• Junio C Hamano's avatar
    git-merge-octopus: use (merge-base A (merge B C D E...)) for stepwise merge · c5dc9a28
    Junio C Hamano authored
    Suppose you have this topology, and you are trying to make an octopus
    across A, B and C (you are at C and merging A and B into your branch).
    The protoccol between "git merge" and merge strategies is for the former
    to pass common ancestor(s), '--' and then commits being merged.
    
    git-merge-octopus does not produce the final merge in one-go.  It
    iteratively produces pairwise merges.  So the first step might be to come
    up with a merge between B and C:
    
                   o---o---o---o---C
                  /                 :
                 /   o---o---o---B..(M)
                /   /
            ---1---2---o---o---o---A
    
    and for that, "1" is used as the merge base, not because it is the base
    across A, B and C but because it is the base between B and C.  For this
    merge, A does not matter.
    
    I drew M in parentheses and lines between B and C to it in dotted line
    because we actually do _not_ create a real commit --- the only thing we
    need is a tree object, in order to proceed to the next step.
    
    Then the final merge result is obtained by merging tree of (M) and A using
    their common ancestor.  For that, we _could_ still use "1" as the merge
    base.
    
    But if you imagine a case where you started from A and M, you would _not_
    pick "1" as the merge base; you would rather use "2" which is a better
    base for this merge.
    
    That is why git-merge-octopus ignores the merge base given by "merge" but
    computes its own.
    
    The comment at the end of git-merge-octopus talks about "merge reference
    commit", that we used to update it to common found in this round, and that
    that updating was pointless.  After the first round of merge to produce
    the tree for M (but without actually creating the commit object M itself),
    in order to figure out the merge base used to merge that with A in the
    second round, we used to use A and "1" (which was merge base between B and
    C).  That was pointless --- "merge-base A 1" is guaranteed to give a base
    that is no better than either "merge-base A B" or "merge-base A C".  So
    the current code keeps using the original head (iow, MRC=C, because in
    this case we are starting from C and merging B and then A into it).
    
    This trickerly was necessary only because we avoided creating the extra
    merge commit object M.
    
    	Side note.  An alternative implementation could have been to
    	actually record it as a real merge commit M, and then let the
    	two-commit merge-base compute the base between A and M when
    	merging A to the result of the previous round, but we avoided
    	creating M, at the expense of potentially using suboptimal base in
    	the later rounds.
    
    But we do not have to be that pessimistic.  We can instead accumulate the
    commits we have merged so far in MRC, and have merge_bases_many() compute
    the merge base for the new head being merged and the heads we have
    processed so far, which can give a better base than what we currently do.
    Signed-off-by: default avatarJunio C Hamano <gitster@pobox.com>
    c5dc9a28
git-merge-octopus.sh 2.02 KB