depot_tools: Add git svn find-rev for safesync_url parsing (commonly LKGR link).

scm.py

Index: scm.py
===================================================================
--- scm.py	(revision 107937)
+++ scm.py	(working copy)
@@ -379,6 +379,27 @@
     root = GIT.Capture(['rev-parse', '--show-cdup'], cwd=cwd).strip()
     return os.path.abspath(os.path.join(cwd, root))
 
+  @staticmethod
+  def GetSha1ForSvnRev(cwd, rev):
+    """Returns a git sha1 for a corresponding Subversion revision from a git-svn
+       remote.  If that Subversion revision isn't found, an empty string is
+       returned."""
+    # This probably won't work if not git-svn. There's a warning in
+    # gclient_scm.py that gives a user an error message and a URL to how to set
+    # up a git-svn remote, so we'll probably never get here but it doesn't hurt.
+    if not GIT.IsGitSvn(cwd=cwd):
+      return ''

[M-A Ruel, 2011/10/31 13:46:51]

return None in case of failure.

[Dan Beam, 2011/11/06 07:34:08]

Done.

+    try:
+      GIT.Capture(['svn', 'rebase', '--local'], cwd=cwd)

[M-A Ruel, 2011/10/31 13:46:51]

rebase (!) You need to use svn fetch instead, then "git svn find-rev 'r%s' %
rev".

[Dan Beam, 2011/10/31 18:32:28]

Yeah, I guess this isn't a good idea, but it's the only way I found to convert
git-svn metadata that seems to be guaranteed to not hit network (other than
calling git svn find-rev once to rebuild and maybe once more to find).  I
haven't been able to find a way to exactly replicate what git svn find-rev does
before it searches for a sha1 *just* using git-svn's public API, but perhaps
this is the wrong thing to do.
Are we OK with git svn fetch possibly requiring a painfully long fetching and
converting of svn revisions linearly until hitting HEAD?  Imagine someone
running this the first time with never fetched SVN remote.  I'd like to try only
locally pulled stuff first, but there's no --local flag for git svn fetch (and I
didn't want to have to start looking at git config --get
<git-svn-remote>.fetch).

I guess I'm assuming people have set up git-svn like we mention in our guide,
but maybe they're not using a fetch = refs/remotes/origin/master to convert
already fetched git revs (which is too bad if so...).  Is it not common for
people to git fetch && git svn rebase --local to convert and apply changes (or
git pull && git svn fetch)?
I'm doing that 4 lines after this.

[M-A Ruel, 2011/10/31 19:17:50]

Ah I see. I just dislike that a function like that has a side effect that can
leave your checkout in a middle of a rebase. I'm fine with hitting the network.

 
I see an optimization though; the network hit should be prepended with this
check:
if Capture("git svn info")['revision'] >= rev:
  # Don't hit network
so that network is not hit unless necessary.

Also, find-rev r123 will fail if the commits are 122 and 124. So you need
fallback code there.

[Dan Beam, 2011/11/06 07:34:08]

Done.

+      # Currently works by just grabbing last line of output from git svn
+      # find-rev after rebuilding the index. This seems to currently always be
+      # what we want (either a sha1 or empty) even if there is error output.
+      lines = GIT.Capture(
+                  ['svn', 'find-rev', 'r' + rev], cwd=cwd).splitlines(True)
+      return lines[-1].strip() if len(lines) else ''
+    except subprocess2.CalledProcessError:
+      return ''
+
   @classmethod
   def AssertVersion(cls, min_version):
     """Asserts git's version is at least min_version."""