[SPARK-56873][CORE] Fix race condition in bounded k-way merge in UnsafeExternalSorter#55891
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[SPARK-56873][CORE] Fix race condition in bounded k-way merge in UnsafeExternalSorter#55891ivoson wants to merge 1 commit into
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Pre-rollout hardening of the bounded multi-round k-way merge added in SPARK-56410. Feature remains gated on `spark.unsafe.sorter.spill.merge.factor` (default -1, disabled), so no production behavior change today. Bug: getSortedIterator() took the merge-input snapshot inside UnsafeSorterBoundedSpillMerger.merge() AFTER publishing readingIterator (SpillableIterator). Once readingIterator is non-null, an external consumer's spill() (e.g. a Photon native consumer requesting memory from a non-task thread) is routed to readingIterator.spill(), which both appends a new writer to the live spillWriters list AND rebinds readingIterator.upstream to that new file. A race between publication and snapshot causes the new file to be fed into the final merge twice: once via the snapshot, once via readingIterator. Result: silent duplicate records in sorted output. Fix: lift snapshot + boundedMerger construction + readingIterator publication into a named seam, prepareBoundedMerge(), that takes the defensive snapshot strictly BEFORE publishing readingIterator. The synchronized(this) around boundedMerger assignment matches the existing read under synchronized(this) in cleanupResources(); the volatile modifier already covered visibility, the lock is a consistency win for future readers. Adds a deterministic regression test, testBoundedMergeSnapshotIsolatedFromConcurrentSpill, that drives the race scenario by direct sequencing via the new seam: prepare -> external-trigger spill -> merge, asserting (a) the snapshot is isolated from live spillWriters mutation and (b) every record appears exactly once in the merged output. Pre-fix code fails this test deterministically. Co-authored-by: Isaac
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