Fix ThreadSanitizer data races between bundle processing and async background writer closing in FileBasedSink and WriteFiles

[stankiewicz]

Fixes ThreadSanitizer data race errors reported during asynchronous writer closing in WriteFiles and FileBasedSink.

Unsynchronized Cross-Thread Lifecycle Access:
In WriteFiles.WriteShardsIntoTempFilesFn, a FileBasedSink.Writer is instantiated, opened (writer.open()), and written to on the bundle processing thread. Once bundle processing completes, closeWriterInBackground(writer) submits asynchronous background tasks (MoreFutures.runAsync) to close each writer on a ForkJoinPool thread. Because plain Java fields (id, destination, outputFile, channel) lack release-acquire barriers across threads, ThreadSanitizer flags reads inside writer.close() as racing with writes inside writer.open().

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[stankiewicz]

R: @sjvanrossum

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[sjvanrossum]

If I understand correctly, open(String) initializes all relevant fields which are thereafter never written to again, correct?
Calling open(String) concurrently looks unsafe, so ideally it should be marked as synchronized along with close() to ensure mutually exclusive initialization after construction and to ensure visibility of those writes.

Alternatively, attempt a CAS on id before writing any of the other fields for exclusive initialization after construction (note that visibility of the other fields is not yet guaranteed) followed by CAS on either resourceId or channel to establish a release barrier. That same field must be acquired (e.g., volatile get or getAcquire) before reading any other field in close().