dianabarsan opened a new issue, #6118:
URL: https://github.com/apache/couchdb/issues/6118
### Version
3.5.2
### Describe the problem you're encountering
We are doing quite extensive purging (through the `_purge`endpoint), we are
also using Nouveau indexes.
In one instance, Nouveau index queries started failing consistently. This is
the error we saw in CouchDb logs:
```
{{case_clause,{error,{conflict,<<"Index updates applied in the wrong order
(current seq: 7439245, match seq: 7439245, attempted seq:
7438959)">>}}},[{couch_btree,stream_kv_node2,9,[{file,"src/couch_btree.erl"},{line,1315}]},{couch_btree,stream_kp_node,8,[{file,"src/couch_btree.erl"},{line,1241}]},{couch_btree,stream_kp_node,9,[{file,"src/couch_btree.erl"},{line,1286}]},{couch_btree,fold,4,[{file,"src/couch_btree.erl"},{line,283}]},{couch_bt_engine,fold_purge_infos,5,[{file,"src/couch_bt_engine.erl"},{line,607}]},{nouveau_index_updater,purge_index,3,[{file,"src/nouveau_index_updater.erl"},{line,288}]},{nouveau_index_updater,update,1,[{file,"src/nouveau_index_updater.erl"},{line,97}]}]}
[error] 2026-09-17T18:45:51.685469Z [email protected] <0.2024.0> --------
nouveau_index_manager: db:**** ddoc:**** failed with:
{{case_clause,{error,{conflict,<<"Index updates applied in the wrong order
(current seq: 7420581, match seq: 7420581, attempted seq:
7420405)">>}}},[{couch_btree,stream_kv_node2,9,[{file,"src/couch_btree.erl"},{line,1315}]},{couch_btree,stream_kp_node,8,[{file,"src/couch_btree.erl"},{line,1241}]},{couch_btree,stream_kp_node,9,[{file,"src/couch_btree.erl"},{line,1286}]},{couch_btree,fold,4,[{file,"src/couch_btree.erl"},{line,283}]},{couch_bt_engine,fold_purge_infos,5,[{file,"src/couch_bt_engine.erl"},{line,607}]},{nouveau_index_updater,purge_index,3,[{file,"src/nouveau_index_updater.erl"},{line,288}]},{nouveau_index_updater,update,1,[{file,"src/nouveau_index_updater.erl"},{line,97}]}]}
[error] 2026-09-17T18:45:51.685586Z [email protected] emulator --------
Error in process <0.111495.0> on node '[email protected]' with exit value:
```
The team deleted the affected reported nouveau index on disk, expecting the
index rebuild to resolve the error. Instead, this is the error that is logged
now:
```
{{invalid_start_purge_seq,0,180213},[{couch_bt_engine,fold_purge_infos,5,[{file,"src/couch_bt_engine.erl"},{line,601}]},{nouveau_index_updater,purge_index,3,[{file,"src/nouveau_index_updater.erl"},{line,288}]},{nouveau_index_updater,update,1,[{file,"src/nouveau_index_updater.erl"},{line,97}]}]}
```
The Nouveau container is not reporting any errors.
### Expected Behaviour
Using _purge should preserve Nouveau indexes.
### Steps to Reproduce
Repeatedly calling the `_purge` over a database that uses Nouveau indexes.
This only appeared on one instance and we were unable to replicate it in
tests. It seems that a race condition is causing this.
### Your Environment
Docker hosted CouchDb + Nouveau 3.5.2.
### Additional Context
Can you please advise on steps to unblock this instance. Deleting one
Nouveau index did not resolve this, but did change the error that was being
thrown.
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