[2.7] Fix RxTask self-deadlock on stream error cleanup

[pcnudde]

Summary

• avoid calling RxTask.stop() while holding RxTask.map_lock in find_or_create_task
• stop the existing task only after leaving the map lock to prevent self-deadlock
• add a regression unit test for the existing-task error path

[greptile-apps]

Greptile Summary

Fixes a critical self-deadlock bug in RxTask.find_or_create_task() where calling task.stop() while holding map_lock caused the thread to deadlock when stop() tried to re-acquire the same lock.

Changes:

• Defers task.stop() call until after releasing map_lock by storing task reference and calling it outside the critical section
• Adds regression test with custom _DeadlockDetectingLock that raises on same-thread re-acquire
• Verifies fix by confirming stop() is called with map_lock unlocked

Impact:
This prevents production hangs when stream errors occur on existing tasks, particularly in the error cleanup path for duplicate or failed streams.

Confidence Score: 5/5

• Safe to merge - clean deadlock fix with excellent test coverage
• The fix is minimal, correct, and follows proper locking discipline. Tests explicitly verify the deadlock scenario and confirm the fix works.
• No files require special attention

Important Files Changed

Filename	Overview
nvflare/fuel/f3/streaming/byte_receiver.py	Correctly fixes self-deadlock by deferring stop() call until after releasing map_lock
tests/unit_test/fuel/f3/streaming/byte_receiver_test.py	Comprehensive regression tests validating deadlock fix with custom lock detector

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[find_or_create_task called] --> B[Acquire map_lock]
    B --> C{Task exists?}
    C -->|No| D{Error message?}
    D -->|Yes| E[Return None]
    D -->|No| F[Create new RxTask<br/>Add to map]
    C -->|Yes| G{Error message?}
    G -->|No| H[Release map_lock<br/>Return task]
    G -->|Yes| I[Store task_to_stop]
    I --> J[Release map_lock]
    J --> K[Call task_to_stop.stop]
    K --> L[stop acquires map_lock<br/>Removes from map<br/>✓ No deadlock]
    L --> M[Return None]
    F --> H
    E --> N[End]
    H --> N
    M --> N
    
    style K fill:#90EE90
    style L fill:#90EE90
    style I fill:#90EE90

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_{Last reviewed commit: 635575e}

[greptile-apps]

_{2 files reviewed, no comments}

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[pcnudde]

/build

[chesterxgchen]

suggest update the tests

[greptile-apps]

_{2 files reviewed, no comments}

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[pcnudde]

/build

[greptile-apps]

_{2 files reviewed, no comments}

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[chesterxgchen]

/build