[zigpy-review-bot]

Summary

Multicast.subscribe() only records a group in its cache after the firmware write is awaited, but it claims a free table slot before that await. Two concurrent calls for the same group both miss the cache, each pop a slot and write it, and the second cache write overwrites the first slot reference. One multicast table slot is leaked per race, and the table is small (bellows asks for 16).

This became reachable with zigpy's subscribe_to_multicast_group() (zigpy/zigpy#1864) and the quirk-side API in zigpy/zha-device-handlers#5209: ZHA initializes devices concurrently, so several devices sharing a quirk (e.g. two BILRESA remotes) request the same group at the same time.

Changes

• Guard Multicast.subscribe() and Multicast.unsubscribe() with one asyncio.Lock, so check, slot claim, firmware write and cache update are atomic with respect to each other. The already-subscribed early return then handles the second caller.
• startup() is unchanged and does not hold the lock, so there is no re-entrancy.

Testing

• Added test_subscribe_concurrent_same_group (two concurrent subscribes use one slot and unsubscribing frees it) and test_subscribe_unsubscribe_serialized. Both fail without the lock.
• Full suite: 766 passed. pre-commit clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (f747e95) to head (0215209).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #771   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files          64       64           
  Lines        4409     4417    +8     
=======================================
+ Hits         4391     4399    +8     
  Misses         18       18           

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