Fix the issue where compaction incorrectly drops a key when there is a snapshot with a sequence number of zero.

[tcwzxx]

The compaction will incorrectly drop a key under the following conditions:

1. Open an empty database.
2. Use the IngestExternalFile API to ingest an SST file (the global sequence number will be 0).
3. Create a snapshot (the snapshot sequence number will be 0).
4. Trigger compaction; the key in the above SST file will be dropped.

The drop condition is found here:

rocksdb/db/compaction/compaction_iterator.cc

Lines 875 to 878 in f20d12a

	} else if (last_snapshot == current_user_key_snapshot_ ||
	(last_snapshot > 0 &&
	last_snapshot < current_user_key_snapshot_)) {
	// If the earliest snapshot is which this key is visible in

The condition does not explicitly check if a previous key exists.

Fix: Add a check of last_sequence != kMaxSequenceNumber to verify if there is a previous key

[cbi42]

Thanks for the fix. Can you add a release note for the bug fix under https://github.com/facebook/rocksdb/tree/main/unreleased_history?

[facebook-github-bot]

@cbi42 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@tcwzxx has updated the pull request. You must reimport the pull request before landing.

[tcwzxx]

Thanks for the fix. Can you add a release note for the bug fix under https://github.com/facebook/rocksdb/tree/main/unreleased_history?

Thanks for the reivew. The release note has been added.

[facebook-github-bot]

@cbi42 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[tcwzxx]

@cbi42 Hi, could you help me merge this PR?