Add a flag ‘--check’ to verify build determinism

The flag ‘--check’ to ‘nix-store -r’ or ‘nix-build’ will cause Nix to
redo the build of a derivation whose output paths are already valid.
If the new output differs from the original output, an error is
printed.  This makes it easier to test if a build is deterministic.
(Obviously this cannot catch all sources of non-determinism, but it
catches the most common one, namely the current time.)

For example:

  $ nix-build '<nixpkgs>' -A patchelf
  ...
  $ nix-build '<nixpkgs>' -A patchelf --check
  error: derivation `/nix/store/1ipvxsdnbhl1rw6siz6x92s7sc8nwkkb-patchelf-0.6' may not be deterministic: hash mismatch in output `/nix/store/4pc1dmw5xkwmc6q3gdc9i5nbjl4dkjpp-patchelf-0.6.drv'

The --check build fails if not all outputs are valid.  Thus the first
call to nix-build is necessary to ensure that all outputs are valid.

The current outputs are left untouched: the new outputs are either put
in a chroot or diverted to a different location in the store using
hash rewriting.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 3017254baf3b..461920693290 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -447,9 +447,9 @@ Paths RemoteStore::importPaths(bool requireSignature, Source & source)
 }
 
 
-void RemoteStore::buildPaths(const PathSet & drvPaths, bool repair)
+void RemoteStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
 {
-    if (repair) throw Error("repairing is not supported when building through the Nix daemon");
+    if (buildMode != bmNormal) throw Error("repairing or checking is not supported when building through the Nix daemon");
     openConnection();
     writeInt(wopBuildPaths, to);
     if (GET_PROTOCOL_MINOR(daemonVersion) >= 13)