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, 41 insertions, 0 deletions

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);
+}