From: Michael Haggerty <mhagger@alum.mit.edu>
Date: Fri, 24 Apr 2015 21:15:09 GMT
Subject: Re: [PATCH 4/5] ref_transaction_commit(): remove the local flags variables
Message-ID: <553AB25D.9090201@alum.mit.edu>
In-Reply-To: <20150424173005.GC30592@peff.net>

On 04/24/2015 07:30 PM, Jeff King wrote:
> On Fri, Apr 24, 2015 at 01:35:48PM +0200, Michael Haggerty wrote:
> 
>> Instead, work directly with update->flags. This has the advantage that
>> the REF_DELETING bit, set in the first loop, can be read in the third
>> loop instead of having to compute the same expression again. Plus, it
>> was kindof confusing having both update->flags and flags, which
>> sometimes had different values.
> 
> Hmm. I think this is losing the distinction of "flags the caller has
> passed in to us" versus "flags we are using locally only during the
> transaction_commit routine". If callers look at the flags in the
> REF_TRANSACTION_CLOSED state, do they care about seeing these new flags?
> 
> My guess is probably not in practice, and "leaking" these flags is an
> acceptable tradeoff for keeping the transaction_commit function simpler.
> But I haven't looked that closely.

"struct ref_update" is opaque to callers outside of the refs module, and
ref_update::flags is not read anywhere outside of
ref_transaction_commit() (and its value is passed to
lock_ref_sha1_basic()). So I don't think we have to be shy about storing
our own internal information there.

In fact, REF_DELETING, REF_ISPRUNING, REF_HAVE_NEW, and REF_HAVE_OLD are
also private to the refs module.

I suppose we could mask out all the "private" bits in the flags
parameter passed by the caller, to make sure that the caller hasn't
accidentally set other bits. I think that would be more defensive than
our usual practice, but I don't mind doing it if people think it would
be prudent.

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu