event/Loop: add flag `alive`

This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().

src/event/Call.cxx

@@ -80,7 +80,7 @@ private:
 void
 BlockingCall(EventLoop &loop, std::function<void()> &&f)
 {
-	if (loop.IsDead() || loop.IsInside()) {
+	if (!loop.IsAlive() || loop.IsInside()) {
 		/* we're already inside the loop - we can simply call
 		   the function */
 		f();

src/event/Loop.cxx

@@ -25,7 +25,13 @@
 
 EventLoop::EventLoop(ThreadId _thread)
 	:SocketMonitor(*this),
-	 quit(false), dead(false),
+	 /* if this instance is hosted by an EventThread (no ThreadId
+	    known yet) then we're not yet alive until the thread is
+	    started; for the main EventLoop instance, we assume it's
+	    already alive, because nobody but EventThread will call
+	    SetAlive() */
+	 alive(!_thread.IsNull()),
+	 quit(false),
 	 thread(_thread)
 {
 	SocketMonitor::Open(SocketDescriptor(wake_fd.Get()));
@@ -143,12 +149,11 @@ EventLoop::Run() noexcept
 
 	assert(IsInside());
 	assert(!quit);
-	assert(!dead);
+	assert(alive);
 	assert(busy);
 
 	SocketMonitor::Schedule(SocketMonitor::READ);
 	AtScopeExit(this) {
-		dead = true;
 		SocketMonitor::Cancel();
 	};
 
@@ -215,7 +220,6 @@ EventLoop::Run() noexcept
 	} while (!quit);
 
 #ifndef NDEBUG
-	assert(!dead);
 	assert(busy);
 	assert(thread.IsInside());
 #endif

src/event/Loop.hxx

@@ -85,12 +85,15 @@ class EventLoop final : SocketMonitor
 
 	std::chrono::steady_clock::time_point now = std::chrono::steady_clock::now();
 
-	std::atomic_bool quit;
-
 	/**
-	 * If this is true, then Run() has returned.
+	 * Is this #EventLoop alive, i.e. can events be scheduled?
+	 * This is used by BlockingCall() to determine whether
+	 * schedule in the #EventThread or to call directly (if
+	 * there's no #EventThread yet/anymore).
 	 */
-	std::atomic_bool dead;
+	bool alive;
+
+	std::atomic_bool quit;
 
 	/**
 	 * True when the object has been modified and another check is
@@ -207,9 +210,12 @@ private:
 	bool OnSocketReady(unsigned flags) noexcept override;
 
 public:
-	gcc_pure
-	bool IsDead() const noexcept {
-		return dead;
+	void SetAlive(bool _alive) noexcept {
+		alive = _alive;
+	}
+
+	bool IsAlive() const noexcept {
+		return alive;
 	}
 
 	/**

src/event/Thread.cxx

@@ -26,8 +26,11 @@
 void
 EventThread::Start()
 {
+	assert(!event_loop.IsAlive());
 	assert(!thread.IsDefined());
 
+	event_loop.SetAlive(true);
+
 	thread.Start();
 }
 
@@ -35,6 +38,9 @@ void
 EventThread::Stop() noexcept
 {
 	if (thread.IsDefined()) {
+		assert(event_loop.IsAlive());
+		event_loop.SetAlive(false);
+
 		event_loop.Break();
 		thread.Join();
 	}