providers-fab: Handle database errors in cleanup_session_middleware Session.remove()

[8silvergun]

Summary

Fix unhandled MySQLdb.OperationalError: (2006, 'Server has gone away') in cleanup_session_middleware when Session.remove() encounters a dead database connection.

Closes: #62335

Problem

PR #61480 introduced cleanup_session_middleware in providers-fab 3.3.0 to fix PendingRollbackError (#59349). The middleware calls Session.remove() in a bare finally block:

finally:
    from airflow import settings
    if settings.Session:
        settings.Session.remove()  # can raise if DB connection is dead

When the underlying database connection has been closed server-side (MySQL timeout, Aurora failover, network interruption), Session.remove() internally attempts a ROLLBACK on the dead connection, raising OperationalError. This unhandled exception propagates as a 500 Internal Server Error — even though the original request completed successfully.

Production error log:

[error] Exception in ASGI application [airflow.providers.fab.auth_manager.fab_auth_manager] loc=fab_auth_manager.py:243
  File ".../fab_auth_manager.py", line 243, in cleanup_session_middleware
    settings.Session.remove()
  ...
MySQLdb.OperationalError: (2006, 'Server has gone away')

Solution

Wrap Session.remove() in a try-except that catches and logs the error as a warning:

finally:
    from airflow import settings
    if settings.Session:
        try:
            settings.Session.remove()
        except Exception:
            log.warning("Failed to remove session during cleanup", exc_info=True)

This is consistent with session cleanup patterns elsewhere in Airflow (e.g., airflow/utils/session.py).

Why this is safe

• Session.remove() is a cleanup operation — if it fails because the connection is already dead, the session will be discarded anyway on the next request
• The warning log preserves visibility for debugging
• The except Exception is intentionally broad since any error during cleanup should not affect the HTTP response

Testing

Added TestFabAuthManagerSessionCleanupErrorHandling with 2 tests:

• test_session_remove_db_error_does_not_propagate: Verifies OperationalError (MySQL 'Server has gone away') is caught
• test_session_remove_generic_error_does_not_propagate: Verifies other exceptions (e.g., RuntimeError) are also caught

Related

• providers-fab: Airflow 3.1.3 sqlalchemy.exc.PendingRollbackError: Can't reconnect until invalid transaction is rolled back #59349 — Original PendingRollbackError issue
• Add session cleanup middleware to FAB FastAPI app #61480 — PR that introduced cleanup_session_middleware (this PR fixes a gap in that implementation)
• sqlalchemy.exc.PendingRollbackError: Can't reconnect until invalid transaction is rolled back #57470, Airflow Web Interface Crashes with 500 Error (FAB Auth, MySQL Timeout) #57859 — Earlier reports of the session lifecycle problem

AI Disclosure

This PR was developed with AI assistance.

[boring-cyborg]

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[vincbeck]

Static checks are failing, running prek should solve them

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.