janl commented on issue #4994:
URL: https://github.com/apache/couchdb/issues/4994#issuecomment-2048319508

   @jcoglan and I have come up with a repro for the 'wrong signature' event: 
https://gist.github.com/jcoglan/0a5feb4af2a496ce10c9b80cf02ea28f. Our theory is 
that when an index is first queried following a v2->v3 upgrade, 
https://github.com/apache/couchdb/blob/3.3.3/src/couch_mrview/src/couch_mrview_index.erl#L121
 will rename the file and return the old signature. however, 
maybe_update_index_file/1 just renames the file, it does not change its 
content. 
https://github.com/apache/couchdb/blob/3.3.3/src/couch_mrview/src/couch_mrview_index.erl#L127
 matches and the normal index update path is followed, but if the index is 
empty then no new content and no new header is written, so the old signature 
remains in the file. the next time the view is queried, 
maybe_update_index_file/1 will do nothing (the old file does not exist) and 
return ok, so we hit this clause where wrong signature appears 
https://github.com/apache/couchdb/blob/3.3.3/src/couch_mrview/src/couch_mrview_index.erl#L139


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