--check: Keep the differing output if -K is given

This makes it easier to investigate the non-determinism, e.g.

  $ nix-build pkgs/stdenv/linux -A stage1.pkgs.zlib --check -K
  error: derivation ‘/nix/store/l54i8wlw22656i4pk05c52ngv9rpl39q-zlib-1.2.8.drv’ may not be deterministic: output ‘/nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8’ differs from ‘/nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8-check’

  $ diffoscope /nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8 /nix/store/11a27shh6n2ivi4a7s964i65ql80cf27-zlib-1.2.8-check
  ...
  ├── lib/libz.a
  │   ├── metadata
  │   │ @@ -1,15 +1,15 @@
  │   │ -rw-r--r-- 30001/30000   3096 Jan 12 15:20 2016 adler32.o
  ...
  │   │ +rw-r--r-- 30001/30000   3096 Jan 12 15:28 2016 adler32.o
  ...

1 files changed, 12 insertions, 2 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 0b6a214efd32..27ba1c84ab94 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2748,8 +2748,18 @@ void DerivationGoal::registerOutputs()
         if (buildMode == bmCheck) {
             if (!store->isValidPath(path)) continue;
             ValidPathInfo info = worker.store.queryPathInfo(path);
-            if (hash.first != info.hash)
-                throw Error(format("derivation ‘%1%’ may not be deterministic: hash mismatch in output ‘%2%’") % drvPath % path);
+            if (hash.first != info.hash) {
+                if (settings.keepFailed) {
+                    Path dst = path + "-check";
+                    if (pathExists(dst)) deletePath(dst);
+                    if (rename(actualPath.c_str(), dst.c_str()))
+                        throw SysError(format("renaming ‘%1%’ to ‘%2%’") % actualPath % dst);
+                    throw Error(format("derivation ‘%1%’ may not be deterministic: output ‘%2%’ differs from ‘%3%’")
+                        % drvPath % path % dst);
+                } else
+                    throw Error(format("derivation ‘%1%’ may not be deterministic: output ‘%2%’ differs")
+                        % drvPath % path);
+            }
             continue;
         }