Fix root creation when the root already exists but is owned by another user

src/hydra-queue-runner/builder.cc

@@ -264,7 +264,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
         assert(stepNr);
 
         for (auto & path : step->drv.outputPaths())
-            writeFile(rootsDir + "/" + baseNameOf(path), "");
+            addRoot(path);
 
         /* Register success in the database for all Build objects that
            have this step as the top-level step. Since the queue
@@ -473,3 +473,10 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
 
     return sDone;
 }
+
+
+void State::addRoot(const Path & storePath)
+{
+    auto root = rootsDir + "/" + baseNameOf(storePath);
+    if (!pathExists(root)) writeFile(root, "");
+}

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

@@ -841,6 +841,8 @@ void State::run(BuildID buildOne)
         jobsetRepeats.emplace(std::make_pair(s2[0], s2[1]), std::stoi(s2[2]));
     }
 
+    addRoot("/nix/store/rk9lahzm21qm0zjclv6mz0m4hy77wk14-vm-test-run-unnamed.drv");
+
     {
         auto conn(dbPool.get());
         clearBusy(*conn, 0);

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

@@ -222,7 +222,7 @@ bool State::getQueuedBuilds(Connection & conn,
             BuildOutput res = getBuildOutputCached(conn, destStore, drv);
 
             for (auto & path : drv.outputPaths())
-                writeFile(rootsDir + "/" + baseNameOf(path), "");
+                addRoot(path);
 
             {
             auto mc = startDbUpdate();

src/hydra-queue-runner/state.hh

@@ -535,6 +535,8 @@ private:
 
     void dumpStatus(Connection & conn, bool log);
 
+    void addRoot(const nix::Path & storePath);
+
 public:
 
     void showStatus();