On Thu, Feb 04, 2016 at 05:12:20PM +0900, Mike Hommey wrote:
> > > It seems like git branch -d ascend the hierarchy (up to
> > > .git/refs/heads/), deleting any empty directories.
> >
> > Yes, though it needs to be coupled with making the branch-creation
> > process more robust to races (since we might create "refs/heads/foo" in
> > order to make "refs/heads/foo/bar" while somebody else is deleting it to
> > get rid of "refs/heads/foo/baz").
>
> Can't we come up with a system that would update packed-refs directly
> instead of creating files?
There are a few reasons not to:
- it breaks backwards compatibility (unless we continue to create the
directory in order to put the dot-lock in it, but then I don't think
we've gained anything)
- the usual update method for packed-refs is to take a dot-lock, do a
whole-file update, and then atomically rename into place. That
makes writing a ref O(# of refs) instead of O(1), and increases lock
contention on the packed-refs file.
- if we abandon atomic renames as the update mechanism and just update
in place via lseek/write, then we need read-locking, or we need to
hope that a reader will never see a sheared write
But if we're willing to break compatibility, we should ditch packed-refs
entirely and move to a _real_ concurrent database. And there is work
underway already to do that (see David Turner's ref-backend-lmdb
series).
-Peff
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