[GitHub] [couchdb] iilyak commented on issue #1600: Sporadic errors on /_all_docs with Cookie auth and system databases

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iilyak commented on issue #1600: Sporadic errors on /_all_docs with Cookie auth and system databases
URL: https://github.com/apache/couchdb/issues/1600#issuecomment-503371250
 
 
   > @wohali Another repro on this: saw it when enabling maintenance mode on a node.
   
   # Theory : Some update events are not delivered when node is in MM.
   
   The `chttpd_auth_cache` server listens for changes to users database. So it can [evict](https://github.com/apache/couchdb/blob/master/src/chttpd/src/chttpd_auth_cache.erl#L165) updated entries from the cache. The changes listener is implemented using `fabric:changes/4`. The `fabric:changes` [iterates](https://github.com/apache/couchdb/blob/master/src/fabric/src/fabric_db_update_listener.erl#L70:L73) over all shards for users database and calls [`start_update_notifier/1`](https://github.com/apache/couchdb/blob/master/src/fabric/src/fabric_db_update_listener.erl#L76) for every shard. The [`start_update_notifier/1`](https://github.com/apache/couchdb/blob/master/src/fabric/src/fabric_db_update_listener.erl#L76:L81) is implemented as follows:
   ```
   start_update_notifier(DbNames) ->
       {Caller, Ref} = get(rexi_from),
       Notify = config:get("couchdb", "maintenance_mode", "false") /= "true",
       State = #cb_state{client_pid = Caller, client_ref = Ref, notify = Notify},
       Options = [{parent, Caller}, {dbnames, DbNames}],
       couch_event:listen(?MODULE, handle_db_event, State, Options).
   ```
   
   The notification about a database update is not send if node is in the maintenance mode. The failure mode is as follows:
   
   - put node in the MM
   - cause changes listener restart (it would be restarted automatically after config:get("httpd", "changes_timeout", "60000")) milliseconds.
   - update user 
   - we would miss db_update event (because Notify would be false)
   - the ets_lru cache backing chttpd_auth_cache would still be holding old entry.

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