[FLINK-40525][network] Remove residual receivedBuffers lock in finishReadRecoveredState to avoid ABBA deadlock

[1996fanrui]

What is the purpose of the change

RecoveredInputChannel.finishReadRecoveredState() wrapped onRecoveredStateBuffer() in
synchronized(receivedBuffers), but that method already locks receivedBuffers internally and, when the
queue was empty, calls notifyChannelNonEmpty(), which acquires inputChannelsWithData while
receivedBuffers is still held. This inverts the task thread's inputChannelsWithData -> receivedBuffers
lock order and can deadlock on the error path (ABBA). Removing the redundant outer lock restores the correct
order with no behavior change, since onRecoveredStateBuffer already provides the required mutual exclusion.

Brief change log

• [FLINK-40525] Drop the outer synchronized(receivedBuffers) in RecoveredInputChannel.finishReadRecoveredState().

Verifying this change

This change is a trivial rework / code cleanup without any test coverage. The ABBA cycle is an error-path
lock-ordering issue that cannot be reproduced deterministically in a unit test; onRecoveredStateBuffer
retains its internal locking, so behavior is otherwise unchanged.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: yes (input channel-state recovery)
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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CI report:

• a6e89f2 Azure: SUCCESS

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