Bug 1660660 - Fix deadlock in gamepad initialization code. r=cmartin

StartGamepadMonitoring() can end up in AddGamepad, and acquire the lock
again on the same thread, effectively dead-locking.

This is a regression from bug 1657404. Relevant stack:

    (gdb) bt
    #0  0x00007fd19bace801 in clock_nanosleep@GLIBC_2.2.5 () at /lib64/libc.so.6
    #1  0x00007fd19bad4157 in nanosleep () at /lib64/libc.so.6
    #2  0x00007fd19bad408e in sleep () at /lib64/libc.so.6
    #3  0x00007fd195233e87 in ah_crap_handler(int) (signum=11) at /home/emilio/src/moz/gecko-4/toolkit/xre/nsSigHandlers.cpp:95
    #4  0x00007fd1952165c4 in nsProfileLock::FatalSignalHandler(int, siginfo_t*, void*) (signo=11, info=0x7fd14abb9db0, context=0x7fd14abb9c80) at /home/emilio/src/moz/gecko-4/toolkit/profile/nsProfileLock.cpp:177
    #5  0x00007fd1964973b2 in WasmTrapHandler(int, siginfo_t*, void*) (signum=11, info=<optimized out>, context=<optimized out>) at /home/emilio/src/moz/gecko-4/js/src/wasm/WasmSignalHandlers.cpp:978
    #6  0x00007fd19bf3ca90 in <signal handler called> () at /lib64/libpthread.so.0
    #7  mozilla::detail::MutexImpl::mutexLock() (this=<optimized out>) at /home/emilio/src/moz/gecko-4/mozglue/misc/Mutex_posix.cpp:118
    #8  mozilla::detail::MutexImpl::lock() (this=<optimized out>) at /home/emilio/src/moz/gecko-4/mozglue/misc/Mutex_posix.cpp:142
    #9  0x00007fd190cc795a in mozilla::OffTheBooksMutex::Lock() (this=0x7fd136649398) at /home/emilio/src/moz/gecko-4/xpcom/threads/BlockingResourceBase.cpp:318
    #10 0x00007fd19326e65e in mozilla::detail::BaseAutoLock<mozilla::Mutex&>::BaseAutoLock(mozilla::Mutex&) (this=<optimized out>, aLock=...) at /home/emilio/src/moz/gecko-4/obj-debug-no-sccache/dist/include/mozilla/Mutex.h:159
    #11 mozilla::dom::GamepadPlatformService::NotifyGamepadChange<mozilla::dom::GamepadAdded>(unsigned int, mozilla::dom::GamepadAdded const&) (this=0x7fd136649380, aIndex=1, aInfo=...)
        at /home/emilio/src/moz/gecko-4/dom/gamepad/GamepadPlatformService.cpp:65
    #12 0x00007fd193269178 in mozilla::dom::GamepadPlatformService::AddGamepad(char const*, mozilla::dom::GamepadMappingType, mozilla::dom::GamepadHand, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int)
        (this=0x7fd136649380, aID=<optimized out>, aMapping=mozilla::dom::GamepadMappingType::_empty, aHand=mozilla::dom::GamepadHand::_empty, aNumButtons=11, aNumAxes=8, aHaptics=0, aNumLightIndicator=0, aNumTouchEvents=0)
        at /home/emilio/src/moz/gecko-4/dom/gamepad/GamepadPlatformService.cpp:96
    #13 0x00007fd19326de4a in (anonymous namespace)::LinuxGamepadService::AddDevice(mozilla::udev_device*) (this=<optimized out>, dev=<optimized out>) at /home/emilio/src/moz/gecko-4/dom/gamepad/linux/LinuxGamepad.cpp:139
    #14 0x00007fd19326a156 in (anonymous namespace)::LinuxGamepadService::ScanForDevices() (this=<optimized out>) at /home/emilio/src/moz/gecko-4/dom/gamepad/linux/LinuxGamepad.cpp:188
    #15 (anonymous namespace)::LinuxGamepadService::Startup() (this=<optimized out>) at /home/emilio/src/moz/gecko-4/dom/gamepad/linux/LinuxGamepad.cpp:233
    #16 mozilla::dom::StartGamepadMonitoring() () at /home/emilio/src/moz/gecko-4/dom/gamepad/linux/LinuxGamepad.cpp:334
    #17 0x00007fd193269c6b in mozilla::dom::GamepadPlatformService::AddChannelParent(mozilla::dom::GamepadEventChannelParent*) (this=<optimized out>, aParent=<optimized out>)
        at /home/emilio/src/moz/gecko-4/dom/gamepad/GamepadPlatformService.cpp:225
    #18 0x00007fd19326d175 in mozilla::dom::GamepadEventChannelParent::Init() (this=0x7fd136e76a00) at /home/emilio/src/moz/gecko-4/dom/gamepad/ipc/GamepadEventChannelParent.cpp:50
    #19 0x00007fd1913ba3a6 in mozilla::ipc::BackgroundParentImpl::RecvPGamepadEventChannelConstructor(mozilla::dom::PGamepadEventChannelParent*) (this=0x7fd13f888000, aActor=0x0)
        at /home/emilio/src/moz/gecko-4/ipc/glue/BackgroundParentImpl.cpp:1109
    #20 0x00007fd1917c7da1 in mozilla::ipc::PBackgroundParent::OnMessageReceived(IPC::Message const&) (this=0x7fd13f888000, msg__=...) at PBackgroundParent.cpp:4967
    #21 0x00007fd1913ea71d in mozilla::ipc::MessageChannel::DispatchAsyncMessage(mozilla::ipc::ActorLifecycleProxy*, IPC::Message const&) (this=0x7fd13f8880f8, aProxy=0x7fd13ff48140, aMsg=...)

Differential Revision: https://phabricator.services.mozilla.com/D87967

dom/gamepad/GamepadPlatformService.cpp

@@ -207,20 +207,22 @@ void GamepadPlatformService::AddChannelParent(
   MOZ_ASSERT(!mChannelParents.Contains(aParent));
 
   // We use mutex here to prevent race condition with monitor thread
-  MutexAutoLock autoLock(mMutex);
-  mChannelParents.AppendElement(aParent);
-
-  // For a new GamepadEventChannel, we have to send the exising GamepadAdded
-  // to it to make it can have the same amount of gamepads with others.
-  if (mChannelParents.Length() > 1) {
-    for (const auto& evt : mGamepadAdded) {
-      GamepadChangeEventBody body(evt.second);
-      GamepadChangeEvent e(evt.first, GamepadServiceType::Standard, body);
-      aParent->DispatchUpdateEvent(e);
+  {
+    MutexAutoLock autoLock(mMutex);
+    mChannelParents.AppendElement(aParent);
+
+    // For a new GamepadEventChannel, we have to send the exising GamepadAdded
+    // to it to make it can have the same amount of gamepads with others.
+    if (mChannelParents.Length() > 1) {
+      for (const auto& evt : mGamepadAdded) {
+        GamepadChangeEventBody body(evt.second);
+        GamepadChangeEvent e(evt.first, GamepadServiceType::Standard, body);
+        aParent->DispatchUpdateEvent(e);
+      }
     }
-  }
 
-  FlushPendingEvents();
+    FlushPendingEvents();
+  }
 
   StartGamepadMonitoring();
 }