Re: [PATCH 12/12] refs: do not invalidate the packed-refs cache unnecessarily

[Jeff King <peff@peff.net>]

On Tue, Jun 11, 2013 at 11:48:32PM +0200, Michael Haggerty wrote:

> Now that we keep track of the packed-refs file metadata, we can detect
> when the packed-refs file has been modified since we last read it, and
> we do so automatically every time that get_packed_ref_cache() is
> called.  So there is no need to invalidate the cache automatically
> when lock_packed_refs() is called; usually the old copy will still be
> valid.
> 
> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
> ---
> This patch is optional.  It makes the assumption that the metadata
> stored in stat_validity are adequate to reliably detect when the
> packed-refs file has changed.  Given that we are about to rewrite the
> file, it is perhaps even more crucial not to make a mistake in this
> codepath than in others.  So if the stat_validity check is not
> considered safe enough, it might be prudent to omit this patch and
> continue to reload the packed-refs data here unconditionally.

I doubt that the risk is very high, but I'd also doubt that this
provides any useful performance improvement, because it is mostly
happening during a `git pack-refs` call. The exception is packing for a
ref deletion, and it could potentially speed up a receive-pack that was
deleting many refs.

I don't have a strong opinion either way.

-Peff