hydra-queue-runner: drop broken connections from pool

Closes #1336

When restarting postgresql, the connections are still reused in
`hydra-queue-runner` causing errors like this

    main thread: Lost connection to the database server.
    queue monitor: Lost connection to the database server.

and no more builds being processed.

`hydra-evaluator` doesn't have that issue since it crashes right away.
We could let it retry indefinitely as well (see below), but I don't
want to change too much.

If the DB is still unreachable 10s later, the process will stop with a
non-zero exit code because of a missing DB connection. This however
isn't such a big deal because it will be immediately restarted
afterwards. With the current configuration, Hydra will never give up,
but restart (and retry) infinitely. To me that seems reasonable, i.e. to
retry DB connections on a long-running process. If this doesn't work
out, the monitoring should fire anyways because the queue fills up, but
I'm open to discuss that.

Please note that this isn't reproducible with the DB and the queue
runner on the same machine when using `services.hydra-dev`, because of
the `Requires=` dependency `hydra-queue-runner.service` ->
`hydra-init.service` -> `postgresql.service` that causes the queue
runner to be restarted on `systemctl restart postgresql`.

Internally, Hydra uses Nix's pool data structure: it basically has N
slots (here DB connections) and whenever a new one is requested, an idle
slot is provided or a new one is created (when N slots are active, it'll
be waited until one slot is free). The issue in the code here is however
that whenever an error is encountered, the slot is released, however the
same broken connection will be reused the next time. By using
`Pool::Handle::markBad`, Nix will drop a broken slot. This is now being
done when `pqxx::broken_connection` was caught.

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

@@ -928,11 +928,18 @@ void State::run(BuildID buildOne)
     while (true) {
         try {
             auto conn(dbPool.get());
+            try {
                 receiver dumpStatus_(*conn, "dump_status");
                 while (true) {
                     conn->await_notification();
                     dumpStatus(*conn);
                 }
+            } catch (pqxx::broken_connection & connEx) {
+                printMsg(lvlError, "main thread: %s", connEx.what());
+                printMsg(lvlError, "main thread: Reconnecting in 10s");
+                conn.markBad();
+                sleep(10);
+            }
         } catch (std::exception & e) {
             printMsg(lvlError, "main thread: %s", e.what());
             sleep(10); // probably a DB problem, so don't retry right away

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

@@ -10,8 +10,14 @@ using namespace nix;
 void State::queueMonitor()
 {
     while (true) {
+        auto conn(dbPool.get());
         try {
-            queueMonitorLoop();
+            queueMonitorLoop(*conn);
+        } catch (pqxx::broken_connection & e) {
+            printMsg(lvlError, "queue monitor: %s", e.what());
+            printMsg(lvlError, "queue monitor: Reconnecting in 10s");
+            conn.markBad();
+            sleep(10);
         } catch (std::exception & e) {
             printError("queue monitor: %s", e.what());
             sleep(10); // probably a DB problem, so don't retry right away
@@ -20,16 +26,14 @@ void State::queueMonitor()
 }
 
 
-void State::queueMonitorLoop()
+void State::queueMonitorLoop(Connection & conn)
 {
-    auto conn(dbPool.get());
-
-    receiver buildsAdded(*conn, "builds_added");
-    receiver buildsRestarted(*conn, "builds_restarted");
-    receiver buildsCancelled(*conn, "builds_cancelled");
-    receiver buildsDeleted(*conn, "builds_deleted");
-    receiver buildsBumped(*conn, "builds_bumped");
-    receiver jobsetSharesChanged(*conn, "jobset_shares_changed");
+    receiver buildsAdded(conn, "builds_added");
+    receiver buildsRestarted(conn, "builds_restarted");
+    receiver buildsCancelled(conn, "builds_cancelled");
+    receiver buildsDeleted(conn, "builds_deleted");
+    receiver buildsBumped(conn, "builds_bumped");
+    receiver jobsetSharesChanged(conn, "jobset_shares_changed");
 
     auto destStore = getDestStore();
 
@@ -39,17 +43,17 @@ void State::queueMonitorLoop()
     while (!quit) {
         localStore->clearPathInfoCache();
 
-        bool done = getQueuedBuilds(*conn, destStore, lastBuildId);
+        bool done = getQueuedBuilds(conn, destStore, lastBuildId);
 
         if (buildOne && buildOneDone) quit = true;
 
         /* Sleep until we get notification from the database about an
            event. */
         if (done && !quit) {
-            conn->await_notification();
+            conn.await_notification();
             nrQueueWakeups++;
         } else
-            conn->get_notifs();
+            conn.get_notifs();
 
         if (auto lowestId = buildsAdded.get()) {
             lastBuildId = std::min(lastBuildId, static_cast<unsigned>(std::stoul(*lowestId) - 1));
@@ -61,11 +65,11 @@ void State::queueMonitorLoop()
         }
         if (buildsCancelled.get() || buildsDeleted.get() || buildsBumped.get()) {
             printMsg(lvlTalkative, "got notification: builds cancelled or bumped");
-            processQueueChange(*conn);
+            processQueueChange(conn);
         }
         if (jobsetSharesChanged.get()) {
             printMsg(lvlTalkative, "got notification: jobset shares changed");
-            processJobsetSharesChange(*conn);
+            processJobsetSharesChange(conn);
         }
     }
 

src/hydra-queue-runner/state.hh

@@ -531,7 +531,7 @@ private:
 
     void queueMonitor();
 
-    void queueMonitorLoop();
+    void queueMonitorLoop(Connection & conn);
 
     /* Check the queue for new builds. */
     bool getQueuedBuilds(Connection & conn,