build: add exit code for hash and check mismatches

Makes it easier to identify the failure reason in other tooling, eg.
differentiate between a non-deterministic --check vs a failed build.

	$ nix-build '<nix/fetchurl.nix>' --argstr url http://example.org --argstr sha256 0000000000000000000000000000000000000000000000000000
	hash mismatch in fixed-output derivation '/nix/store/nzi9ck45rwlxzcwr25is7qlf3hs5xl83-example.org':
	  wanted: sha256:0000000000000000000000000000000000000000000000000000
	  got:    sha256:08y4734bm2zahw75b16bcmcg587vvyvh0n11gwiyir70divwp1rm
	$ echo $?
	102

	$ nix-build -E 'with import <nixpkgs> {}; runCommand "foo" {} "date +%s > $out"' --check
	warning: rewriting hashes in '/nix/store/g3k47g0399fvjmbm0p0mnad74k4w8vkz-foo'; cross fingers
	error: derivation '/nix/store/mggc8dz13ackb49qca6m23zq4fpq132q-foo.drv' may not be deterministic: output '/nix/store/g3k47g0399fvjmbm0p0mnad74k4w8vkz-foo' differs
	$ echo $?
	104

src/libstore/build.cc

@@ -266,6 +266,12 @@ public:
     /* Set if at least one derivation had a timeout. */
     bool timedOut;
 
+    /* Set if at least one derivation fails with a hash mismatch. */
+    bool hashMismatch;
+
+    /* Set if at least one derivation is not deterministic in check mode. */
+    bool checkMismatch;
+
     LocalStore & store;
 
     std::unique_ptr<HookInstance> hook;
@@ -3213,6 +3219,7 @@ void DerivationGoal::registerOutputs()
 
                 /* Throw an error after registering the path as
                    valid. */
+                worker.hashMismatch = true;
                 delayedException = std::make_exception_ptr(
                     BuildError("hash mismatch in fixed-output derivation '%s':\n  wanted: %s\n  got:    %s",
                         dest, h.to_string(), h2.to_string()));
@@ -3255,6 +3262,7 @@ void DerivationGoal::registerOutputs()
             if (!worker.store.isValidPath(path)) continue;
             auto info = *worker.store.queryPathInfo(path);
             if (hash.first != info.narHash) {
+                worker.checkMismatch = true;
                 if (settings.runDiffHook || settings.keepFailed) {
                     Path dst = worker.store.toRealPath(path + checkSuffix);
                     deletePath(dst);
@@ -3266,10 +3274,10 @@ void DerivationGoal::registerOutputs()
                         buildUser ? buildUser->getGID() : getgid(),
                         path, dst, drvPath, tmpDir);
 
-                    throw Error(format("derivation '%1%' may not be deterministic: output '%2%' differs from '%3%'")
+                    throw NotDeterministic(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")
+                    throw NotDeterministic(format("derivation '%1%' may not be deterministic: output '%2%' differs")
                         % drvPath % path);
             }
 
@@ -4101,6 +4109,8 @@ Worker::Worker(LocalStore & store)
     lastWokenUp = steady_time_point::min();
     permanentFailure = false;
     timedOut = false;
+    hashMismatch = false;
+    checkMismatch = false;
 }
 
 
@@ -4461,7 +4471,7 @@ void Worker::waitForInput()
 
 unsigned int Worker::exitStatus()
 {
-    return timedOut ? 101 : (permanentFailure ? 100 : 1);
+    return checkMismatch ? 104 : (hashMismatch ? 102 : (timedOut ? 101 : (permanentFailure ? 100 : 1)));
 }
 
 

src/libstore/download.cc

@@ -855,10 +855,11 @@ CachedDownloadResult Downloader::downloadCached(
     }
 
     if (expectedStorePath != "" && storePath != expectedStorePath) {
+        unsigned int statusCode = 102;
         Hash gotHash = request.unpack
             ? hashPath(request.expectedHash.type, store->toRealPath(storePath)).first
             : hashFile(request.expectedHash.type, store->toRealPath(storePath));
-        throw nix::Error("hash mismatch in file downloaded from '%s':\n  wanted: %s\n  got:    %s",
+        throw nix::Error(statusCode, "hash mismatch in file downloaded from '%s':\n  wanted: %s\n  got:    %s",
             url, request.expectedHash.to_string(), gotHash.to_string());
     }