daemon: Add "builtin:download" derivation builder.

This ensures that 1) the derivation doesn't change when Guix changes;
2) the derivation closure doesn't contain Guix and its dependencies; 3)
we don't have to rely on ugly chroot hacks.

Adapted from Nix commit 0a2bee307b20411f5b0dda0c662b1f9bb9e0e131.

* nix/libstore/build.cc (DerivationGoal::runChild): Add special case for
'isBuiltin(drv)'.  Disable chroot when 'isBuiltin(drv)'.
* nix/libstore/builtins.cc, nix/libstore/builtins.hh,
nix/scripts/download.in, guix/scripts/perform-download.scm: New files.
* guix/ui.scm (show-guix-help)[internal?]: Add 'perform-download'.
* nix/local.mk (libstore_a_SOURCES): Add builtins.cc.
(libstore_headers): Add builtins.hh.
(nodist_pkglibexec_SCRIPTS): Add 'scripts/download'.
* config-daemon.ac: Emit 'scripts/download'.
* Makefile.am (MODULES): Add 'guix/scripts/perform-download.scm'.
* tests/derivations.scm ("unknown built-in builder")
("'download' built-in builder")
("'download' built-in builder, invalid hash")
("'download' built-in builder, not found")
("'download' built-in builder, not fixed-output"): New tests.

Co-authored-by: Eelco Dolstra <eelco.dolstra@logicblox.com>

3 files changed, 139 insertions, 7 deletions

diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index ae78e65199..889ee3d2bd 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -8,6 +8,7 @@
 #include "util.hh"
 #include "archive.hh"
 #include "affinity.hh"
+#include "builtins.hh"
 
 #include <map>
 #include <sstream>
@@ -2047,7 +2048,12 @@ void DerivationGoal::runChild()
         commonChildInit(builderOut);
 
 #if CHROOT_ENABLED
-        if (useChroot) {
+	/* Note: built-in builders are *not* running in a chroot environment
+	   so that we can easily implement them in Guile without having it as
+	   a derivation input (they are running under a separate build user,
+	   though).  */
+
+        if (useChroot && !isBuiltin(drv)) {
             /* Initialise the loopback interface. */
             AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
             if (fd == -1) throw SysError("cannot open IP socket");
@@ -2255,6 +2261,28 @@ void DerivationGoal::runChild()
                 throw SysError("setuid failed");
         }
 
+        restoreSIGPIPE();
+
+        /* Indicate that we managed to set up the build environment. */
+        writeFull(STDERR_FILENO, "\n");
+
+        /* Execute the program.  This should not return. */
+        if (isBuiltin(drv)) {
+            try {
+                logType = ltFlat;
+
+		auto buildDrv = lookupBuiltinBuilder(drv.builder);
+                if (buildDrv != NULL)
+                    buildDrv(drv, drvPath);
+                else
+                    throw Error(format("unsupported builtin function '%1%'") % string(drv.builder, 8));
+                _exit(0);
+            } catch (std::exception & e) {
+                writeFull(STDERR_FILENO, "error: " + string(e.what()) + "\n");
+                _exit(1);
+            }
+        }
+
         /* Fill in the arguments. */
         Strings args;
         string builderBasename = baseNameOf(drv.builder);
@@ -2262,12 +2290,6 @@ void DerivationGoal::runChild()
         foreach (Strings::iterator, i, drv.args)
             args.push_back(rewriteHashes(*i, rewritesToTmp));
 
-        restoreSIGPIPE();
-
-        /* Indicate that we managed to set up the build environment. */
-        writeFull(STDERR_FILENO, "\n");
-
-        /* Execute the program.  This should not return. */
         execve(drv.builder.c_str(), stringsToCharPtrs(args).data(), stringsToCharPtrs(envStrs).data());
 
         throw SysError(format("executing `%1%'") % drv.builder);
diff --git a/nix/libstore/builtins.cc b/nix/libstore/builtins.cc
new file mode 100644
index 0000000000..605e44079a
--- /dev/null
+++ b/nix/libstore/builtins.cc
@@ -0,0 +1,69 @@
+/* GNU Guix --- Functional package management for GNU
+   Copyright (C) 2016 Ludovic Courtès <ludo@gnu.org>
+
+   This file is part of GNU Guix.
+
+   GNU Guix is free software; you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or (at
+   your option) any later version.
+
+   GNU Guix is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <builtins.hh>
+#include <util.hh>
+#include <globals.hh>
+
+#include <unistd.h>
+
+namespace nix {
+
+static void builtinDownload(const Derivation &drv,
+			    const std::string &drvPath)
+{
+    /* Invoke 'guix perform-download'.  */
+    Strings args;
+    args.push_back("perform-download");
+    args.push_back(drvPath);
+
+    /* Close all other file descriptors. */
+    closeMostFDs(set<int>());
+
+    const char *const argv[] = { "download", drvPath.c_str(), NULL };
+
+    /* XXX: Hack our way to use the 'download' script from 'LIBEXECDIR/guix'
+       or just 'LIBEXECDIR', depending on whether we're running uninstalled or
+       not.  */
+    const string subdir = getenv("GUIX_UNINSTALLED") != NULL
+	? "" : "/guix";
+
+    const string program = settings.nixLibexecDir + subdir + "/download";
+    execv(program.c_str(), (char *const *) argv);
+
+    throw SysError(format("failed to run download program '%1%'") % program);
+}
+
+static const std::map<std::string, derivationBuilder> builtins =
+{
+    { "download", builtinDownload }
+};
+
+derivationBuilder lookupBuiltinBuilder(const std::string & name)
+{
+    if (name.substr(0, 8) == "builtin:")
+    {
+	auto realName = name.substr(8);
+	auto builder = builtins.find(realName);
+	return builder == builtins.end() ? NULL : builder->second;
+    }
+    else
+	return NULL;
+}
+
+}
diff --git a/nix/libstore/builtins.hh b/nix/libstore/builtins.hh
new file mode 100644
index 0000000000..0c6db651ab
--- /dev/null
+++ b/nix/libstore/builtins.hh
@@ -0,0 +1,41 @@
+/* GNU Guix --- Functional package management for GNU
+   Copyright (C) 2016 Ludovic Courtès <ludo@gnu.org>
+
+   This file is part of GNU Guix.
+
+   GNU Guix is free software; you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or (at
+   your option) any later version.
+
+   GNU Guix is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Interface to built-in derivation builders.  */
+
+#pragma once
+
+#include <derivations.hh>
+#include <map>
+#include <string>
+
+namespace nix {
+
+    inline bool isBuiltin(const Derivation & drv)
+    {
+	return string(drv.builder, 0, 8) == "builtin:";
+    }
+
+    /* Build DRV, which lives at DRVPATH.  */
+    typedef void (*derivationBuilder) (const Derivation &drv,
+				       const std::string &drvPath);
+
+    /* Return the built-in builder called BUILDER, or NULL if none was
+       found.  */
+    derivationBuilder lookupBuiltinBuilder(const std::string &builder);
+}