Fix ABBA deadlock between waitForTransform and testTransformableRequests (backport #982)

[mergify]

Description

As mentioned in #979, an ABBA deadlock situation was introduced in #966 where testTransformableRequests() locks transformable_requests_mutex_ and tries to run cb which requires timer_to_request_map_mutex_ while parallely waitForTransform acquires timer_to_request_map_mutex_ and tries to call addTransformableRequest which internally needs transformable_requests_mutex_ resulting in ABBA situation.

My apologies for not noticing this issue previously and a special thanks to @jplapp and @mini-1235 for bringing it to my attention.

@mini-1235 has already opened #979 to fix this issue. I agree with the approach in that PR (to unlock mutex before running callbacks and lock again) but I think restarting the i and going through the beginning of the requests for every callback would add unnecessary computation time. More precisely, the cost is that if M callbacks fire per setTransform and N requests are pending, you get O(M·N) work instead of O(N).

I tried to improve that by going through the requests first, extracting the pending callbacks and then running them. That way we keep the swap-and-pop behavior as before.

I have also copied the test @jplapp added in #980 here as well. @mini-1235 could you please also check this in your nav2 setup where this problem became highlighted?

Credits to @jplap and @mini-1235 for their idea and suggested changes.

Did you use Generative AI?

Yes, Claude Sonned 4.6

---

This is an automatic backport of pull request #982 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   tf2_ros/test/test_buffer.cpp

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	both modified:   tf2/src/buffer_core.cpp

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[ahcorde]

Pulls: #990
Gist: https://gist.githubusercontent.com/ahcorde/af8f401662d2acc02bbdf26b78cba4f6/raw/bfc68c1d9d564382c81b89268cfba95dbee4f99c/ros2.repos
BUILD args: --packages-above-and-dependencies tf2 tf2_ros
TEST args: --packages-above tf2 tf2_ros
ROS Distro: humble
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/20365

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[ahcorde]

Pulls: #990
Gist: https://gist.githubusercontent.com/ahcorde/80dbedfa5cdb8d4dde67b2a38fca3a54/raw/bfc68c1d9d564382c81b89268cfba95dbee4f99c/ros2.repos
BUILD args: --packages-above-and-dependencies tf2 tf2_ros
TEST args: --packages-above tf2 tf2_ros
ROS Distro: humble
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/20398

• Linux
• Linux-aarch64
• Linux-rhel
• Windows