#2705 fixed simultaneous captures colliding on the built-in camera path,
keyed by printer IP through capture_camera_frame_bytes(). External
cameras reach the same collision through a different function -
external_camera.capture_frame() - that #2705 didn't touch, and a V4L2
USB device allows exactly one open handle just like Bambu's own RTSP
limit.
Nothing coalesced two one-shot capturers here either: Obico polling,
the in-print frame bank, the finish-photo moment, plate detection and
the notification snapshot could each open their own connection to the
same USB camera and collide - is_stream_active() only stops a
capturer from competing with an attached viewer, not with another
capturer (that's what #2707 fixed).
capture_frame() is now a single-flight coalescing wrapper (actual
dispatch moved to _capture_frame_uncoalesced), keyed by (url,
camera_type, snapshot_url) - snapshot_url is part of the key since
#1177's override routes to a completely different endpoint. Mirrors
#2705's shape: coalesces, doesn't cache (a call after the previous one
finishes always captures fresh); each caller keeps its own timeout via
wait_for(shield(...)) rather than inheriting the leader's; a follower
whose leader fails takes its own turn instead of inheriting a failure
it never had a chance to avoid, bounded at two rounds; cancellation is
disambiguated via leader.cancelled() so a follower's own cancellation
still propagates while a cancelled leader is treated as a failed one.
12 tests mirroring test_camera_capture_coalescing.py's structure.
maziggy
c9f99b35d1