internal/compact: DeletableValueMerger needDelete path corrupts iterator state #6168

Summary

When a merger implementing base.DeletableValueMerger returns delete=true from DeletableFinish during a compaction, the compaction iterator's needDelete handling elides the merge result via continue without performing the skip/position bookkeeping that the normal returned-key path performs. Depending on arrangement this causes either:

• Silent data corruption: the just-consumed base SET (and any older same-key entries the pending skip was supposed to elide) are re-processed as fresh keys, resurrecting a value for a key the merger declared non-existent; or
• A panic: compaction iterator has skip=true, but iterator is at iterPosNext on a subsequent Iter.Next() call.

Reproduced on v2.1.4, v2.1.5, v2.1.6; the relevant code is unchanged on master.

Jira issue: PEBBLE-1469