path: root/src/libstore/db.cc

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <memory>

#include "db.hh"
#include "util.hh"
#include "pathlocks.hh"


/* Wrapper class to ensure proper destruction. */
class DestroyDbc 
{
    Dbc * dbc;
public:
    DestroyDbc(Dbc * _dbc) : dbc(_dbc) { }
    ~DestroyDbc() { dbc->close(); /* close() frees dbc */ }
};


static void rethrow(DbException & e)
{
    throw Error(e.what());
}


Transaction::Transaction()
    : txn(0)
{
}


Transaction::Transaction(Database & db)
{
    db.requireEnv();
    try {
        db.env->txn_begin(0, &txn, 0);
    } catch (DbException e) { rethrow(e); }
}


Transaction::~Transaction()
{
    if (txn) abort();
}


void Transaction::commit()
{
    if (!txn) throw Error("commit called on null transaction");
    debug(format("committing transaction %1%") % (void *) txn);
    DbTxn * txn2 = txn;
    txn = 0;
    try {
        txn2->commit(0);
    } catch (DbException e) { rethrow(e); }
}


void Transaction::abort()
{
    if (!txn) throw Error("abort called on null transaction");
    debug(format("aborting transaction %1%") % (void *) txn);
    DbTxn * txn2 = txn;
    txn = 0;
    try {
        txn2->abort();
    } catch (DbException e) { rethrow(e); }
}


void Transaction::moveTo(Transaction & t)
{
    if (t.txn) throw Error("target txn already exists");
    t.txn = txn;
    txn = 0;
}


void Database::requireEnv()
{
    checkInterrupt();
    if (!env) throw Error("database environment not open");
}


Db * Database::getDb(TableId table)
{
    map<TableId, Db *>::iterator i = tables.find(table);
    if (i == tables.end())
        throw Error("unknown table id");
    return i->second;
}


Database::Database()
    : env(0)
    , nextId(1)
{
}


Database::~Database()
{
    close();
}


int getAccessorCount(int fd)
{
    if (lseek(fd, 0, SEEK_SET) == -1)
        throw SysError("seeking accessor count");
    char buf[128];
    int len;
    if ((len = read(fd, buf, sizeof(buf) - 1)) == -1)
        throw SysError("reading accessor count");
    buf[len] = 0;
    int count;
    if (sscanf(buf, "%d", &count) != 1) {
        debug(format("accessor count is invalid: `%1%'") % buf);
        return -1;
    }
    return count;
}


void setAccessorCount(int fd, int n)
{
    if (lseek(fd, 0, SEEK_SET) == -1)
        throw SysError("seeking accessor count");
    string s = (format("%1%") % n).str();
    const char * s2 = s.c_str();
    if (write(fd, s2, strlen(s2)) != (ssize_t) strlen(s2) ||
        ftruncate(fd, strlen(s2)) != 0)
        throw SysError("writing accessor count");
}


void openEnv(DbEnv * env, const string & path, u_int32_t flags)
{
    env->open(path.c_str(),
        DB_INIT_LOCK | DB_INIT_LOG | DB_INIT_MPOOL | DB_INIT_TXN |
        DB_CREATE | flags,
        0666);
}


static int my_fsync(int fd)
{
    return 0;
}


void Database::open(const string & path)
{
    if (env) throw Error(format("environment already open"));

    try {

        debug(format("opening database environment"));


        /* Create the database environment object. */
        DbEnv * env = 0; /* !!! close on error */
        env = new DbEnv(0);

        /* Smaller log files. */
        env->set_lg_bsize(32 * 1024); /* default */
        env->set_lg_max(256 * 1024); /* must be > 4 * lg_bsize */

        /* Write the log, but don't sync.  This protects transactions
           against application crashes, but if the system crashes,
           some transactions may be undone.  An acceptable risk, I
           think. */
        env->set_flags(DB_TXN_WRITE_NOSYNC | DB_LOG_AUTOREMOVE, 1);

        /* Increase the locking limits.  If you ever get `Dbc::get:
           Cannot allocate memory' or similar, especially while
           running `nix-store --verify', just increase the following
           number, then run db_recover on the database to remove the
           existing DB environment (since changes only take effect on
           new environments). */
        env->set_lk_max_locks(4000);
        env->set_lk_max_lockers(4000);
        env->set_lk_max_objects(4000);
        env->set_lk_detect(DB_LOCK_DEFAULT);

        /* Dangerous, probably, but from the docs it *seems* that BDB
           shouldn't sync when DB_TXN_WRITE_NOSYNC is used, but it
           still fsync()s sometimes. */
        db_env_set_func_fsync(my_fsync);
        

        /* The following code provides automatic recovery of the
           database environment.  Recovery is necessary when a process
           dies while it has the database open.  To detect this,
           processes atomically increment a counter when they open the
           database, and decrement it when they close it.  If we see
           that counter is > 0 but no processes are accessing the
           database---determined by attempting to obtain a write lock
           on a lock file on which all accessors have a read lock---we
           must run recovery.  Note that this also ensures that we
           only run recovery when there are no other accessors (which
           could cause database corruption). */

        /* !!! close fdAccessors / fdLock on exception */

        /* Open the accessor count file. */
        string accessorsPath = path + "/accessor_count";
        fdAccessors = ::open(accessorsPath.c_str(), O_RDWR | O_CREAT, 0666);
        if (fdAccessors == -1)
            throw SysError(format("opening file `%1%'") % accessorsPath);

        /* Open the lock file. */
        string lockPath = path + "/access_lock";
        fdLock = ::open(lockPath.c_str(), O_RDWR | O_CREAT, 0666);
        if (fdLock == -1)
            throw SysError(format("opening lock file `%1%'") % lockPath);

        /* Try to acquire a write lock. */
        debug(format("attempting write lock on `%1%'") % lockPath);
        if (lockFile(fdLock, ltWrite, false)) { /* don't wait */

            debug(format("write lock granted"));

            /* We have a write lock, which means that there are no
               other readers or writers. */

            int n = getAccessorCount(fdAccessors);

            if (n != 0) {
                printMsg(lvlTalkative,
                    format("accessor count is %1%, running recovery") % n);

                /* Open the environment after running recovery. */
                openEnv(env, path, DB_RECOVER);
            }
            
            else 
                /* Open the environment normally. */
                openEnv(env, path, 0);

            setAccessorCount(fdAccessors, 1);

            /* Downgrade to a read lock. */
            debug(format("downgrading to read lock on `%1%'") % lockPath);
            lockFile(fdLock, ltRead, true);

        } else {
            /* There are other accessors. */ 
            debug(format("write lock refused"));

            /* Acquire a read lock. */
            debug(format("acquiring read lock on `%1%'") % lockPath);
            lockFile(fdLock, ltRead, true); /* wait indefinitely */

            /* Increment the accessor count. */
            lockFile(fdAccessors, ltWrite, true);
            int n = getAccessorCount(fdAccessors) + 1;
            setAccessorCount(fdAccessors, n);
            debug(format("incremented accessor count to %1%") % n);
            lockFile(fdAccessors, ltNone, true);

            /* Open the environment normally. */
            openEnv(env, path, 0);
        }

        this->env = env;
        
    } catch (DbException e) { rethrow(e); }
}


void Database::close()
{
    if (!env) return;

    /* Close the database environment. */
    debug(format("closing database environment"));

    try {

        for (map<TableId, Db *>::iterator i = tables.begin();
             i != tables.end(); i++)
        {
            Db * db = i->second;
            db->close(DB_NOSYNC);
            delete db;
        }

        /* Do a checkpoint every 128 kilobytes, or every 5 minutes. */
        env->txn_checkpoint(128, 5, 0);
        
        env->close(0);

    } catch (DbException e) { rethrow(e); }

    delete env;

    /* Decrement the accessor count. */
    lockFile(fdAccessors, ltWrite, true);
    int n = getAccessorCount(fdAccessors) - 1;
    setAccessorCount(fdAccessors, n);
    debug(format("decremented accessor count to %1%") % n);
    lockFile(fdAccessors, ltNone, true);

    ::close(fdAccessors);
    ::close(fdLock);
}


TableId Database::openTable(const string & tableName)
{
    requireEnv();
    TableId table = nextId++;

    try {

        Db * db = new Db(env, 0);

        try {
            db->open(0, tableName.c_str(), 0, 
                DB_HASH, DB_CREATE | DB_AUTO_COMMIT, 0666);
        } catch (...) {
            delete db;
            throw;
        }

        tables[table] = db;

    } catch (DbException e) { rethrow(e); }

    return table;
}


bool Database::queryString(const Transaction & txn, TableId table, 
    const string & key, string & data)
{
    checkInterrupt();

    try {
        Db * db = getDb(table);

        Dbt kt((void *) key.c_str(), key.length());
        Dbt dt;

        int err = db->get(txn.txn, &kt, &dt, 0);
        if (err) return false;

        if (!dt.get_data())
            data = "";
        else
            data = string((char *) dt.get_data(), dt.get_size());
    
    } catch (DbException e) { rethrow(e); }

    return true;
}


bool Database::queryStrings(const Transaction & txn, TableId table, 
    const string & key, Strings & data)
{
    string d;
    if (!queryString(txn, table, key, d))
        return false;
    data = unpackStrings(d);
    return true;
}


void Database::setString(const Transaction & txn, TableId table,
    const string & key, const string & data)
{
    checkInterrupt();
    try {
        Db * db = getDb(table);
        Dbt kt((void *) key.c_str(), key.length());
        Dbt dt((void *) data.c_str(), data.length());
        db->put(txn.txn, &kt, &dt, 0);
    } catch (DbException e) { rethrow(e); }
}


void Database::setStrings(const Transaction & txn, TableId table,
    const string & key, const Strings & data, bool deleteEmpty)
{
    if (deleteEmpty && data.size() == 0)
        delPair(txn, table, key);
    else
        setString(txn, table, key, packStrings(data));
}


void Database::delPair(const Transaction & txn, TableId table,
    const string & key)
{
    checkInterrupt();
    try {
        Db * db = getDb(table);
        Dbt kt((void *) key.c_str(), key.length());
        db->del(txn.txn, &kt, 0);
        /* Non-existence of a pair with the given key is not an
           error. */
    } catch (DbException e) { rethrow(e); }
}


void Database::enumTable(const Transaction & txn, TableId table,
    Strings & keys)
{
    try {
        Db * db = getDb(table);

        Dbc * dbc;
        db->cursor(txn.txn, &dbc, 0);
        DestroyDbc destroyDbc(dbc);

        Dbt kt, dt;
        while (dbc->get(&kt, &dt, DB_NEXT) != DB_NOTFOUND) {
            checkInterrupt();
            keys.push_back(
                string((char *) kt.get_data(), kt.get_size()));
        }

    } catch (DbException e) { rethrow(e); }
}