[libcamera-devel] [PATCH 02/19] libcamera: bound_method: Avoid deadlock with ConnectionTypeBlocking

[Laurent Pinchart]

ConnectionTypeBlocking always invokes the method through inter-thread
message passing, which results in deadlocks if the sender and receiver
live in the same thread. The deadlock can easily be avoided by turning
the invocation into a direct call in this case. Do so to make
ConnectionTypeBlocking easier to use when some of the senders live in
the same thread as the receiver while the other senders don't.

Extend the object-invoke test to cover this usage.

While at it reformat the documentation to avoid long \brief lines.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
---
 src/libcamera/bound_method.cpp | 22 ++++++++++++++--------
 test/object-invoke.cpp         | 20 ++++++++++++++++++++
 2 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/src/libcamera/bound_method.cpp b/src/libcamera/bound_method.cpp
index e18c2eb4c68e..9aa59dc3678f 100644
--- a/src/libcamera/bound_method.cpp
+++ b/src/libcamera/bound_method.cpp
@@ -35,16 +35,19 @@ namespace libcamera {
  * thread.
  *
  * \var ConnectionType::ConnectionTypeQueued
- * \brief The receiver is invoked asynchronously in its thread when control
- * returns to the thread's event loop. The sender proceeds without waiting for
- * the invocation to complete.
+ * \brief The receiver is invoked asynchronously
+ *
+ * Invoke the receiver asynchronously in its thread when control returns to the
+ * thread's event loop. The sender proceeds without waiting for the invocation
+ * to complete.
  *
  * \var ConnectionType::ConnectionTypeBlocking
- * \brief The receiver is invoked asynchronously in its thread when control
- * returns to the thread's event loop. The sender blocks until the receiver
- * signals the completion of the invocation. This connection type shall not be
- * used when the sender and receiver live in the same thread, otherwise
- * deadlock will occur.
+ * \brief The receiver is invoked synchronously
+ *
+ * If the sender and the receiver live in the same thread, this is equivalent to
+ * ConnectionTypeDirect. Otherwise, the receiver is invoked asynchronously in
+ * its thread when control returns to the thread's event loop. The sender
+ * blocks until the receiver signals the completion of the invocation.
  */
 
 /**
@@ -71,6 +74,9 @@ bool BoundMethodBase::activatePack(std::shared_ptr<BoundMethodPackBase> pack,
 			type = ConnectionTypeDirect;
 		else
 			type = ConnectionTypeQueued;
+	} else if (type == ConnectionTypeBlocking) {
+		if (Thread::current() == object_->thread())
+			type = ConnectionTypeDirect;
 	}
 
 	switch (type) {
diff --git a/test/object-invoke.cpp b/test/object-invoke.cpp
index 8e2055ca620f..fa162c838c78 100644
--- a/test/object-invoke.cpp
+++ b/test/object-invoke.cpp
@@ -100,6 +100,26 @@ protected:
 			return TestFail;
 		}
 
+		/*
+		 * Test that blocking invocation is delivered directly when the
+		 * caller and callee live in the same thread.
+		 */
+		object_.reset();
+
+		object_.invokeMethod(&InvokedObject::method,
+				     ConnectionTypeBlocking, 42);
+
+		switch (object_.status()) {
+		case InvokedObject::NoCall:
+			cout << "Method not invoked for main thread (blocking)" << endl;
+			return TestFail;
+		case InvokedObject::InvalidThread:
+			cout << "Method invoked in incorrect thread for main thread (blocking)" << endl;
+			return TestFail;
+		default:
+			break;
+		}
+
 		/*
 		 * Move the object to a thread and verify that auto method
 		 * invocation is delivered in the correct thread.
-- 
Regards,

Laurent Pinchart