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

[Michael Haggerty <mhagger@alum.mit.edu>]

On 02/13/2015 10:53 PM, Junio C Hamano wrote:
> Michael Haggerty <mhagger@alum.mit.edu> 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.

Yes. The first is approximately the status quo, except that you would
like explicitly to *suppress* creating reflogs for symbolic refs other
than HEAD even if a reference is altered via the symbolic ref.

The second makes sense, though I think reflogs for remote HEADs are far
less useful than those for HEAD. So I think this is a low-priority project.

>> 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.

I forgot to mention that if we want to retain lock-compatibility with
older clients, then we *also* need to lock the reference pointed at by a
symbolic ref when modifying the symbolic ref's reflog. This is often
implied by the previous rule, but not when we reseat a symbolic
reference or when we expire a symbolic reference's reflog.

I will look at how hard this is to implement. If it is at all involved,
then I might drop this patch from the current patch series and defer it
to another one.

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu