[libcamera-devel] [PATCH 3/3] base: thread: Fix recursive calls to dispatchMessages()

[Laurent Pinchart]

There are use cases for calling the dispatchMessages() function
recursively, from within a message handler. This can be used, for
instance, to force delivery of messages posted to a thread concurrently
to stopping the thread. This currently causes access, in the outer
dispatchMessages() call, to iterators that have been invalidated by
erasing list elements in the recursive call, leading to undefined
behaviour (most likely double-free or other crashes).

Fix it by only erasing messages from the list at the end of the outer
call, identified using a recursion counter. We need to keep track of the
first non-null entry in the posted messages list to restrict the
deletion to the

Bug: https://bugs.libcamera.org/show_bug.cgi?id=26
Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 src/libcamera/base/thread.cpp | 43 +++++++++++++++++++++++++++--------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/src/libcamera/base/thread.cpp b/src/libcamera/base/thread.cpp
index 7f79115222e8..9eb488d46db8 100644
--- a/src/libcamera/base/thread.cpp
+++ b/src/libcamera/base/thread.cpp
@@ -126,6 +126,11 @@ public:
 	 * \brief Protects the \ref list_
 	 */
 	Mutex mutex_;
+	/**
+	 * \brief The recursion level for recursive Thread::dispatchMessages()
+	 * calls
+	 */
+	unsigned int recursion_ = 0;
 };
 
 /**
@@ -595,30 +600,34 @@ void Thread::removeMessages(Object *receiver)
  * Messages shall only be dispatched from the current thread, typically within
  * the thread from the run() function. Calling this function outside of the
  * thread results in undefined behaviour.
+ *
+ * This function is not thread-safe, but it may be called recursively in the
+ * same thread from an object's message handler. It guarantees in all cases
+ * delivery of messages in the order they have been posted.
  */
 void Thread::dispatchMessages(Message::Type type)
 {
 	ASSERT(data_ == ThreadData::current());
 
+	++data_->messages_.recursion_;
+
 	MutexLocker locker(data_->messages_.mutex_);
 
 	std::list<std::unique_ptr<Message>> &messages = data_->messages_.list_;
 
-	for (auto iter = messages.begin(); iter != messages.end(); ) {
-		std::unique_ptr<Message> &msg = *iter;
-
-		if (!msg) {
-			iter = data_->messages_.list_.erase(iter);
+	for (std::unique_ptr<Message> &msg : messages) {
+		if (!msg)
 			continue;
-		}
 
-		if (type != Message::Type::None && msg->type() != type) {
-			++iter;
+		if (type != Message::Type::None && msg->type() != type)
 			continue;
-		}
 
+		/*
+		 * Move the message, setting the entry in the list to null. It
+		 * will cause recursive calls to ignore the entry, and the erase
+		 * loop at the end of the function to delete it from the list.
+		 */
 		std::unique_ptr<Message> message = std::move(msg);
-		iter = data_->messages_.list_.erase(iter);
 
 		Object *receiver = message->receiver_;
 		ASSERT(data_ == receiver->thread()->data_);
@@ -629,6 +638,20 @@ void Thread::dispatchMessages(Message::Type type)
 		message.reset();
 		locker.lock();
 	}
+
+	/*
+	 * If the recursion level is 0, erase all null messages in the list. We
+	 * can't do so during recursion, as it would invalidate the iterator of
+	 * the outer calls.
+	 */
+	if (!--data_->messages_.recursion_) {
+		for (auto iter = messages.begin(); iter != messages.end(); ) {
+			if (!*iter)
+				iter = messages.erase(iter);
+			else
+				++iter;
+		}
+	}
 }
 
 /**
-- 
Regards,

Laurent Pinchart