[webkit-dev] Faster Git SVN updates

Ojan Vafai ojan at chromium.org
Mon Nov 22 16:58:22 PST 2010

http://trac.webkit.org/changeset/72575 for having scm.py do the right thing.

On Thu, Nov 18, 2010 at 4:11 PM, Eric Seidel <eric at webkit.org> wrote:

> OK.  Sounds we should make this setup default.  I'll see if we can't
> educate update-webkit and webkitpy/common/checkout/scm.py about detecting
> this setup and doing the right thing in that case.
> I'll file a separate bug about making scm.py's Git object use --no-rebase
> during dcommit.
> -eric
> On Thu, Nov 18, 2010 at 4:08 PM, Evan Martin <evan at chromium.org> wrote:
>> 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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