Support testing build determinism

Builds can now specify the attribute "isDeterministic = true" to tell
Hydra to build with build-repeat > 0. If there is a mismatch between
rounds, the step / build fails with a suitable status.

Maybe this should be a meta attribute, but that makes it invisible to
hydra-queue-runner, and it seems reasonable to make a claim of
mandatory determinism part of the derivation (since e.g. enabling this
flag should trigger a rebuild).

src/hydra-queue-runner/build-remote.cc

@@ -169,7 +169,7 @@ void State::buildRemote(ref<Store> destStore,
         unsigned int remoteVersion;
 
         try {
-            to << SERVE_MAGIC_1 << 0x202;
+            to << SERVE_MAGIC_1 << 0x203;
             to.flush();
 
             unsigned int magic = readInt(from);
@@ -180,6 +180,8 @@ void State::buildRemote(ref<Store> destStore,
                 throw Error(format("unsupported ‘nix-store --serve’ protocol version on ‘%1%’") % machine->sshName);
             if (GET_PROTOCOL_MINOR(remoteVersion) >= 1)
                 sendDerivation = false;
+            if (GET_PROTOCOL_MINOR(remoteVersion) < 3 && step->isDeterministic)
+                throw Error("machine ‘%1%’ does not support deterministic builds; please upgrade it to Nix 1.12", machine->sshName);
 
         } catch (EndOfFile & e) {
             child.pid.wait(true);
@@ -261,6 +263,9 @@ void State::buildRemote(ref<Store> destStore,
         to << maxSilentTime << buildTimeout;
         if (GET_PROTOCOL_MINOR(remoteVersion) >= 2)
             to << 64 * 1024 * 1024; // == maxLogSize
+        if (GET_PROTOCOL_MINOR(remoteVersion) >= 3)
+            // FIXME: make the number of repeats configurable.
+            to << (step->isDeterministic ? 1 : 0);
         to.flush();
 
         result.startTime = time(0);
@@ -325,6 +330,11 @@ void State::buildRemote(ref<Store> destStore,
                 case BuildResult::LogLimitExceeded:
                     result.stepStatus = bsLogLimitExceeded;
                     break;
+                case BuildResult::NotDeterministic:
+                    result.stepStatus = bsNotDeterministic;
+                    result.canRetry = false;
+                    result.canCache = true;
+                    break;
                 default:
                     result.stepStatus = bsAborted;
                     break;

src/hydra-queue-runner/queue-monitor.cc

@@ -418,6 +418,7 @@ Step::ptr State::createStep(ref<Store> destStore,
     step->drv = readDerivation(drvPath);
 
     step->preferLocalBuild = step->drv.willBuildLocally();
+    step->isDeterministic = get(step->drv.env, "isDetermistic", "0") == "1";
 
     step->systemType = step->drv.platform;
     {

src/hydra-queue-runner/state.hh

@@ -35,6 +35,7 @@ typedef enum {
     bsUnsupported = 9,
     bsLogLimitExceeded = 10,
     bsNarSizeLimitExceeded = 11,
+    bsNotDeterministic = 12,
     bsBusy = 100, // not stored
 } BuildStatus;
 
@@ -139,6 +140,7 @@ struct Step
     nix::Derivation drv;
     std::set<std::string> requiredSystemFeatures;
     bool preferLocalBuild;
+    bool isDeterministic;
     std::string systemType; // concatenation of drv.platform and requiredSystemFeatures
 
     struct State