Mercurial Pushlog Is Now Robust Against Interrupts

December 30, 2014 at 12:25 PM | categories: Mozilla, Firefox | View Comments

hg.mozilla.org - Mozilla's Mercurial server - has functionality called the pushlog which records who pushed what when. Essentially, it's a log of when a repository was changed. This is separate from the commit log because the commit log can be spoofed and the commit log doesn't record when commits were actually pushed.

Since its inception, the pushlog has suffered from data consistency issues. If you aborted the push at a certain time, data was not inserted in the pushlog. If you aborted the push at another time, data existed in the pushlog but not in the repository (the repository would get rolled back but the pushlog data wouldn't).

I'm pleased to announce that the pushlog is now robust against interruptions and its updates are consistent with what is recorded by Mercurial. The pushlog database commit/rollback is tied to Mercurial's own transaction API. What Mercurial does to the push transaction, the pushlog follows.

This former inconsistency has caused numerous problems over the years. When data was inconsistent, we often had to close trees until someone could SSH into the machines and manually run SQL to fix the problems. This also contributed to a culture of don't press ctrl+c during push: it could corrupt Mercurial. (Ctrl+c should be safe to press any time: if it isn't, there is a bug to be filed.)

Any time you remove a source of tree closures is a cause for celebration. Please join me in celebrating your new freedom to abort pushes without concern for data inconsistency.

In case you want to test things out, aborting pushes and (and rolling back the pushlog) should now result in something like:

pushing to ssh://hg.mozilla.org/mozilla-central
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
Trying to insert into pushlog.
Inserted into the pushlog db successfully.
^C
rolling back pushlog
transaction abort!
rollback completed
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