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https://issues.apache.org/jira/browse/COUCHDB-1363?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13170547#comment-13170547
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Randall Leeds commented on COUCHDB-1363:
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Thanks for the review. I'll explain more (I was distracted by talking after the
CouchBase meetup).
I've run into this during the course of my work for COUCHDB-1350. As part of
that, I had to change the replicator_db test and make the
continuous_replication_survives_restart use bare db names instead of urls (the
url changes after /_restart). I'm guessing I uncovered this race because the
timing changes as a result of this different path.
In any case, unless I'm missing something fundamental it's most definitely a
bug and this patch definitely fixes it. I tested by removing the line to stop
CouchDB after the tests, so I could leave the replication running after the
failure, which was line 700 of replicator_db.js. The replication stays open in
active tasks but the fourth change, to doc 'foo1000', isn't replicated until I
PUT another change to the source database. That is because the Db handle that's
passed into handle_changes is pointing at an old header and the update_notifier
hasn't been started yet. The patch re-opens the db after subscribing to
notifications, which ensures we can't miss an update and stall in this way.
It probably doesn't bite many people, though (replication starts again as soon
as the next change comes), but I can think of situations I've seen where this
could be a problem.
Thanks for catching the shadowing issue. I didn't notice the warning when I
compiled. I thought that the DbName in the fun header would be matched against
the DbName in the handle_changes header instead of shadowing. My intention was
not to change that functionality and I should probably have left it alone.
Let me know if it's okay to commit (- the shadowing). I don't see a better fix.
Do you?
> Race condition edge case when pulling local changes
> ---------------------------------------------------
>
> Key: COUCHDB-1363
> URL: https://issues.apache.org/jira/browse/COUCHDB-1363
> Project: CouchDB
> Issue Type: Bug
> Components: Database Core
> Affects Versions: 1.0.3, 1.1.1
> Reporter: Randall Leeds
> Assignee: Filipe Manana
> Priority: Minor
> Fix For: 1.2, 1.3
>
> Attachments: 0001-Fix-a-race-condition-starting-replications.patch
>
>
> It's necessary to re-open the #db after subscribing to notifications so that
> updates are not lost. In practice, this is rarely problematic because the
> next change will cause everything to catch up, but if a quick burst of
> changes happens while replication is starting the replication can go stale.
> Detected by intermittent replicator_db js test failures.
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