[libcamera-devel] [PATCH v2] cam: event_loop: Stop processing calls when event loop is exiting

[Niklas Söderlund]

Terminating the event loop with EventLoop::exit() does not grantee that
it will terminate. If the event loops 'call later' queue can be kept
non-empty by continuously queuing new calls using EventLoop::callLater()
either from a different thread or from callbacks in the loop itself
EventLoop::dispatchCalls() will never complete and the loop will run
forever.

Solve this by evaluating the exit condition between each callback and
only enter the idle loop if there are no callbacks to process.

Reported-by: Sebastian Fricke <sebastian.fricke at posteo.net>
Fixes: f49e93338b6309a6 ("cam: event_loop: Add deferred calls support")
Signed-off-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
---
* Changes since v1
- Was [PATCH 0/2] cam: Fix races in event loop and long request
  processing times
- Rework dispatchCalls() instead of callLater() for a simpler fix.
---
 src/cam/event_loop.cpp | 22 +++++++++++++---------
 src/cam/event_loop.h   |  2 +-
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/src/cam/event_loop.cpp b/src/cam/event_loop.cpp
index 94c5d1d362455f33..18786b4500be9b6f 100644
--- a/src/cam/event_loop.cpp
+++ b/src/cam/event_loop.cpp
@@ -41,7 +41,9 @@ int EventLoop::exec()
 	exit_.store(false, std::memory_order_release);
 
 	while (!exit_.load(std::memory_order_acquire)) {
-		dispatchCalls();
+		if (dispatchCall())
+			continue;
+
 		event_base_loop(event_, EVLOOP_NO_EXIT_ON_EMPTY);
 	}
 
@@ -70,17 +72,19 @@ void EventLoop::callLater(const std::function<void()> &func)
 	interrupt();
 }
 
-void EventLoop::dispatchCalls()
+bool EventLoop::dispatchCall()
 {
 	std::unique_lock<std::mutex> locker(lock_);
 
-	for (auto iter = calls_.begin(); iter != calls_.end(); ) {
-		std::function<void()> call = std::move(*iter);
+	if (calls_.empty())
+		return false;
 
-		iter = calls_.erase(iter);
+	auto iter = calls_.begin();
+	std::function<void()> call = std::move(*iter);
+	calls_.erase(iter);
 
-		locker.unlock();
-		call();
-		locker.lock();
-	}
+	locker.unlock();
+	call();
+
+	return true;
 }
diff --git a/src/cam/event_loop.h b/src/cam/event_loop.h
index 408073c50594d09d..b84e1c9e432086e3 100644
--- a/src/cam/event_loop.h
+++ b/src/cam/event_loop.h
@@ -38,7 +38,7 @@ private:
 	std::mutex lock_;
 
 	void interrupt();
-	void dispatchCalls();
+	bool dispatchCall();
 };
 
 #endif /* __CAM_EVENT_LOOP_H__ */
-- 
2.30.0