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, 4 insertions, 3 deletions

diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index e969a3b71..aef55f5c9 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -104,12 +104,13 @@ static PathSet realisePath(const Path & path, bool build = true)
 static void opRealise(Strings opFlags, Strings opArgs)
 {
     bool dryRun = false;
-    bool repair = false;
+    BuildMode buildMode = bmNormal;
     bool ignoreUnknown = false;
 
     foreach (Strings::iterator, i, opFlags)
         if (*i == "--dry-run") dryRun = true;
-        else if (*i == "--repair") repair = true;
+        else if (*i == "--repair") buildMode = bmRepair;
+        else if (*i == "--check") buildMode = bmCheck;
         else if (*i == "--ignore-unknown") ignoreUnknown = true;
         else throw UsageError(format("unknown flag `%1%'") % *i);
 
@@ -137,7 +138,7 @@ static void opRealise(Strings opFlags, Strings opArgs)
     if (dryRun) return;
 
     /* Build all paths at the same time to exploit parallelism. */
-    store->buildPaths(PathSet(paths.begin(), paths.end()), repair);
+    store->buildPaths(PathSet(paths.begin(), paths.end()), buildMode);
 
     if (!ignoreUnknown)
         foreach (Paths::iterator, i, paths) {