event/Loop: try to avoid the WakeFD when adding DeferredMonitor

Add a flag that indicates whether the EventLoop is currently "busy".
As long as that flag is set, it does not need to be woken up - we can
simply add the DeferredMonitor to the list, and it will be caught by
EventLoop very soon.  This eliminates nearly all of the
DeferredMonitor overhead when compared to IdleMonitor, rendering
IdleMonitor mostly obsolete.

src/event/Loop.cxx

@@ -31,7 +31,7 @@
 EventLoop::EventLoop(Default)
 	:SocketMonitor(*this),
 	 now_ms(::MonotonicClockMS()),
-	 quit(false),
+	 quit(false), busy(true),
 	 thread(ThreadId::Null())
 {
 	SocketMonitor::Open(wake_fd.Get());
@@ -122,6 +122,7 @@ EventLoop::Run()
 	thread = ThreadId::GetCurrent();
 
 	assert(!quit);
+	assert(busy);
 
 	do {
 		now_ms = ::MonotonicClockMS();
@@ -161,6 +162,13 @@ EventLoop::Run()
 				return;
 		}
 
+		/* try to handle DeferredMonitors without WakeFD
+		   overhead */
+		mutex.lock();
+		HandleDeferred();
+		busy = false;
+		mutex.unlock();
+
 		if (again)
 			/* re-evaluate timers because one of the
 			   IdleMonitors may have added a new
@@ -173,6 +181,10 @@ EventLoop::Run()
 
 		now_ms = ::MonotonicClockMS();
 
+		mutex.lock();
+		busy = true;
+		mutex.unlock();
+
 		/* invoke sockets */
 		for (int i = 0; i < poll_result.GetSize(); ++i) {
 			auto events = poll_result.GetEvents(i);
@@ -190,6 +202,7 @@ EventLoop::Run()
 	} while (!quit);
 
 #ifndef NDEBUG
+	assert(busy);
 	assert(thread.IsInside());
 	thread = ThreadId::Null();
 #endif
@@ -209,10 +222,11 @@ EventLoop::AddDeferred(DeferredMonitor &d)
 
 	/* we don't need to wake up the EventLoop if another
 	   DeferredMonitor has already done it */
-	const bool must_wake = deferred.empty();
+	const bool must_wake = !busy && deferred.empty();
 
 	d.pending = true;
 	deferred.push_back(&d);
+	again = true;
 	mutex.unlock();
 
 	if (must_wake)

src/event/Loop.hxx

@@ -90,6 +90,14 @@ class EventLoop final : SocketMonitor
 	 */
 	bool again;
 
+	/**
+	 * True when handling callbacks, false when waiting for I/O or
+	 * timeout.
+	 *
+	 * Protected with #mutex.
+	 */
+	bool busy;
+
 	PollGroup poll_group;
 	PollResult poll_result;