[BACKPORT] uORB: fix several race conditions during topic initialization #13589
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_generation is read in a multi-threaded context w/o locking.
No semantic change (yet)
Possible race conditions (they all happen between the check of existence of a topic and trying to create the node): - single instance, with multiple advertisers during the first advertise: both advertisers see the topic as non-existent and try to advertise it. One of them will fail, leading to an error message. This is the cause for telemetry_status advert failure seen in SITL in rare cases. - multi-instance: subscription to non-existing instance -> px4_open fails, and the subscriber tries to create the node. If during that time a publisher publishes that instance, the subscriber will get (instance+1) (or fails if the max number of instances is exceeded). This is a race that goes pretty much unnoticed. - multi-instance: 2 publishers can get the same instance (if is_published() is false in case both have not published data yet). This can also go unnoticed. Therefore the patch changes where _advertised is set: it is now set directly during the advertisement instead of during publication.
Exclusive open is not required, but we now need to ensure the queue size is set atomically. It avoids a race condition between 2 single-instance advertisers, where one of them would fail to open the node with -EBUSY.
julianoes
approved these changes
Nov 25, 2019
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Backport of #13560 to the 1.10 release branch.