Bug 1747526 - Return a handle from asyncShutdown to observe and cance…

…l thread shutdown progress, r=xpcom-reviewers,KrisWright

This change introduces a new interface, nsIThreadShutdown, which is used
to handle the relevant state for communicating thread shutdown state
between the joining and terminating threads. This type is now returned
from `nsIThread::AsyncShutdown` and can be used to register callbacks
for when thread shutdown is complete, as well as cancel shutdown
entirely, leading to the underlying PRThread never being joined using
PR_JoinThread. This leaking limitation may be avoidable if support for
detaching PRThreads is added to NSPR, or nsThread switches to a more
feature-complete threading API.

This patch also uses the new interface to rework nsThreadPool's Shutdown
and ShutdownWithTimeout methods to avoid poking at nsThread internals
and instead use the publicly facing methods. This allows us to start
async shutdown for all threads and spin the event loop until they all
complete, or a timeout timer fires.

Differential Revision: https://phabricator.services.mozilla.com/D136045

xpcom/threads/LazyIdleThread.cpp

@@ -473,6 +473,13 @@ LazyIdleThread::AsyncShutdown() {
   return NS_ERROR_NOT_IMPLEMENTED;
 }
 
+NS_IMETHODIMP
+LazyIdleThread::BeginShutdown(nsIThreadShutdown** aShutdown) {
+  ASSERT_OWNING_THREAD();
+  *aShutdown = nullptr;
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 NS_IMETHODIMP
 LazyIdleThread::Shutdown() {
   ASSERT_OWNING_THREAD();

xpcom/threads/moz.build

@@ -18,6 +18,7 @@ XPIDL_SOURCES += [
     "nsIThreadInternal.idl",
     "nsIThreadManager.idl",
     "nsIThreadPool.idl",
+    "nsIThreadShutdown.idl",
     "nsITimer.idl",
 ]
 

xpcom/threads/nsIThread.idl

@@ -5,9 +5,11 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsISerialEventTarget.idl"
+#include "nsIThreadShutdown.idl"
 
 %{C++
 #include "mozilla/AlreadyAddRefed.h"
+
 namespace mozilla {
 class TimeStamp;
 class TimeDurationValueCalculator;
@@ -130,11 +132,17 @@ interface nsIThread : nsISerialEventTarget
    * @throws NS_ERROR_UNEXPECTED
    *   Indicates that this method was erroneously called when this thread was
    *   the current thread, that this thread was not created with a call to
-   *   nsIThreadManager::NewThread, or if this method was called more than once
-   *   on the thread object.
+   *   nsIThreadManager::NewNamedThread, or that this method was called more
+   *   than once on the thread object.
    */
   void asyncShutdown();
 
+  /**
+   * Like `asyncShutdown`, but also returns a nsIThreadShutdown instance to
+   * allow observing and controlling the thread's async shutdown progress.
+   */
+  nsIThreadShutdown beginShutdown();
+
   /**
    * Dispatch an event to a specified queue for the thread.  This function
    * may be called from any thread, and it may be called re-entrantly.

xpcom/threads/nsIThreadShutdown.idl

@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIRunnable;
+
+/**
+ * Handle for the ongoing shutdown progress of a given thread which can be used
+ * to observe and interrupt async shutdown progress. Methods on this interface
+ * may generally only be used on the thread which called
+ * `nsIThread::beginShutdown`.
+ */
+[scriptable, builtinclass, uuid(70a43748-6130-4ea6-a440-7c74e1b7dd7c)]
+interface nsIThreadShutdown : nsISupports
+{
+  /**
+   * Register a runnable to be executed when the thread has completed shutdown,
+   * or shutdown has been cancelled due to `stopWaitingAndLeakThread()`.
+   *
+   * If the thread has already completed or cancelled shutdown, the runnable
+   * may be executed synchronously.
+   *
+   * May only be called on the thread which invoked `nsIThread::beginShutdown`.
+   */
+  void onCompletion(in nsIRunnable aEvent);
+
+  /**
+   * Check if the target thread has completed shutdown.
+   *
+   * May only be accessed on the thread which called `nsIThread::beginShutdown`.
+   */
+  [infallible] readonly attribute boolean completed;
+
+  /**
+   * Give up on waiting for the shutting down thread to exit. Calling this
+   * method will allow the thread to continue running, no longer block shutdown,
+   * and the thread will never be joined or have its resources reclaimed.
+   *
+   * Completion callbacks attached to this `nsIThreadShutdown` may be executed
+   * during this call.
+   *
+   * This method should NOT be called except in exceptional circumstances during
+   * shutdown, as it will cause resources for the shutting down thread to be
+   * leaked.
+   *
+   * May only be called on the thread which called `nsIThread::beginShutdown`
+   *
+   * @throws NS_ERROR_NOT_AVAILABLE
+   *   Indicates that the target thread has already stopped running and a
+   *   request to be joined is already being dispatched to the waiting thread.
+   */
+  void stopWaitingAndLeakThread();
+};