NIFI-15519 - Skip flow save during shutdown to preserve processor auto-resume states

[pvillard31]

Summary

NIFI-15519 - Skip flow save during shutdown to preserve processor auto-resume states

OK, thanks to the reporter and the data shared with me, I was able to track down the issue. At a high level, during graceful shutdown, SaveReportingTask can execute after FlowController.shutdown() has already set all processors' desired state to STOPPED.

If this happens, VersionedDataflowMapper maps the STOPPED state to ENABLED in flow.json.gz. On the next startup, VersionedFlowSynchronizer reads ENABLED (not RUNNING) and does not add the processors to startConnectablesAfterInitialization, so onFlowInitialized() starts zero components.

This is a race condition: the SaveReportingTask runs on a scheduled executor every 500ms and also runs explicitly at the end of StandardFlowService.stop(). If a save request (saveHolder) is pending when shutdown begins, the save executes after processors are stopped, overwriting the previously saved RUNNING states with ENABLED.

The issue might happen more frequently when using flow registry versioning which triggers frequent background saveFlowChanges() calls, making it highly likely that a save is pending at shutdown time.

The system test easily reproduces the issue and the test would fail without the fix.

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[exceptionfactory]

Thanks for tracking this down @pvillard31, the adjustments to the save process on shutdown make sense, and the additional test is helpful. +1 merging