daemon: Allow builds to be repeated.

This makes it easy to detect non-deterministic builds.

* nix/libstore/build.cc (DerivationGoal): Remove 'InodesSeen'; add
'curRound', 'nrRound', and 'prevInfos'.
(DerivationGoal::inputsRealised): Initialize 'nrRound'.
(NotDeterministic): New error type.
(DerivationGoal::buildDone): Check whether we need to repeat.
(DerivationGoal::startBuilder): Adjust message.
(DerivationGoal::registerOutputs): Check whether we get the same result.
* nix/libstore/globals.cc (Settings::get(const string & name, int def)):
New method.
* nix/libstore/globals.hh (Settings): Add it.
* nix/libstore/store-api.hh (ValidPathInfo): Add operator ==.
* nix/nix-daemon/nix-daemon.cc (performOp): Allow "build-repeat" for
"untrusted" users.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

nix/libstore/store-api.hh

@@ -88,10 +88,17 @@ 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;