[webkit-dev] Faster Git SVN updates

Evan Martin evan at chromium.org
Thu Nov 18 16:08:57 PST 2010

On Thu, Nov 18, 2010 at 3:36 PM, David Levin <levin at google.com> wrote:
>> It's some magical setup by which your git svn fetchs will be much faser.
>>  But I've heard it's buggy?  Can lead to local repository corruption?
>> Can someone set me straight?

No magic, just standard git: complicated, but logical once you
understand how it works. :\

It means that both "git pull" and "git svn fetch" will be updating the
same branch.  When the latter sees the former has pulled down new
stuff (quickly, via the fast git protocol), it knows to rebuild its
metadata from the new stuff you fetched (rather than fetching it all
over again via the slow svn protocol).

There's a catch: if you "git svn fetch", that creates new commits
locally.  When you later "git pull", you get the commits that were
constructed by git.webkit.org, which don't match (due to differing
timestamps).  This may screw up rebase, but I believe it's smart
enough to recognize the commits are "the same" (applied the same diff;
in git jargon, they have the same patch-id).  In practice you don't
even run "git svn fetch" except when the git server is behind, so I
don't know if there are corner cases here that I haven't run into.  (I
also haven't tried this on Windows in a while -- kind of terrified of
git there, though I hear it works for others.)

In particular for bots that are not committing, I see no catch, other
than that they will be behind whenever git.webkit.org is behind.

>> The current git svn fetch is *super* slow.  Especially if you're behind by
>> more than a day or two.
>> If there was a way to make this faster method safe, by wrapping it in some
>> other (error-checking) command which knew how to fall back to git svn
>> rebase, etc. when necessary I would love to make it the default method for
>> all WebKit get users.

I have instructed all Chrome git users to use this method since
(checking the commit history...) Feb 2009 and it seems to work for us.
 Note that you need git >= 1.6.1 or so for it to work properly.  I
also use this method for working on WebKit (though I've only committed
around 60 patches so I mostly use it for pulling down new code).

PS: our tools also run svn dcommit with "--no-rebase" to avoid
needlessly going out to svn again after committing.

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