printHash16or32(h2));
+        }
+
+        /* Get rid of all weird permissions.  This also checks that
+           all files are owned by the build user, if applicable. */
+        canonicalisePathMetaData(actualPath,
+            buildUser.enabled() && !rewritten ? buildUser.getUID() : -1, inodesSeen);
+
+        /* For this output path, find the references to other paths
+           contained in it.  Compute the SHA-256 NAR hash at the same
+           time.  The hash is stored in the database so that we can
+           verify later on whether nobody has messed with the store. */
+        HashResult hash;
+        PathSet references = scanForReferences(actualPath, allPaths, hash);
+
+        if (buildMode == bmCheck) {
+            ValidPathInfo info = worker.store.queryPathInfo(path);
+            if (hash.first != info.hash)
+                throw Error(format("derivation `%2%' may not be deterministic: hash mismatch in output `%1%'") % drvPath % path);
+            continue;
+        }
+
+        /* For debugging, print out the referenced and unreferenced
+           paths. */
+        foreach (PathSet::iterator, i, inputPaths) {
+            PathSet::iterator j = references.find(*i);
+            if (j == references.end())
+                debug(format("unreferenced input: `%1%'") % *i);
+            else
+                debug(format("referenced input: `%1%'") % *i);
+        }
+
+        /* If the derivation specifies an `allowedReferences'
+           attribute (containing a list of paths that the output may
+           refer to), check that all references are in that list.  !!!
+           allowedReferences should really be per-output. */
+        if (drv.env.find("allowedReferences") != drv.env.end()) {
+            PathSet allowed = parseReferenceSpecifiers(drv, get(drv.env, "allowedReferences"));
+            foreach (PathSet::iterator, i, references)
+                if (allowed.find(*i) == allowed.end())
+                    throw BuildError(format("output is not allowed to refer to path `%1%'") % *i);
+        }
+
+        worker.store.optimisePath(path); // FIXME: combine with scanForReferences()
+
+        worker.store.markContentsGood(path);
+
+        ValidPathInfo info;
+        info.path = path;
+        info.hash = hash.first;
+        info.narSize = hash.second;
+        info.references = references;
+        info.deriver = drvPath;
+        infos.push_back(info);
+    }
+
+    if (buildMode == bmCheck) return;
+
+    /* Register each output path as valid, and register the sets of
+       paths referenced by each of them.  If there are cycles in the
+       outputs, this will fail. */
+    worker.store.registerValidPaths(infos);
+}
+
+
+string drvsLogDir = "drvs";
+
+
+Path DerivationGoal::openLogFile()
+{
+    logSize = 0;
+
+    if (!settings.keepLog) return "";
+
+    string baseName = baseNameOf(drvPath);
+
+    /* Create a log file. */
+    Path dir = (format("%1%/%2%/%3%/") % settings.nixLogDir % drvsLogDir % string(baseName, 0, 2)).str();
+    createDirs(dir);
+
+    if (settings.compressLog) {
+
+        Path logFileName = (format("%1%/%2%.bz2") % dir % string(baseName, 2)).str();
+        AutoCloseFD fd = open(logFileName.c_str(), O_CREAT | O_WRONLY | O_TRUNC, 0666);
+        if (fd == -1) throw SysError(format("creating log file `%1%'") % logFileName);
+        closeOnExec(fd);
+
+        if (!(fLogFile = fdopen(fd.borrow(), "w")))
+            throw SysError(format("opening file `%1%'") % logFileName);
+
+        int err;
+        if (!(bzLogFile = BZ2_bzWriteOpen(&err, fLogFile, 9, 0, 0)))
+            throw Error(format("cannot open compressed log file `%1%'") % logFileName);
+
+        return logFileName;
+
+    } else {
+        Path logFileName = (format("%1%/%2%") % dir % string(baseName, 2)).str();
+        fdLogFile = open(logFileName.c_str(), O_CREAT | O_WRONLY | O_TRUNC, 0666);
+        if (fdLogFile == -1) throw SysError(format("creating log file `%1%'") % logFileName);
+        closeOnExec(fdLogFile);
+        return logFileName;
+    }
+}
+
+
+void DerivationGoal::closeLogFile()
+{
+    if (bzLogFile) {
+        int err;
+        BZ2_bzWriteClose(&err, bzLogFile, 0, 0, 0);
+        bzLogFile = 0;
+        if (err != BZ_OK) throw Error(format("cannot close compressed log file (BZip2 error = %1%)") % err);
+    }
+
+    if (fLogFile) {
+        fclose(fLogFile);
+        fLogFile = 0;
+    }
+
+    fdLogFile.close();
+}
+
+
+void DerivationGoal::deleteTmpDir(bool force)
+{
+    if (tmpDir != "") {
+        if (settings.keepFailed && !force) {
+            printMsg(lvlError,
+                format("note: keeping build directory `%2%'")
+                % drvPath % tmpDir);
+            chmod(tmpDir.c_str(), 0755);
+        }
+        else
+            deletePath(tmpDir);
+        tmpDir = "";
+    }
+}
+
+
+void DerivationGoal::handleChildOutput(int fd, const string & data)
+{
+    if ((hook && fd == hook->builderOut.readSide) ||
+        (!hook && fd == builderOut.readSide))
+    {
+        logSize += data.size();
+        if (settings.maxLogSize && logSize > settings.maxLogSize) {
+            printMsg(lvlError,
+                format("%1% killed after writing more than %2% bytes of log output")
+                % getName() % settings.maxLogSize);
+            cancel(true); // not really a timeout, but close enough
+            return;
+        }
+        if (verbosity >= settings.buildVerbosity)
+            writeToStderr(data);
+        if (bzLogFile) {
+            int err;
+            BZ2_bzWrite(&err, bzLogFile, (unsigned char *) data.data(), data.size());
+            if (err != BZ_OK) throw Error(format("cannot write to compressed log file (BZip2 error = %1%)") % err);
+        } else if (fdLogFile != -1)
+            writeFull(fdLogFile, (unsigned char *) data.data(), data.size());
+    }
+
+    if (hook && fd == hook->fromHook.readSide)
+        writeToStderr(data);
+}
+
+
+void DerivationGoal::handleEOF(int fd)
+{
+    worker.wakeUp(shared_from_this());
+}
+
+
+PathSet DerivationGoal::checkPathValidity(bool returnValid, bool checkHash)
+{
+    PathSet result;
+    foreach (DerivationOutputs::iterator, i, drv.outputs) {
+        if (!wantOutput(i->first, wantedOutputs)) continue;
+        bool good =
+            worker.store.isValidPath(i->second.path) &&
+            (!checkHash || worker.store.pathContentsGood(i->second.path));
+        if (good == returnValid) result.insert(i->second.path);
+    }
+    return result;
+}
+
+
+bool DerivationGoal::pathFailed(const Path & path)
+{
+    if (!settings.cacheFailure) return false;
+
+    if (!worker.store.hasPathFailed(path)) return false;
+
+    printMsg(lvlError, format("builder for `%1%' failed previously (cached)") % path);
+
+    if (settings.printBuildTrace)
+        printMsg(lvlError, format("@ build-failed %1% - cached") % drvPath);
+
+    worker.permanentFailure = true;
+    amDone(ecFailed);
+
+    return true;
+}
+
+
+Path DerivationGoal::addHashRewrite(const Path & path)
+{
+    string h1 = string(path, settings.nixStore.size() + 1, 32);
+    string h2 = string(printHash32(hashString(htSHA256, "rewrite:" + drvPath + ":" + path)), 0, 32);
+    Path p = settings.nixStore + "/" + h2 + string(path, settings.nixStore.size() + 33);
+    if (pathExists(p)) deletePath(p);
+    assert(path.size() == p.size());
+    rewritesToTmp[h1] = h2;
+    rewritesFromTmp[h2] = h1;
+    redirectedOutputs[path] = p;
+    return p;
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+class SubstitutionGoal : public Goal
+{
+    friend class Worker;
+
+private:
+    /* The store path that should be realised through a substitute. */
+    Path storePath;
+
+    /* The remaining substituters. */
+    Paths subs;
+
+    /* The current substituter. */
+    Path sub;
+
+    /* Whether any substituter can realise this path */
+    bool hasSubstitute;
+
+    /* Path info returned by the substituter's query info operation. */
+    SubstitutablePathInfo info;
+
+    /* Pipe for the substituter's standard output. */
+    Pipe outPipe;
+
+    /* Pipe for the substituter's standard error. */
+    Pipe logPipe;
+
+    /* The process ID of the builder. */
+    Pid pid;
+
+    /* Lock on the store path. */
+    std::shared_ptr<PathLocks> outputLock;
+
+    /* Whether to try to repair a valid path. */
+    bool repair;
+
+    /* Location where we're downloading the substitute.  Differs from
+       storePath when doing a repair. */
+    Path destPath;
+
+    typedef void (SubstitutionGoal::*GoalState)();
+    GoalState state;
+
+public:
+    SubstitutionGoal(const Path & storePath, Worker & worker, bool repair = false);
+    ~SubstitutionGoal();
+
+    void cancel(bool timeout);
+
+    void work();
+
+    /* The states. */
+    void init();
+    void tryNext();
+    void gotInfo();
+    void referencesValid();
+    void tryToRun();
+    void finished();
+
+    /* Callback used by the worker to write to the log. */
+    void handleChildOutput(int fd, const string & data);
+    void handleEOF(int fd);
+
+    Path getStorePath() { return storePath; }
+};
+
+
+SubstitutionGoal::SubstitutionGoal(const Path & storePath, Worker & worker, bool repair)
+    : Goal(worker)
+    , hasSubstitute(false)
+    , repair(repair)
+{
+    this->storePath = storePath;
+    state = &SubstitutionGoal::init;
+    name = (format("substitution of `%1%'") % storePath).str();
+    trace("created");
+}
+
+
+SubstitutionGoal::~SubstitutionGoal()
+{
+    if (pid != -1) worker.childTerminated(pid);
+}
+
+
+void SubstitutionGoal::cancel(bool timeout)
+{
+    if (settings.printBuildTrace && timeout)
+        printMsg(lvlError, format("@ substituter-failed %1% timeout") % storePath);
+    if (pid != -1) {
+        pid_t savedPid = pid;
+        pid.kill();
+        worker.childTerminated(savedPid);
+    }
+    amDone(ecFailed);
+}
+
+
+void SubstitutionGoal::work()
+{
+    (this->*state)();
+}
+
+
+void SubstitutionGoal::init()
+{
+    trace("init");
+
+    worker.store.addTempRoot(storePath);
+
+    /* If the path already exists we're done. */
+    if (!repair && worker.store.isValidPath(storePath)) {
+        amDone(ecSuccess);
+        return;
+    }
+
+    if (settings.readOnlyMode)
+        throw Error(format("cannot substitute path `%1%' - no write access to the Nix store") % storePath);
+
+    subs = settings.substituters;
+
+    tryNext();
+}
+
+
+void SubstitutionGoal::tryNext()
+{
+    trace("trying next substituter");
+
+    if (subs.size() == 0) {
+        /* None left.  Terminate this goal and let someone else deal
+           with it. */
+        debug(format("path `%1%' is required, but there is no substituter that can build it") % storePath);
+        /* Hack: don't indicate failure if there were no substituters.
+           In that case the calling derivation should just do a
+           build. */
+        amDone(hasSubstitute ? ecFailed : ecNoSubstituters);
+        return;
+    }
+
+    sub = subs.front();
+    subs.pop_front();
+
+    SubstitutablePathInfos infos;
+    PathSet dummy(singleton<PathSet>(storePath));
+    worker.store.querySubstitutablePathInfos(sub, dummy, infos);
+    SubstitutablePathInfos::iterator k = infos.find(storePath);
+    if (k == infos.end()) { tryNext(); return; }
+    info = k->second;
+    hasSubstitute = true;
+
+    /* To maintain the closure invariant, we first have to realise the
+       paths referenced by this one. */
+    foreach (PathSet::iterator, i, info.references)
+        if (*i != storePath) /* ignore self-references */
+            addWaitee(worker.makeSubstitutionGoal(*i));
+
+    if (waitees.empty()) /* to prevent hang (no wake-up event) */
+        referencesValid();
+    else
+        state = &SubstitutionGoal::referencesValid;
+}
+
+
+void SubstitutionGoal::referencesValid()
+{
+    trace("all references realised");
+
+    if (nrFailed > 0) {
+        debug(format("some references of path `%1%' could not be realised") % storePath);
+        amDone(nrNoSubstituters > 0 || nrIncompleteClosure > 0 ? ecIncompleteClosure : ecFailed);
+        return;
+    }
+
+    foreach (PathSet::iterator, i, info.references)
+        if (*i != storePath) /* ignore self-references */
+            assert(worker.store.isValidPath(*i));
+
+    state = &SubstitutionGoal::tryToRun;
+    worker.wakeUp(shared_from_this());
+}
+
+
+void SubstitutionGoal::tryToRun()
+{
+    trace("trying to run");
+
+    /* Make sure that we are allowed to start a build.  Note that even
+       is maxBuildJobs == 0 (no local builds allowed), we still allow
+       a substituter to run.  This is because substitutions cannot be
+       distributed to another machine via the build hook. */
+    if (worker.getNrLocalBuilds() >= (settings.maxBuildJobs == 0 ? 1 : settings.maxBuildJobs)) {
+        worker.waitForBuildSlot(shared_from_this());
+        return;
+    }
+
+    /* Maybe a derivation goal has already locked this path
+       (exceedingly unlikely, since it should have used a substitute
+       first, but let's be defensive). */
+    outputLock.reset(); // make sure this goal's lock is gone
+    if (pathIsLockedByMe(storePath)) {
+        debug(format("restarting substitution of `%1%' because it's locked by another goal")
+            % storePath);
+        worker.waitForAnyGoal(shared_from_this());
+        return; /* restart in the tryToRun() state when another goal finishes */
+    }
+
+    /* Acquire a lock on the output path. */
+    outputLock = std::shared_ptr<PathLocks>(new PathLocks);
+    if (!outputLock->lockPaths(singleton<PathSet>(storePath), "", false)) {
+        worker.waitForAWhile(shared_from_this());
+        return;
+    }
+
+    /* Check again whether the path is invalid. */
+    if (!repair && worker.store.isValidPath(storePath)) {
+        debug(format("store path `%1%' has become valid") % storePath);
+        outputLock->setDeletion(true);
+        amDone(ecSuccess);
+        return;
+    }
+
+    printMsg(lvlInfo, format("fetching path `%1%'...") % storePath);
+
+    outPipe.create();
+    logPipe.create();
+
+    destPath = repair ? storePath + ".tmp" : storePath;
+
+    /* Remove the (stale) output path if it exists. */
+    if (pathExists(destPath))
+        deletePath(destPath);
+
+    worker.store.setSubstituterEnv();
+
+    /* Fill in the arguments. */
+    Strings args;
+    args.push_back(baseNameOf(sub));
+    args.push_back("--substitute");
+    args.push_back(storePath);
+    args.push_back(destPath);
+    const char * * argArr = strings2CharPtrs(args);
+
+    /* Fork the substitute program. */
+    pid = maybeVfork();
+
+    switch (pid) {
+
+    case -1:
+        throw SysError("unable to fork");
+
+    case 0:
+        try { /* child */
+
+            commonChildInit(logPipe);
+
+            if (dup2(outPipe.writeSide, STDOUT_FILENO) == -1)
+                throw SysError("cannot dup output pipe into stdout");
+
+            execv(sub.c_str(), (char * *) argArr);
+
+            throw SysError(format("executing `%1%'") % sub);
+
+        } catch (std::exception & e) {
+            writeToStderr("substitute error: " + string(e.what()) + "\n");
+        }
+        _exit(1);
+    }
+
+    /* parent */
+    pid.setSeparatePG(true);
+    pid.setKillSignal(SIGTERM);
+    outPipe.writeSide.close();
+    logPipe.writeSide.close();
+    worker.childStarted(shared_from_this(),
+        pid, singleton<set<int> >(logPipe.readSide), true, true);
+
+    state = &SubstitutionGoal::finished;
+
+    if (settings.printBuildTrace)
+        printMsg(lvlError, format("@ substituter-started %1% %2%") % storePath % sub);
+}
+
+
+void SubstitutionGoal::finished()
+{
+    trace("substitute finished");
+
+    /* Since we got an EOF on the logger pipe, the substitute is
+       presumed to have terminated.  */
+    pid_t savedPid = pid;
+    int status = pid.wait(true);
+
+    /* So the child is gone now. */
+    worker.childTerminated(savedPid);
+
+    /* Close the read side of the logger pipe. */
+    logPipe.readSide.close();
+
+    /* Get the hash info from stdout. */
+    string dummy = readLine(outPipe.readSide);
+    string expectedHashStr = statusOk(status) ? readLine(outPipe.readSide) : "";
+    outPipe.readSide.close();
+
+    /* Check the exit status and the build result. */
+    HashResult hash;
+    try {
+
+        if (!statusOk(status))
+            throw SubstError(format("fetching path `%1%' %2%")
+                % storePath % statusToString(status));
+
+        if (!pathExists(destPath))
+            throw SubstError(format("substitute did not produce path `%1%'") % destPath);
+
+        hash = hashPath(htSHA256, destPath);
+
+        /* Verify the expected hash we got from the substituer. */
+        if (expectedHashStr != "") {
+            size_t n = expectedHashStr.find(':');
+            if (n == string::npos)
+                throw Error(format("bad hash from substituter: %1%") % expectedHashStr);
+            HashType hashType = parseHashType(string(expectedHashStr, 0, n));
+            if (hashType == htUnknown)
+                throw Error(format("unknown hash algorithm in `%1%'") % expectedHashStr);
+            Hash expectedHash = parseHash16or32(hashType, string(expectedHashStr, n + 1));
+            Hash actualHash = hashType == htSHA256 ? hash.first : hashPath(hashType, destPath).first;
+            if (expectedHash != actualHash)
+                throw SubstError(format("hash mismatch in downloaded path `%1%': expected %2%, got %3%")
+                    % storePath % printHash(expectedHash) % printHash(actualHash));
+        }
+
+    } catch (SubstError & e) {
+
+        printMsg(lvlInfo, e.msg());
+
+        if (settings.printBuildTrace) {
+            printMsg(lvlError, format("@ substituter-failed %1% %2% %3%")
+                % storePath % status % e.msg());
+        }
+
+        /* Try the next substitute. */
+        state = &SubstitutionGoal::tryNext;
+        worker.wakeUp(shared_from_this());
+        return;
+    }
+
+    if (repair) replaceValidPath(storePath, destPath);
+
+    canonicalisePathMetaData(storePath, -1);
+
+    worker.store.optimisePath(storePath); // FIXME: combine with hashPath()
+
+    ValidPathInfo info2;
+    info2.path = storePath;
+    info2.hash = hash.first;
+    info2.narSize = hash.second;
+    info2.references = info.references;
+    info2.deriver = info.deriver;
+    worker.store.registerValidPath(info2);
+
+    outputLock->setDeletion(true);
+    outputLock.reset();
+
+    worker.store.markContentsGood(storePath);
+
+    printMsg(lvlChatty,
+        format("substitution of path `%1%' succeeded") % storePath);
+
+    if (settings.printBuildTrace)
+        printMsg(lvlError, format("@ substituter-succeeded %1%") % storePath);
+
+    amDone(ecSuccess);
+}
+
+
+void SubstitutionGoal::handleChildOutput(int fd, const string & data)
+{
+    assert(fd == logPipe.readSide);
+    if (verbosity >= settings.buildVerbosity) writeToStderr(data);
+    /* Don't write substitution output to a log file for now.  We
+       probably should, though. */
+}
+
+
+void SubstitutionGoal::handleEOF(int fd)
+{
+    if (fd == logPipe.readSide) worker.wakeUp(shared_from_this());
+}
+
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+static bool working = false;
+
+
+Worker::Worker(LocalStore & store)
+    : store(store)
+{
+    /* Debugging: prevent recursive workers. */
+    if (working) abort();
+    working = true;
+    nrLocalBuilds = 0;
+    lastWokenUp = 0;
+    permanentFailure = false;
+}
+
+
+Worker::~Worker()
+{
+    working = false;
+
+    /* Explicitly get rid of all strong pointers now.  After this all
+       goals that refer to this worker should be gone.  (Otherwise we
+       are in trouble, since goals may call childTerminated() etc. in
+       their destructors). */
+    topGoals.clear();
+}
+
+
+GoalPtr Worker::makeDerivationGoal(const Path & path, const StringSet & wantedOutputs, BuildMode buildMode)
+{
+    GoalPtr goal = derivationGoals[path].lock();
+    if (!goal) {
+        goal = GoalPtr(new DerivationGoal(path, wantedOutputs, *this, buildMode));
+        derivationGoals[path] = goal;
+        wakeUp(goal);
+    } else
+        (dynamic_cast<DerivationGoal *>(goal.get()))->addWantedOutputs(wantedOutputs);
+    return goal;
+}
+
+
+GoalPtr Worker::makeSubstitutionGoal(const Path & path, bool repair)
+{
+    GoalPtr goal = substitutionGoals[path].lock();
+    if (!goal) {
+        goal = GoalPtr(new SubstitutionGoal(path, *this, repair));
+        substitutionGoals[path] = goal;
+        wakeUp(goal);
+    }
+    return goal;
+}
+
+
+static void removeGoal(GoalPtr goal, WeakGoalMap & goalMap)
+{
+    /* !!! inefficient */
+    for (WeakGoalMap::iterator i = goalMap.begin();
+         i != goalMap.end(); )
+        if (i->second.lock() == goal) {
+            WeakGoalMap::iterator j = i; ++j;
+            goalMap.erase(i);
+            i = j;
+        }
+        else ++i;
+}
+
+
+void Worker::removeGoal(GoalPtr goal)
+{
+    nix::removeGoal(goal, derivationGoals);
+    nix::removeGoal(goal, substitutionGoals);
+    if (topGoals.find(goal) != topGoals.end()) {
+        topGoals.erase(goal);
+        /* If a top-level goal failed, then kill all other goals
+           (unless keepGoing was set). */
+        if (goal->getExitCode() == Goal::ecFailed && !settings.keepGoing)
+            topGoals.clear();
+    }
+
+    /* Wake up goals waiting for any goal to finish. */
+    foreach (WeakGoals::iterator, i, waitingForAnyGoal) {
+        GoalPtr goal = i->lock();
+        if (goal) wakeUp(goal);
+    }
+
+    waitingForAnyGoal.clear();
+}
+
+
+void Worker::wakeUp(GoalPtr goal)
+{
+    goal->trace("woken up");
+    addToWeakGoals(awake, goal);
+}
+
+
+unsigned Worker::getNrLocalBuilds()
+{
+    return nrLocalBuilds;
+}
+
+
+void Worker::childStarted(GoalPtr goal,
+    pid_t pid, const set<int> & fds, bool inBuildSlot,
+    bool respectTimeouts)
+{
+    Child child;
+    child.goal = goal;
+    child.fds = fds;
+    child.timeStarted = child.lastOutput = time(0);
+    child.inBuildSlot = inBuildSlot;
+    child.respectTimeouts = respectTimeouts;
+    children[pid] = child;
+    if (inBuildSlot) nrLocalBuilds++;
+}
+
+
+void Worker::childTerminated(pid_t pid, bool wakeSleepers)
+{
+    assert(pid != -1); /* common mistake */
+
+    Children::iterator i = children.find(pid);
+    assert(i != children.end());
+
+    if (i->second.inBuildSlot) {
+        assert(nrLocalBuilds > 0);
+        nrLocalBuilds--;
+    }
+
+    children.erase(pid);
+
+    if (wakeSleepers) {
+
+        /* Wake up goals waiting for a build slot. */
+        foreach (WeakGoals::iterator, i, wantingToBuild) {
+            GoalPtr goal = i->lock();
+            if (goal) wakeUp(goal);
+        }
+
+        wantingToBuild.clear();
+    }
+}
+
+
+void Worker::waitForBuildSlot(GoalPtr goal)
+{
+    debug("wait for build slot");
+    if (getNrLocalBuilds() < settings.maxBuildJobs)
+        wakeUp(goal); /* we can do it right away */
+    else
+        addToWeakGoals(wantingToBuild, goal);
+}
+
+
+void Worker::waitForAnyGoal(GoalPtr goal)
+{
+    debug("wait for any goal");
+    addToWeakGoals(waitingForAnyGoal, goal);
+}
+
+
+void Worker::waitForAWhile(GoalPtr goal)
+{
+    debug("wait for a while");
+    addToWeakGoals(waitingForAWhile, goal);
+}
+
+
+void Worker::run(const Goals & _topGoals)
+{
+    foreach (Goals::iterator, i,  _topGoals) topGoals.insert(*i);
+
+    startNest(nest, lvlDebug, format("entered goal loop"));
+
+    while (1) {
+
+        checkInterrupt();
+
+        /* Call every wake goal. */
+        while (!awake.empty() && !topGoals.empty()) {
+            WeakGoals awake2(awake);
+            awake.clear();
+            foreach (WeakGoals::iterator, i, awake2) {
+                checkInterrupt();
+                GoalPtr goal = i->lock();
+                if (goal) goal->work();
+                if (topGoals.empty()) break;
+            }
+        }
+
+        if (topGoals.empty()) break;
+
+        /* Wait for input. */
+        if (!children.empty() || !waitingForAWhile.empty())
+            waitForInput();
+        else {
+            if (awake.empty() && settings.maxBuildJobs == 0) throw Error(
+                "unable to start any build; either increase `--max-jobs' "
+                "or enable distributed builds");
+            assert(!awake.empty());
+        }
+    }
+
+    /* If --keep-going is not set, it's possible that the main goal
+       exited while some of its subgoals were still active.  But if
+       --keep-going *is* set, then they must all be finished now. */
+    assert(!settings.keepGoing || awake.empty());
+    assert(!settings.keepGoing || wantingToBuild.empty());
+    assert(!settings.keepGoing || children.empty());
+}
+
+
+void Worker::waitForInput()
+{
+    printMsg(lvlVomit, "waiting for children");
+
+    /* Process output from the file descriptors attached to the
+       children, namely log output and output path creation commands.
+       We also use this to detect child termination: if we get EOF on
+       the logger pipe of a build, we assume that the builder has
+       terminated. */
+
+    bool useTimeout = false;
+    struct timeval timeout;
+    timeout.tv_usec = 0;
+    time_t before = time(0);
+
+    /* If we're monitoring for silence on stdout/stderr, or if there
+       is a build timeout, then wait for input until the first
+       deadline for any child. */
+    assert(sizeof(time_t) >= sizeof(long));
+    time_t nearest = LONG_MAX; // nearest deadline
+    foreach (Children::iterator, i, children) {
+        if (!i->second.respectTimeouts) continue;
+        if (settings.maxSilentTime != 0)
+            nearest = std::min(nearest, i->second.lastOutput + settings.maxSilentTime);
+        if (settings.buildTimeout != 0)
+            nearest = std::min(nearest, i->second.timeStarted + settings.buildTimeout);
+    }
+    if (nearest != LONG_MAX) {
+        timeout.tv_sec = std::max((time_t) 1, nearest - before);
+        useTimeout = true;
+        printMsg(lvlVomit, format("sleeping %1% seconds") % timeout.tv_sec);
+    }
+
+    /* If we are polling goals that are waiting for a lock, then wake
+       up after a few seconds at most. */
+    if (!waitingForAWhile.empty()) {
+        useTimeout = true;
+        if (lastWokenUp == 0)
+            printMsg(lvlError, "waiting for locks or build slots...");
+        if (lastWokenUp == 0 || lastWokenUp > before) lastWokenUp = before;
+        timeout.tv_sec = std::max((time_t) 1, (time_t) (lastWokenUp + settings.pollInterval - before));
+    } else lastWokenUp = 0;
+
+    using namespace std;
+    /* Use select() to wait for the input side of any logger pipe to
+       become `available'.  Note that `available' (i.e., non-blocking)
+       includes EOF. */
+    fd_set fds;
+    FD_ZERO(&fds);
+    int fdMax = 0;
+    foreach (Children::iterator, i, children) {
+        foreach (set<int>::iterator, j, i->second.fds) {
+            FD_SET(*j, &fds);
+            if (*j >= fdMax) fdMax = *j + 1;
+        }
+    }
+
+    if (select(fdMax, &fds, 0, 0, useTimeout ? &timeout : 0) == -1) {
+        if (errno == EINTR) return;
+        throw SysError("waiting for input");
+    }
+
+    time_t after = time(0);
+
+    /* Process all available file descriptors. */
+
+    /* Since goals may be canceled from inside the loop below (causing
+       them go be erased from the `children' map), we have to be
+       careful that we don't keep iterators alive across calls to
+       cancel(). */
+    set<pid_t> pids;
+    foreach (Children::iterator, i, children) pids.insert(i->first);
+
+    foreach (set<pid_t>::iterator, i, pids) {
+        checkInterrupt();
+        Children::iterator j = children.find(*i);
+        if (j == children.end()) continue; // child destroyed
+        GoalPtr goal = j->second.goal.lock();
+        assert(goal);
+
+        set<int> fds2(j->second.fds);
+        foreach (set<int>::iterator, k, fds2) {
+            if (FD_ISSET(*k, &fds)) {
+                unsigned char buffer[4096];
+                ssize_t rd = read(*k, buffer, sizeof(buffer));
+                if (rd == -1) {
+                    if (errno != EINTR)
+                        throw SysError(format("reading from %1%")
+                            % goal->getName());
+                } else if (rd == 0) {
+                    debug(format("%1%: got EOF") % goal->getName());
+                    goal->handleEOF(*k);
+                    j->second.fds.erase(*k);
+                } else {
+                    printMsg(lvlVomit, format("%1%: read %2% bytes")
+                        % goal->getName() % rd);
+                    string data((char *) buffer, rd);
+                    j->second.lastOutput = after;
+                    goal->handleChildOutput(*k, data);
+                }
+            }
+        }
+
+        if (goal->getExitCode() == Goal::ecBusy &&
+            settings.maxSilentTime != 0 &&
+            j->second.respectTimeouts &&
+            after - j->second.lastOutput >= (time_t) settings.maxSilentTime)
+        {
+            printMsg(lvlError,
+                format("%1% timed out after %2% seconds of silence")
+                % goal->getName() % settings.maxSilentTime);
+            goal->cancel(true);
+        }
+
+        else if (goal->getExitCode() == Goal::ecBusy &&
+            settings.buildTimeout != 0 &&
+            j->second.respectTimeouts &&
+            after - j->second.timeStarted >= (time_t) settings.buildTimeout)
+        {
+            printMsg(lvlError,
+                format("%1% timed out after %2% seconds")
+                % goal->getName() % settings.buildTimeout);
+            goal->cancel(true);
+        }
+    }
+
+    if (!waitingForAWhile.empty() && lastWokenUp + settings.pollInterval <= after) {
+        lastWokenUp = after;
+        foreach (WeakGoals::iterator, i, waitingForAWhile) {
+            GoalPtr goal = i->lock();
+            if (goal) wakeUp(goal);
+        }
+        waitingForAWhile.clear();
+    }
+}
+
+
+unsigned int Worker::exitStatus()
+{
+    return permanentFailure ? 100 : 1;
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+
+void LocalStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
+{
+    startNest(nest, lvlDebug,
+        format("building %1%") % showPaths(drvPaths));
+
+    Worker worker(*this);
+
+    Goals goals;
+    foreach (PathSet::const_iterator, i, drvPaths) {
+        DrvPathWithOutputs i2 = parseDrvPathWithOutputs(*i);
+        if (isDerivation(i2.first))
+            goals.insert(worker.makeDerivationGoal(i2.first, i2.second, buildMode));
+        else
+            goals.insert(worker.makeSubstitutionGoal(*i, buildMode));
+    }
+
+    worker.run(goals);
+
+    PathSet failed;
+    foreach (Goals::iterator, i, goals)
+        if ((*i)->getExitCode() == Goal::ecFailed) {
+            DerivationGoal * i2 = dynamic_cast<DerivationGoal *>(i->get());
+            if (i2) failed.insert(i2->getDrvPath());
+            else failed.insert(dynamic_cast<SubstitutionGoal *>(i->get())->getStorePath());
+        }
+
+    if (!failed.empty())
+        throw Error(format("build of %1% failed") % showPaths(failed), worker.exitStatus());
+}
+
+
+void LocalStore::ensurePath(const Path & path)
+{
+    /* If the path is already valid, we're done. */
+    if (isValidPath(path)) return;
+
+    Worker worker(*this);
+    GoalPtr goal = worker.makeSubstitutionGoal(path);
+    Goals goals = singleton<Goals>(goal);
+
+    worker.run(goals);
+
+    if (goal->getExitCode() != Goal::ecSuccess)
+        throw Error(format("path `%1%' does not exist and cannot be created") % path, worker.exitStatus());
+}
+
+
+void LocalStore::repairPath(const Path & path)
+{
+    Worker worker(*this);
+    GoalPtr goal = worker.makeSubstitutionGoal(path, true);
+    Goals goals = singleton<Goals>(goal);
+
+    worker.run(goals);
+
+    if (goal->getExitCode() != Goal::ecSuccess)
+        throw Error(format("cannot repair path `%1%'") % path, worker.exitStatus());
+}
+
+
+}