Add option to verify build determinism

Passing "--option build-repeat <N>" will cause every build to be
repeated N times. If the build output differs between any round, the
build is rejected, and the output paths are not registered as
valid. This is primarily useful to verify build determinism. (We
already had a --check option to repeat a previously succeeded
build. However, with --check, non-deterministic builds are registered
in the DB. Preventing that is useful for Hydra to ensure that
non-deterministic builds don't end up getting published at all.)

doc/manual/command-ref/conf-file.xml

@@ -616,6 +616,18 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>
   </varlistentry>
 
 
+  <varlistentry xml:id="conf-build-repeat"><term><literal>build-repeat</literal></term>
+
+    <listitem><para>How many times to repeat builds to check whether
+    they are deterministic. The default value is 0. If the value is
+    non-zero, every build is repeated the specified number of
+    times. If the contents of any of the runs differs from the
+    previous ones, the build is rejected and the resulting store paths
+    are not registered as “valid” in Nix’s database.</para></listitem>
+
+  </varlistentry>
+
+
 </variablelist>
 
 </para>

src/libstore/build.cc

@@ -791,13 +791,19 @@ class DerivationGoal : public Goal
        temporary paths. */
     PathSet redirectedBadOutputs;
 
-    /* Set of inodes seen during calls to canonicalisePathMetaData()
-       for this build's outputs.  This needs to be shared between
-       outputs to allow hard links between outputs. */
-    InodesSeen inodesSeen;
-
     BuildResult result;
 
+    /* The current round, if we're building multiple times. */
+    unsigned int curRound = 1;
+
+    unsigned int nrRounds;
+
+    /* Path registration info from the previous round, if we're
+       building multiple times. Since this contains the hash, it
+       allows us to compare whether two rounds produced the same
+       result. */
+    ValidPathInfos prevInfos;
+
 public:
     DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs,
         Worker & worker, BuildMode buildMode = bmNormal);
@@ -1238,6 +1244,10 @@ void DerivationGoal::inputsRealised()
     for (auto & i : drv->outputs)
         if (i.second.hash == "") fixedOutput = false;
 
+    /* Don't repeat fixed-output derivations since they're already
+       verified by their output hash.*/
+    nrRounds = fixedOutput ? 1 : settings.get("build-repeat", 0) + 1;
+
     /* Okay, try to build.  Note that here we don't wait for a build
        slot to become available, since we don't need one if there is a
        build hook. */
@@ -1420,6 +1430,9 @@ void replaceValidPath(const Path & storePath, const Path tmpPath)
 }
 
 
+MakeError(NotDeterministic, BuildError)
+
+
 void DerivationGoal::buildDone()
 {
     trace("build done");
@@ -1519,6 +1532,15 @@ void DerivationGoal::buildDone()
 
         deleteTmpDir(true);
 
+        /* Repeat the build if necessary. */
+        if (curRound++ < nrRounds) {
+            outputLocks.unlock();
+            buildUser.release();
+            state = &DerivationGoal::tryToBuild;
+            worker.wakeUp(shared_from_this());
+            return;
+        }
+
         /* It is now safe to delete the lock files, since all future
            lockers will see that the output paths are valid; they will
            not create new lock files with the same names as the old
@@ -1552,6 +1574,7 @@ void DerivationGoal::buildDone()
                     % drvPath % 1 % e.msg());
 
             st =
+                dynamic_cast<NotDeterministic*>(&e) ? BuildResult::NotDeterministic :
                 statusOk(status) ? BuildResult::OutputRejected :
                 fixedOutput || diskFull ? BuildResult::TransientFailure :
                 BuildResult::PermanentFailure;
@@ -1678,10 +1701,13 @@ int childEntry(void * arg)
 
 void DerivationGoal::startBuilder()
 {
-    startNest(nest, lvlInfo, format(
-            buildMode == bmRepair ? "repairing path(s) %1%" :
-            buildMode == bmCheck ? "checking path(s) %1%" :
-            "building path(s) %1%") % showPaths(missingPaths));
+    auto f = format(
+        buildMode == bmRepair ? "repairing path(s) %1%" :
+        buildMode == bmCheck ? "checking path(s) %1%" :
+        nrRounds > 1 ? "building path(s) %1% (round %2%/%3%)" :
+        "building path(s) %1%");
+    f.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
+    startNest(nest, lvlInfo, f % showPaths(missingPaths) % curRound % nrRounds);
 
     /* Right platform? */
     if (!canBuildLocally(*drv)) {
@@ -1693,6 +1719,7 @@ void DerivationGoal::startBuilder()
     }
 
     /* Construct the environment passed to the builder. */
+    env.clear();
 
     /* Most shells initialise PATH to some default (/bin:/usr/bin:...) when
        PATH is not set.  We don't want this, so we fill it in with some dummy
@@ -1873,6 +1900,8 @@ void DerivationGoal::startBuilder()
         PathSet dirs2 = tokenizeString<StringSet>(settings.get("build-extra-chroot-dirs", string("")));
         dirs.insert(dirs2.begin(), dirs2.end());
 
+        dirsInChroot.clear();
+
         for (auto & i : dirs) {
             size_t p = i.find('=');
             if (p == string::npos)
@@ -2582,6 +2611,11 @@ void DerivationGoal::registerOutputs()
 
     ValidPathInfos infos;
 
+    /* Set of inodes seen during calls to canonicalisePathMetaData()
+       for this build's outputs.  This needs to be shared between
+       outputs to allow hard links between outputs. */
+    InodesSeen inodesSeen;
+
     /* 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. */
@@ -2753,6 +2787,16 @@ void DerivationGoal::registerOutputs()
 
     if (buildMode == bmCheck) return;
 
+    if (curRound > 1 && prevInfos != infos)
+        throw NotDeterministic(
+            format("result of ‘%1%’ differs from previous round; rejecting as non-deterministic")
+            % drvPath);
+
+    if (curRound < nrRounds) {
+        prevInfos = infos;
+        return;
+    }
+
     /* Register each output path as valid, and register the sets of
        paths referenced by each of them.  If there are cycles in the
        outputs, this will fail. */

src/libstore/store-api.hh

@@ -87,10 +87,17 @@ struct ValidPathInfo
     Path deriver;
     Hash hash;
     PathSet references;
-    time_t registrationTime;
-    unsigned long long narSize; // 0 = unknown
+    time_t registrationTime = 0;
+    unsigned long long narSize = 0; // 0 = unknown
     unsigned long long id; // internal use only
-    ValidPathInfo() : registrationTime(0), narSize(0) { }
+
+    bool operator == (const ValidPathInfo & i) const
+    {
+        return
+            path == i.path
+            && hash == i.hash
+            && references == i.references;
+    }
 };
 
 typedef list<ValidPathInfo> ValidPathInfos;
@@ -114,6 +121,7 @@ struct BuildResult
         MiscFailure,
         DependencyFailed,
         LogLimitExceeded,
+        NotDeterministic,
     } status = MiscFailure;
     std::string errorMsg;
     //time_t startTime = 0, stopTime = 0;