overwrite=True causes unnecessary re-cloning of git repositories

[Colin Watson]

If codetree is asked to fetch a git branch with overwrite=True, then it always removes any existing repository. This makes some Mojo specs very slow to run on slow connections (e.g. doing even simple operations on lp:~ubuntuone-pqm-team/canonical-mojo-specs/mojo-ols-snap-store takes me half an hour), because they keep re-cloning fairly large charm repositories over and over again.

The relevant chunk of code looks like this:

            if options.get("overwrite"):
                shutil.rmtree(dest)
                if not log_failure(clone_command,
                                   "Cloning repository at {} from {}".format(dest, self.source)):
                    return result
            else:
                if not self.check_and_update(dest):
                    return result
                if ref is None: # default to master for the checkout of any updated repo
                    ref = 'master'

This is just one possibility for an improved structure and I haven't tested it, but I think that something like this would work better:

            try:
                updated = self.check_and_update(dest)
            except (NotABranch, NotSameBranch):
                if options.get("overwrite"):
                    updated = False
                else:
                    raise
            if updated:
                if ref is None: # default to master for the checkout of any updated repo
                    ref = 'master'
            elif options.get("overwrite"):
                shutil.rmtree(dest)
                if not log_failure(clone_command,
                                   "Cloning repository at {} from {}".format(dest, self.source)):
                    return result
            else:
                return result

[Tom Haddon (mthaddon)]

Seems like a reasonable change, would obviously need testing.