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>

1 files changed, 29 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);