nickva opened a new pull request #3739:
URL: https://github.com/apache/couchdb/pull/3739
Previously, if the source db purge sequence > `purge_infos_limit`, shard
splitting would crash with the `{{invalid_start_purge_seq,0},
[{couch_bt_engine,fold_purge_infos,5...` error. That was because purge
sequences where always copied starting from 0. That would only work as long as
the total number of purges stayed below the purge_infos_limit threshold. In
order to correctly gather and the purge sequences, the start sequence must be
based off of the actual oldest sequence currently available.
An example of how it should be done is in the `mem_rpc` module, when
[loading purge
infos](https://github.com/apache/couchdb/blob/4ea9f1ea1a2078162d0e281948b56469228af3f7/src/mem3/src/mem3_rpc.erl#L206-L207),
so here we do exactly the same. The `MinSeq - 1` logic is also evident by
inspecting the
[fold_purge_infos](https://github.com/apache/couchdb/blob/3.x/src/couch/src/couch_bt_engine.erl#L625-L637)
function.
The test sets up the exact scenario as described above: reduces the purge
info limit to 10 then purges 20 documents. By purging more than the limit, we
ensure the starting sequence is now != 0. However, the purge sequence btree is
actually trimmed down during compaction. That is why there are a few extra
helper functions to ensure compaction runs and finishes before shard splitting
starts.
Fixes: https://github.com/apache/couchdb/issues/3738
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