apps: common: event_loop: Use single event source for deferred calls

Instead of calling `event_base_once()` every time a deferred call
is added to the loop, create an event source at construction, and
simply trigger that when a new deferred call is scheduled.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

src/apps/common/event_loop.cpp

@@ -21,12 +21,35 @@ EventLoop::EventLoop()
 	evthread_use_pthreads();
 	base_ = event_base_new();
 	instance_ = this;
+
+	callsTrigger_ = event_new(base_, -1, EV_PERSIST, [](evutil_socket_t, short, void *closure) {
+		auto *self = static_cast<EventLoop *>(closure);
+
+		for (;;) {
+			std::function<void()> call;
+
+			{
+				std::lock_guard locker(self->lock_);
+				if (self->calls_.empty())
+					break;
+
+				call = std::move(self->calls_.front());
+				self->calls_.pop_front();
+			}
+
+			call();
+		}
+	}, this);
+	assert(callsTrigger_);
+	event_add(callsTrigger_, nullptr);
 }
 
 EventLoop::~EventLoop()
 {
 	instance_ = nullptr;
 
+	event_free(callsTrigger_);
+
 	events_.clear();
 	event_base_free(base_);
 	libevent_global_shutdown();
@@ -57,7 +80,7 @@ void EventLoop::callLater(std::function<void()> &&func)
 		calls_.push_back(std::move(func));
 	}
 
-	event_base_once(base_, -1, EV_TIMEOUT, dispatchCallback, this, nullptr);
+	event_active(callsTrigger_, 0, 0);
 }
 
 void EventLoop::addFdEvent(int fd, EventType type,
@@ -108,29 +131,6 @@ void EventLoop::addTimerEvent(const std::chrono::microseconds period,
 	events_.push_back(std::move(event));
 }
 
-void EventLoop::dispatchCallback([[maybe_unused]] evutil_socket_t fd,
-				 [[maybe_unused]] short flags, void *param)
-{
-	EventLoop *loop = static_cast<EventLoop *>(param);
-	loop->dispatchCall();
-}
-
-void EventLoop::dispatchCall()
-{
-	std::function<void()> call;
-
-	{
-		std::unique_lock<std::mutex> locker(lock_);
-		if (calls_.empty())
-			return;
-
-		call = calls_.front();
-		calls_.pop_front();
-	}
-
-	call();
-}
-
 EventLoop::Event::Event(std::function<void()> &&callback)
 	: callback_(std::move(callback)), event_(nullptr)
 {

src/apps/common/event_loop.h

@@ -65,11 +65,8 @@ private:
 	int exitCode_;
 
 	std::deque<std::function<void()>> calls_;
+	struct event *callsTrigger_ = nullptr;
 
 	std::list<std::unique_ptr<Event>> events_;
 	std::mutex lock_;
-
-	static void dispatchCallback(evutil_socket_t fd, short flags,
-				     void *param);
-	void dispatchCall();
 };