Don't wait forever to acquire the send lock

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

@@ -229,7 +229,8 @@ void State::buildRemote(ref<Store> destStore,
         /* Copy the input closure. */
         if (/* machine->sshName != "localhost" */ true) {
             auto mc1 = std::make_shared<MaintainCount>(nrStepsWaiting);
-            std::lock_guard<std::mutex> sendLock(machine->state->sendLock);
+            std::unique_lock<std::timed_mutex> sendLock(
+                machine->state->sendLock, std::chrono::seconds(600));
             mc1.reset();
             MaintainCount mc2(nrStepsCopyingTo);
             printMsg(lvlDebug, format("sending closure of ‘%1%’ to ‘%2%’") % step->drvPath % machine->sshName);

src/hydra-queue-runner/state.hh

@@ -231,7 +231,7 @@ struct Machine
 
         /* Mutex to prevent multiple threads from sending data to the
            same machine (which would be inefficient). */
-        std::mutex sendLock;
+        std::timed_mutex sendLock;
     };
 
     State::ptr state;