PerChatUpdateProcessor awaited the predecessor's tail future directly, so
cancelling a waiting same-chat update also cancelled that future (gate
BR3 r1 2c + quality, Medium). The running predecessor then raised
InvalidStateError from done.set_result() after its handler succeeded, and
the next waiter started while the predecessor still ran (probe order:
start A, start C). Shield the await and, for a cancelled waiter, release
its turn only once its predecessor finishes.
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