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, 11 insertions, 0 deletions

diff --git a/nix/scripts/download.in b/nix/scripts/download.in
new file mode 100644
index 0000000000..4d7088a993
--- /dev/null
+++ b/nix/scripts/download.in
@@ -0,0 +1,11 @@
+#!@SHELL@
+# A shorthand for "guix perform-download", for use by the daemon.
+
+if test "x$GUIX_UNINSTALLED" = "x"
+then
+    prefix="@prefix@"
+    exec_prefix="@exec_prefix@"
+    exec "@bindir@/guix" perform-download "$@"
+else
+    exec guix perform-download "$@"
+fi