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.

2 files changed, 3 insertions, 3 deletions

diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index e2781e540bf2..333a7fe8c0ca 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -714,7 +714,7 @@ static void opSet(Globals & globals,
         PathSet paths = singleton<PathSet>(drv.queryDrvPath());
         printMissing(*store, paths);
         if (globals.dryRun) return;
-        store->buildPaths(paths, globals.state.repair);
+        store->buildPaths(paths, globals.state.repair ? bmRepair : bmNormal);
     }
     else {
         printMissing(*store, singleton<PathSet>(drv.queryOutPath()));
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index 3a73f2647a6b..75f5b54abf0a 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -38,7 +38,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
             drvsToBuild.insert(i->queryDrvPath());
 
     debug(format("building user environment dependencies"));
-    store->buildPaths(drvsToBuild, state.repair);
+    store->buildPaths(drvsToBuild, state.repair ? bmRepair : bmNormal);
 
     /* Construct the whole top level derivation. */
     PathSet references;
@@ -125,7 +125,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
 
     /* Realise the resulting store expression. */
     debug("building user environment");
-    store->buildPaths(singleton<PathSet>(topLevelDrv), state.repair);
+    store->buildPaths(singleton<PathSet>(topLevelDrv), state.repair ? bmRepair : bmNormal);
 
     /* Switch the current user environment to the output path. */
     PathLocks lock;