Subject: Re: [PATCH 8/8] reflog_expire(): lock symbolic refs themselves, not their referent

From: Junio C Hamano <[email-masked]>
Date: Fri, 13 Feb 2015 13:53:08 -0800

Michael Haggerty <[email-masked]> writes:

> Now back to the real world. Currently, if R is changed *through* a
> symbolic reference S, then the reflogs for both R and S are updated, but
> not the reflogs for any other symbolic references that might point at R.
> If R is changed directly, then no symref's reflogs are affected, except
> for the special case that HEAD's reflog is changed if it points directly
> at R. This limitation is a hack to avoid having to walk symrefs
> backwards to find any symrefs that might be pointing at R.

Yup.

> It might actually not be extremely expensive to follow symrefs
> backwards. Symbolic references cannot be packed, so we would only have
> to scan the loose references; we could ignore packed refs. But it would
> still be a lot more expensive than just updating one file. I don't know
> that it's worth it, given that symbolic references are used so sparingly.

I personally do not think it is worth it.  I further think that it
would be perfectly OK to do one of the following:

    - We only maintain reflogs for $GIT_DIR/HEAD; no other symrefs
      get their own reflog, and we only check $GIT_DIR/HEAD when
      updating refs/heads/* and no other refs for direct reference
      (i.e. HEAD -> refs/something/else -> refs/heads/master symref
      chain is ignored).

    - In addition to the above, we also maintain reflogs for
      $GIT_DIR/refs/remotes/*/HEAD but support only when they
      directly point into a remote tracking branch in the same
      hierarchy.  $GIT_DIR/refs/remotes/foo/HEAD that points at
      $GIT_DIR/refs/remotes/bar/master is ignored and will get an
      undefined behaviour.

> I think that the rule about locks as expressed above can be carried over
> the the real world:
>
>     We should hold the locks on exactly those references (symbolic
>     or regular) whose reflogs we plan to change. We should acquire all
>     of the locks before making any changes.

Sure.