hydra-queue-runner: don't dispatch until the machines parser has completed one run

Periodically, I have seen tests fail because of out of order queue runner behavior:

    checking the queue for builds > 0...
    loading build 1 (tests:basic:empty_dir)
    aborting unsupported build step '...-empty-dir.drv' (type 'x86_64-linux')
    marking build 1 as failed
    adding new machine ‘localhost’

This patch should prevent the dispatcher from running before any machines are
made available.

src/hydra-queue-runner/dispatcher.cc

@@ -31,8 +31,10 @@ void State::makeRunnable(Step::ptr step)
 
 void State::dispatcher()
 {
-    while (true) {
+    printMsg(lvlDebug, "Waiting for the machines parsing to have completed at least once");
+    machinesReadyLock.lock();
 
+    while (true) {
         try {
             printMsg(lvlDebug, "dispatcher woken up");
             nrDispatcherWakeups++;

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

@@ -158,6 +158,7 @@ void State::monitorMachinesFile()
             (settings.thisSystem == "x86_64-linux" ? "x86_64-linux,i686-linux" : settings.thisSystem.get())
             + " - " + std::to_string(settings.maxBuildJobs) + " 1 "
             + concatStringsSep(",", settings.systemFeatures.get()));
+        machinesReadyLock.unlock();
         return;
     }
 
@@ -203,9 +204,15 @@ void State::monitorMachinesFile()
         parseMachines(contents);
     };
 
+    auto firstParse = true;
+
     while (true) {
         try {
             readMachinesFiles();
+            if (firstParse) {
+                machinesReadyLock.unlock();
+                firstParse = false;
+            }
             // FIXME: use inotify.
             sleep(30);
         } catch (std::exception & e) {
@@ -770,6 +777,7 @@ void State::run(BuildID buildOne)
         dumpStatus(*conn);
     }
 
+    machinesReadyLock.lock();
     std::thread(&State::monitorMachinesFile, this).detach();
 
     std::thread(&State::queueMonitor, this).detach();

src/hydra-queue-runner/state.hh

@@ -342,6 +342,7 @@ private:
     nix::Pool<Connection> dbPool;
 
     /* The build machines. */
+    std::mutex machinesReadyLock;
     typedef std::map<std::string, Machine::ptr> Machines;
     nix::Sync<Machines> machines; // FIXME: use atomic_shared_ptr