Bug 1351655 - Log which SharedThreadPools are not yet destroyed on shutdown-threads. r=froydnj

We risk a shutdown hang without any clues if a dangling reference somewhere
keeps a SharedThreadPool alive past xpcom-shutdown-threads. This because
xpcom-shutdown-threads waits for all SharedThreadPool to be destroyed before
advancing shutdown further. nsCycleCollector::Shutdown comes after
xpcom-shutdown-threads so it is possible that a reference-cycle unexpectedly
keeps a SharedThreadPool alive and blocks shutdown indefinitely.

This patch attempts to help diagnose future cases like this by printing which
SharedThreadPools we are going to wait for to stderr.

MozReview-Commit-ID: CwTApYUMikA

--HG--
extra : rebase_source : f29d04508cf492c82d65f324d7b1990849a0da13

xpcom/threads/SharedThreadPool.cpp

@@ -47,6 +47,19 @@ SharedThreadPoolShutdownObserver::Observe(nsISupports* aSubject, const char *aTo
                                           const char16_t *aData)
 {
   MOZ_RELEASE_ASSERT(!strcmp(aTopic, "xpcom-shutdown-threads"));
+#ifdef EARLY_BETA_OR_EARLIER
+  {
+    ReentrantMonitorAutoEnter mon(*sMonitor);
+    if (!sPools->Iter().Done()) {
+      nsAutoCString str;
+      for (auto i = sPools->Iter(); !i.Done(); i.Next()) {
+        str.AppendPrintf("\"%s\" ", nsAutoCString(i.Key()).get());
+      }
+      printf_stderr("SharedThreadPool in xpcom-shutdown-threads. Waiting for "
+                    "pools %s\n", str.get());
+    }
+  }
+#endif
   SharedThreadPool::SpinUntilEmpty();
   sMonitor = nullptr;
   sPools = nullptr;