6 files changed, 43 insertions, 33 deletions
diff --git a/doc/manual/release-notes/rl-2.0.xml b/doc/manual/release-notes/rl-2.0.xml
index 0d485c36928c..32cdb1d0cefc 100644
--- a/doc/manual/release-notes/rl-2.0.xml
+++ b/doc/manual/release-notes/rl-2.0.xml
@@ -99,11 +99,11 @@
   </listitem>
 
   <listitem>
-    <para>New build mode <command>nix-build --hash</command> that
-    builds a derivation, computes the hash of the output, and moves
-    the output to the store path corresponding to what a fixed-output
-    derivation with that hash would produce.
-    (Add docs and examples; see d367b8e7875161e655deaa96bf8a5dd0bcf8229e)</para>
+    <para>If a fixed-output derivation produces a result with an
+    incorrect hash, the output path will be moved to the location
+    corresponding to the actual hash and registered as valid. Thus, a
+    subsequent build of the fixed-output derivation with the correct
+    hash is unnecessary.</para>
   </listitem>
 
   <listitem>
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index cca51f17ee26..d4b93b5104c1 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1124,11 +1124,6 @@ void DerivationGoal::haveDerivation()
         return;
     }
 
-    /* Reject doing a hash build of anything other than a fixed-output
-       derivation. */
-    if (buildMode == bmHash && !drv->isFixedOutput())
-        throw Error("cannot do a hash build of non-fixed-output derivation '%1%'", drvPath);
-
     /* We are first going to try to create the invalid output paths
        through substitutes.  If that doesn't work, we'll build
        them. */
@@ -1320,9 +1315,7 @@ void DerivationGoal::inputsRealised()
     allPaths.insert(inputPaths.begin(), inputPaths.end());
 
     /* Is this a fixed-output derivation? */
-    fixedOutput = true;
-    for (auto & i : drv->outputs)
-        if (i.second.hash == "") fixedOutput = false;
+    fixedOutput = drv->isFixedOutput();
 
     /* Don't repeat fixed-output derivations since they're already
        verified by their output hash.*/
@@ -3019,6 +3012,8 @@ void DerivationGoal::registerOutputs()
     bool runDiffHook = settings.runDiffHook;
     bool keepPreviousRound = settings.keepFailed || runDiffHook;
 
+    std::exception_ptr delayedException;
+
     /* Check whether the output paths were created, and grep each
        output path to determine what other paths it references.  Also make all
        output paths read-only. */
@@ -3093,7 +3088,7 @@ void DerivationGoal::registerOutputs()
         /* Check that fixed-output derivations produced the right
            outputs (i.e., the content hash should match the specified
            hash). */
-        if (i.second.hash != "") {
+        if (fixedOutput) {
 
             bool recursive; Hash h;
             i.second.parseHashInfo(recursive, h);
@@ -3109,27 +3104,34 @@ void DerivationGoal::registerOutputs()
             /* Check the hash. In hash mode, move the path produced by
                the derivation to its content-addressed location. */
             Hash h2 = recursive ? hashPath(h.type, actualPath).first : hashFile(h.type, actualPath);
-            if (buildMode == bmHash) {
-                Path dest = worker.store.makeFixedOutputPath(recursive, h2, drv->env["name"]);
-                printError(format("build produced path '%1%' with %2% hash '%3%'")
-                    % dest % printHashType(h.type) % printHash16or32(h2));
-                if (worker.store.isValidPath(dest))
-                    return;
+
+            Path dest = worker.store.makeFixedOutputPath(recursive, h2, drv->env["name"]);
+
+            if (h != h2) {
+
+                /* Throw an error after registering the path as
+                   valid. */
+                delayedException = std::make_exception_ptr(
+                    BuildError("fixed-output derivation produced path '%s' with %s hash '%s' instead of the expected hash '%s'",
+                        dest, printHashType(h.type), printHash16or32(h2), printHash16or32(h)));
+
                 Path actualDest = worker.store.toRealPath(dest);
+
+                if (worker.store.isValidPath(dest))
+                    std::rethrow_exception(delayedException);
+
                 if (actualPath != actualDest) {
                     PathLocks outputLocks({actualDest});
                     deletePath(actualDest);
                     if (rename(actualPath.c_str(), actualDest.c_str()) == -1)
                         throw SysError(format("moving '%1%' to '%2%'") % actualPath % dest);
                 }
+
                 path = dest;
                 actualPath = actualDest;
-            } else {
-                if (h != h2)
-                    throw BuildError(
-                        format("output path '%1%' has %2% hash '%3%' when '%4%' was expected")
-                        % path % i.second.hashAlgo % printHash16or32(h2) % printHash16or32(h));
             }
+            else
+                assert(path == dest);
 
             info.ca = makeFixedOutputCA(recursive, h2);
         }
@@ -3306,6 +3308,11 @@ void DerivationGoal::registerOutputs()
        paths referenced by each of them.  If there are cycles in the
        outputs, this will fail. */
     worker.store.registerValidPaths(infos);
+
+    /* In case of a fixed-output derivation hash mismatch, throw an
+       exception now that we have registered the output as valid. */
+    if (delayedException)
+        std::rethrow_exception(delayedException);
 }
 
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index b21d3dd8a93b..70f23e1fcaf4 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -192,7 +192,7 @@ struct ValidPathInfo
 typedef list<ValidPathInfo> ValidPathInfos;
 
 
-enum BuildMode { bmNormal, bmRepair, bmCheck, bmHash };
+enum BuildMode { bmNormal, bmRepair, bmCheck };
 
 
 struct BuildResult
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index 3d02276bf420..1581c282c75c 100755
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -167,9 +167,6 @@ void mainWrapped(int argc, char * * argv)
             buildMode = bmRepair;
         }
 
-        else if (*arg == "--hash")
-            buildMode = bmHash;
-
         else if (*arg == "--run-env") // obsolete
             runEnv = true;
 
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index f6f276dd1798..4fc3421c0dde 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -122,7 +122,6 @@ static void opRealise(Strings opFlags, Strings opArgs)
         if (i == "--dry-run") dryRun = true;
         else if (i == "--repair") buildMode = bmRepair;
         else if (i == "--check") buildMode = bmCheck;
-        else if (i == "--hash") buildMode = bmHash;
         else if (i == "--ignore-unknown") ignoreUnknown = true;
         else throw UsageError(format("unknown flag '%1%'") % i);
 
diff --git a/tests/fixed.sh b/tests/fixed.sh
index cac3f0be91b0..8f51403a7071 100644
--- a/tests/fixed.sh
+++ b/tests/fixed.sh
@@ -5,15 +5,22 @@ clearStore
 export IMPURE_VAR1=foo
 export IMPURE_VAR2=bar
 
+path=$(nix-store -q $(nix-instantiate fixed.nix -A good.0))
+
+echo 'testing bad...'
+nix-build fixed.nix -A bad --no-out-link && fail "should fail"
+
+# Building with the bad hash should produce the "good" output path as
+# a side-effect.
+[[ -e $path ]]
+nix path-info --json $path | grep fixed:md5:2qk15sxzzjlnpjk9brn7j8ppcd
+
 echo 'testing good...'
 nix-build fixed.nix -A good --no-out-link
 
 echo 'testing good2...'
 nix-build fixed.nix -A good2 --no-out-link
 
-echo 'testing bad...'
-nix-build fixed.nix -A bad --no-out-link && fail "should fail"
-
 echo 'testing reallyBad...'
 nix-instantiate fixed.nix -A reallyBad && fail "should fail"