Bug 896896. Use MsgWaitForMultipleObjectsEx instead of WaitMessage. r=roc

widget/windows/WinUtils.cpp

@@ -270,6 +270,28 @@ WinUtils::GetMessage(LPMSG aMsg, HWND aWnd, UINT aFirstMessage,
   return ::GetMessageW(aMsg, aWnd, aFirstMessage, aLastMessage);
 }
 
+/* static */
+void
+WinUtils::WaitForMessage()
+{
+  DWORD result = ::MsgWaitForMultipleObjectsEx(0, NULL, INFINITE, QS_ALLINPUT,
+                                               MWMO_INPUTAVAILABLE);
+  NS_WARN_IF_FALSE(result != WAIT_FAILED, "Wait failed");
+
+  // This idiom is taken from the Chromium ipc code, see
+  // ipc/chromium/src/base/message+puimp_win.cpp:270.
+  // The intent is to avoid a busy wait when MsgWaitForMultipleObjectsEx
+  // returns quickly but PeekMessage would not return a message.
+  if (result == WAIT_OBJECT_0) {
+    MSG msg = {0};
+    DWORD queue_status = ::GetQueueStatus(QS_MOUSE);
+    if (HIWORD(queue_status) & QS_MOUSE &&
+        !PeekMessage(&msg, NULL, WM_MOUSEFIRST, WM_MOUSELAST, PM_NOREMOVE)) {
+      ::WaitMessage();
+    }
+  }
+}
+
 /* static */
 bool
 WinUtils::GetRegistryKey(HKEY aRoot,

widget/windows/WinUtils.h

@@ -95,6 +95,19 @@ public:
                           UINT aLastMessage, UINT aOption);
   static bool GetMessage(LPMSG aMsg, HWND aWnd, UINT aFirstMessage,
                          UINT aLastMessage);
+
+  /**
+   * Wait until a message is ready to be processed.
+   * Prefer using this method to directly calling ::WaitMessage since
+   * ::WaitMessage will wait if there is an unread message in the queue.
+   * That can cause freezes until another message enters the queue if the
+   * message is marked read by a call to PeekMessage which the caller is
+   * not aware of (e.g., from a different thread).
+   * Note that this method may cause sync dispatch of sent (as opposed to
+   * posted) messages.
+   */
+  static void WaitForMessage();
+
   /**
    * Gets the value of a string-typed registry value.
    *

widget/windows/nsAppShell.cpp

@@ -308,7 +308,7 @@ nsAppShell::ProcessNextNativeEvent(bool mayWait)
       mozilla::HangMonitor::Suspend();
       {
         GeckoProfilerSleepRAII profiler_sleep;
-        ::WaitMessage();
+        WinUtils::WaitForMessage();
       }
     }
   } while (!gotMessage && mayWait);