event/Loop: split InjectBreak() from Break()

src/event/Loop.cxx

@@ -87,17 +87,6 @@ EventLoop::GetUring() noexcept
 
 #endif
 
-void
-EventLoop::Break() noexcept
-{
-	if (quit.exchange(true))
-		return;
-
-#ifdef HAVE_THREADED_EVENT_LOOP
-	wake_fd.Write();
-#endif
-}
-
 bool
 EventLoop::AddFD(int fd, unsigned events, SocketEvent &event) noexcept
 {
@@ -286,6 +275,7 @@ EventLoop::Run() noexcept
 
 	assert(IsInside());
 	assert(!quit);
+	assert(!quit_injected);
 #ifdef HAVE_THREADED_EVENT_LOOP
 	assert(alive);
 	assert(busy);
@@ -437,6 +427,11 @@ EventLoop::OnSocketReady([[maybe_unused]] unsigned flags) noexcept
 
 	wake_fd.Read();
 
+	if (quit_injected) {
+		Break();
+		return;
+	}
+
 	const std::scoped_lock<Mutex> lock(mutex);
 	HandleInject();
 }

src/event/Loop.hxx

@@ -115,7 +115,7 @@ class EventLoop final
 	bool alive;
 #endif
 
-	std::atomic_bool quit{false};
+	bool quit = false;
 
 	/**
 	 * True when the object has been modified and another check is
@@ -124,6 +124,8 @@ class EventLoop final
 	bool again;
 
 #ifdef HAVE_THREADED_EVENT_LOOP
+	bool quit_injected = false;
+
 	/**
 	 * True when handling callbacks, false when waiting for I/O or
 	 * timeout.
@@ -187,11 +189,30 @@ public:
 #endif
 
 	/**
-	 * Stop execution of this #EventLoop at the next chance.  This
-	 * method is thread-safe and non-blocking: after returning, it
-	 * is not guaranteed that the EventLoop has really stopped.
+	 * Stop execution of this #EventLoop at the next chance.
+	 *
+	 * This method is not thread-safe.  For stopping the
+	 * #EventLoop from within another thread, use InjectBreak().
 	 */
-	void Break() noexcept;
+	void Break() noexcept {
+		quit = true;
+	}
+
+#ifdef HAVE_THREADED_EVENT_LOOP
+	/**
+	 * Like Break(), but thread-safe.  It is also non-blocking:
+	 * after returning, it is not guaranteed that the EventLoop
+	 * has really stopped.
+	 */
+	void InjectBreak() noexcept {
+		{
+			const std::scoped_lock lock{mutex};
+			quit_injected = true;
+		}
+
+		wake_fd.Write();
+	}
+#endif // HAVE_THREADED_EVENT_LOOP
 
 	bool AddFD(int fd, unsigned events, SocketEvent &event) noexcept;
 	bool ModifyFD(int fd, unsigned events, SocketEvent &event) noexcept;

src/event/Thread.cxx

@@ -45,7 +45,7 @@ EventThread::Stop() noexcept
 		assert(event_loop.IsAlive());
 		event_loop.SetAlive(false);
 
-		event_loop.Break();
+		event_loop.InjectBreak();
 		thread.Join();
 	}
 }