[I] [client] SortMergeReader stops after deleted snapshot rows [fluss]

["luoyuxia (via GitHub)" <gi...@apache.org>]

luoyuxia opened a new issue, #3134:
URL: https://github.com/apache/fluss/issues/3134

   ### Describe the bug
   `SortMergeReader.SnapshotMergedRowIteratorWrapper#hasNext()` can terminate iteration early when a snapshot row is deleted by the changelog.
   
   When `sortMergeWithChangeLog()` returns `SortMergeRows.EMPTY`, `hasNext()` leaves `currentMergedRows` as `null` and immediately returns `false`. The caller then treats the snapshot iterator as exhausted, so all remaining snapshot rows are skipped.
   
   ### To Reproduce
   A minimal reproduction is:
   1. Build a snapshot iterator with ordered primary keys, for example `0..9`.
   2. Build a changelog iterator that contains `DELETE` records for keys that exist in the snapshot, for example `2`, `5`, and `8`.
   3. Create `SortMergeReader.readBatch()` with the snapshot iterator and the changelog iterator.
   4. Iterate through the returned reader.
   
   Expected keys: `0,1,3,4,6,7,9`
   
   Actual behavior before the fix: iteration stops at the first deleted snapshot key and later snapshot rows are never returned.
   
   ### Expected behavior
   Deleted snapshot rows should be skipped, but iteration should continue until the snapshot iterator is actually exhausted.
   
   ### Root cause
   The bug is in `SnapshotMergedRowIteratorWrapper#hasNext()` in `fluss-client/src/main/java/org/apache/fluss/client/table/scanner/SortMergeReader.java`.
   
   The method only tries to fetch one `SortMergeRows` result. If that result is empty, it does not advance to the next snapshot row and returns `false` immediately.
   
   ### Proposed fix
   Change `hasNext()` to keep advancing until it either:
   - finds a non-empty merged result and returns the next row; or
   - truly exhausts the snapshot iterator.
   
   This means empty merge results produced by changelog deletes are skipped instead of terminating the whole scan.
   
   ### Additional context
   I prepared a companion fix and regression test in `SortMergeReaderTest` that covers the delete scenario.
   


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Re: [I] [client] SortMergeReader stops after deleted snapshot rows [fluss]

["luoyuxia (via GitHub)" <gi...@apache.org>]

luoyuxia closed issue #3134: [client] SortMergeReader stops after deleted snapshot rows
URL: https://github.com/apache/fluss/issues/3134


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