Subject: Re: RFH: unexpected reflog behavior with --since=

From: Eric Raible <[email-masked]>
Date: Wed, 9 Nov 2011 23:48:58 -0800

On 11/9/2011 2:01 PM, Jeff King wrote:
> On Tue, Nov 08, 2011 at 04:22:41PM -0800, Eric Raible wrote:
> 
> [explanation how --since is used to limits traversal omitted]

Yes, all that is as expected, and makes sense.

> Now let's look at reflog walking. It's kind of bolted on to the side
> of the revision traversal machinery. We walk through the reflog
> backwards and pretend that entry N's parent is entry N-1 (you can see
> this if you do "git log -g -p", for example; you see the patch versus
> the last reflog entry, not the patch against the commit's true parent).
> 
> In the case of rewound history (like the reset you showed above), this
> means that the history graph will appear to have bad clock skew. The
> timestamp of HEAD@{0} is going to be much earlier than its pretend
> parent, HEAD@{1}. And the "--since" optimization is going to cut off
> traversal, even though there are more interesting commits to be shown.
> 
> So in that sense, I think it's a bug, and we should probably disable the
> exit-early-from-traversal optimization when we're walking reflogs.

Indeed.  Seems like a case of an optimization leading to an incorrect result.

> But it may also be a misfeature, because it's not clear what you're
> actually trying to limit by. We have commit timestamps, of course, but
> when we are walking reflogs, we also have reflog timestamps. Did you
> actually want to say "show me all commits in the reflog, in reverse
> reflog order, omitting commits that happened before time t"? Or did you
> really mean "show me the reflog entries that happened before time t,
> regardless of their commit timestamp"?

I meant "show me the reflog entries that happened *since* time t,
regardless of their commit timestamp.

> In the latter case, we would either need a new specifier (like
> "--reflog-since"), or to rewrite the commit timestamp when we rewrite
> the parent pointers.
> 
> The latter has a certain elegance to it (we are making a pretend linear
> history graph out of the reflog, so faking the timestamps to be sensible
> and in order is a logical thing to do) but I worry about lying too much
> in the output. Something like "git log -g --format=%cd" would now have
> the fake timestamp in the output. But then, we already show the fake
> parents in the output, so I don't know that this is any worse.

Since -g is asking specifying for the reflog, and since the reflog has
its own timestamps, I would expect that those timestamps be used.