hydra-queue-runner: Handle status queries on the main thread

Doing it on the queue monitor thread was problematic because
processing the queue can take a while.

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

@@ -84,6 +84,23 @@ struct Connection : pqxx::connection
 };
 
 
+struct receiver : public pqxx::notification_receiver
+{
+    bool status = false;
+    receiver(pqxx::connection_base & c, const std::string & channel)
+        : pqxx::notification_receiver(c, channel) { }
+    void operator() (const string & payload, int pid) override
+    {
+        status = true;
+    };
+    bool get() {
+        bool b = status;
+        status = false;
+        return b;
+    }
+};
+
+
 typedef unsigned int BuildID;
 
 
@@ -435,26 +452,9 @@ void State::queueMonitorLoop()
 {
     auto conn(dbPool.get());
 
-    struct receiver : public pqxx::notification_receiver
-    {
-        bool status = false;
-        receiver(pqxx::connection_base & c, const std::string & channel)
-            : pqxx::notification_receiver(c, channel) { }
-        void operator() (const string & payload, int pid) override
-        {
-            status = true;
-        };
-        bool get() {
-            bool b = status;
-            status = false;
-            return b;
-        }
-    };
-
     receiver buildsAdded(*conn, "builds_added");
     receiver buildsRestarted(*conn, "builds_restarted");
     receiver buildsCancelled(*conn, "builds_cancelled");
-    receiver dumpStatus(*conn, "dump_status");
 
     auto store = openStore(); // FIXME: pool
 
@@ -479,8 +479,6 @@ void State::queueMonitorLoop()
             removeCancelledBuilds(*conn);
         }
 
-        if (dumpStatus.get())
-            State::dumpStatus();
     }
 }
 
@@ -1314,6 +1312,22 @@ void State::run()
 
     std::thread(&State::dispatcher, this).detach();
 
+    while (true) {
+        try {
+            auto conn(dbPool.get());
+            receiver dumpStatus(*conn, "dump_status");
+            while (true) {
+                conn->await_notification();
+                if (dumpStatus.get())
+                    State::dumpStatus();
+            }
+        } catch (std::exception & e) {
+            printMsg(lvlError, format("main thread: %1%") % e.what());
+            sleep(10); // probably a DB problem, so don't retry right away
+        }
+    }
+
+    // Never reached.
     queueMonitorThread.join();
 }