• Kevin Bracey's avatar
    mergetools/p4merge: create a base if none available · 4549162e
    Kevin Bracey authored
    Originally, with no base, Git gave P4Merge $LOCAL as a dummy base:
    
       p4merge "$LOCAL" "$LOCAL" "$REMOTE" "$MERGED"
    
    Commit 0a0ec7bd changed this to:
    
       p4merge "empty file" "$LOCAL" "$REMOTE" "$MERGED"
    
    to avoid the problem of being unable to save in some circumstances with
    similar inputs.
    
    Unfortunately this approach produces much worse results on differing
    inputs. P4Merge really regards the blank file as the base, and once you
    have just a couple of differences between the two branches you end up
    with one a massive full-file conflict. The 3-way diff is not readable,
    and you have to invoke "difftool MERGE_HEAD HEAD" manually to get a
    useful view.
    
    The original approach appears to have invoked special 2-way merge
    behaviour in P4Merge that occurs only if the base filename is "" or
    equal to the left input.  You get a good visual comparison, and it does
    not auto-resolve differences. (Normally if one branch matched the base,
    it would autoresolve to the other branch).
    
    But there appears to be no way of getting this 2-way behaviour and being
    able to reliably save. Having base==left appears to be triggering other
    assumptions. There are tricks the user can use to force the save icon
    on, but it's not intuitive.
    
    So we now follow a suggestion given in the original patch's discussion:
    generate a virtual base, consisting of the lines common to the two
    branches. This is the same as the technique used in resolve and octopus
    merges, so we relocate that code to a shared function.
    
    Note that if there are no differences at the same location, this
    technique can lead to automatic resolution without conflict, combining
    everything from the 2 files.  As with the other merges using this
    technique, we assume the user will inspect the result before saving.
    Signed-off-by: default avatarKevin Bracey <kevin@bracey.fi>
    Reviewed-by: David Aguilar's avatarDavid Aguilar <davvid@gmail.com>
    Signed-off-by: default avatarJunio C Hamano <gitster@pobox.com>
    4549162e
p4merge 651 Bytes