Re: [PATCH 9/9] rerere forget path: forget recorded resolution

On Montag, 11. Januar 2010, Junio C Hamano wrote:

I ended up doing this myself.  As we are dropping the postimage and adding
a new MERGE_RR entry, I also think that it is safer to update the preimage
with the conflict we got for this round, so I added that as well.

Thank you, it appears to work as expected. It is actually very important to 
update the preimage as well, otherwise, the new postimage can contain 
unrelated additional changes.

However, I think there is a room for improvement in preimage handling.

Currently, the rerere database is indexed with the conflict hash and for
each conflict hash you can record a single preimage-postimage pair to
replay.  But you can have conflicts with the same conflict hash, but with
slightly different contexts outside the conflicted region, and the right
resolution can be different depending on the outside context.

I did encounter a case where the same resolution would apply to all conflicts 
that have the same conflict hash, so it's not quite what you talk about. But 
not all conflicts were automatically resolved. I haven't yet analyzed what 
happened - it could just be that the xdl_merge call fails due to the 
differences in the text immediately outside the conflict markers.

In the traditional implementation, I punted this issue by noticing
conflicts in the three-way merge between pre, post and this images.  If
preimage is too different from the conflicted contents we got during this
merge, then the previous resolution should not apply.

But I think the right solution would be to have more than one preimage and
postimage pairs (preimage.0 vs postimage.0,... etc.) and try to use each
of them in handle_path() until it finds one that can be used to cleanly
merge with the conflict we got in thisimage during this round.

The situation happens rarely, so I don't know if we should care. OTOH, *when* 
the situation arises, and a recorded resolution is applied incorrectly, it 
may be quite annoying. Dunno.

-- Hannes