benwtrent commented on issue #13127:
URL: https://github.com/apache/lucene/issues/13127#issuecomment-2264272195
OK, I added a bunch of logging and it seems like the issue is around
`DWPTP#getAndLock`.
I can see the following occurring, new DWPTs being created, each with the
first generation deleteQueue.
```
1> new Writer: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_0,
aborted=false, numDocsInRAM=0, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t0 20 getNextSequenceNumber 1
called from stack:...
1> new Writer: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_1,
aborted=false, numDocsInRAM=0, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
1> new Writer: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_5,
aborted=false, numDocsInRAM=0, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
1> new Writer: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_2,
aborted=false, numDocsInRAM=0, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
1> new Writer: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_6,
aborted=false, numDocsInRAM=0, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
1> new Writer: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_3,
aborted=false, numDocsInRAM=0, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
1> new Writer: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_4,
aborted=false, numDocsInRAM=0, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
```
Then reset of the deleteQueue occurs to bump its generation, and correctly,
it sees 7 pending actions, and a base seqNo of 1. All ending up with a maxSeqNo
of 9.
```
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t020 advanceQueue called
from stack with maxSeq 9 lastSeqNo: 1 maxNumPendingOps: 7:
```
Now, we get some more `getNextSequenceNumber` calls from all the other
threads that were waiting. Each called on the zeroth generation:
```
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t525 getNextSequenceNumber 2
called from stack:
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t828 getNextSequenceNumber 3
called from stack:
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t727 getNextSequenceNumber 4
called from stack:
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t626 getNextSequenceNumber 5
called from stack:
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t424 getNextSequenceNumber 6
called from stack:
1> DWDQ: [ generation: 0 ] id: 245403753 tid: t323 getNextSequenceNumber 7
called from stack:
```
However, then `getAndLock` is called again, and returns one of those
previously created threads. Meaning, they haven't been removed yet via flush
and consequently still reference the generation 0 deleteQueue
```
1> getAndLock: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_0,
aborted=false, numDocsInRAM=1, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
1> getAndLock: DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_3,
aborted=false, numDocsInRAM=1, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds]
```
Both of these increment `getNextSequenceNumber` and consequently push us
over the maxSeqNo and this trips the assertion.
Later, we see flush finishing and the threads being removed:
```
1> DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_2,
aborted=false, numDocsInRAM=1, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds] checkout to remove
1> DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_3,
aborted=false, numDocsInRAM=2, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds] checkout to remove
1> DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_6,
aborted=false, numDocsInRAM=1, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds] checkout to remove
1> DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_4,
aborted=false, numDocsInRAM=1, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds] checkout to remove
1> DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_5,
aborted=false, numDocsInRAM=1, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds] checkout to remove
1> DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_1,
aborted=false, numDocsInRAM=1, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds] checkout to remove
1> DocumentsWriterPerThread [pendingDeletes=gen=0, segment=_0,
aborted=false, numDocsInRAM=2, deleteQueue=DWDQ: [ generation: 0 ], 0 deleted
docIds] checkout to remove
```
Here is a branch with my debug logging & this issues replication:
https://github.com/apache/lucene/compare/main...benwtrent:lucene:test-replicate-and-debug-13127?expand=1
I honestly don't know how big a deal it is for a previous deleteQueue
generation returning seqNo from the next generation. But this is definitely a
tricky concurrency issue.
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