Proposal for new git Merge Strategy

Proposal for new git Merge Strategy

[Jon Loeliger]

Folks,

The other day, I was talking to some other folks else-list
about git's approach to merges and mentioned that there was
some structure in place to handle different merge strategies.

One person observed that Perforce had a really good
approach to merging and conflict resolution that allowed
user interaction during the process specifically to
help select the individual files and hunks that contributed
to the final result.  I confess that I have never used
Perforce, so this is all hear-say and interpretation. :-)

However, it does seem like an approach that we could
easily add to git -- not as the default of course.
(Just think how dead we'd all be if Linus had to manually
interact with every merge he performed at the tip of the
Linux Pyramid. :-)

But for complex or critical merges, a "guided merge"
strategy seems like it might be a useful tool.  Basically,
it would offer options to select Stage 1 or Stage 2
revisions, or step in and offer hunks from Stage 1 and 2,
revert to "ours" or "theirs", or "revert to 'ours' or 'theirs'
for all remaining files".  Things like that maybe.

Any thoughts down this line?  Good idea?  Bad idea?

Thanks,
jdl

PS -- Please keep mwm on the CC: list as he doesn't
      directly subscribe to the git list, but would
      like to participate in the thread.  Thanks!

Re: Proposal for new git Merge Strategy

[Jakub Narebski]

Jon Loeliger wrote:

> The other day, I was talking to some other folks else-list
> about git's approach to merges and mentioned that there was
> some structure in place to handle different merge strategies.
> 
> One person observed that Perforce had a really good
> approach to merging and conflict resolution that allowed
> user interaction during the process specifically to
> help select the individual files and hunks that contributed
> to the final result.  I confess that I have never used
> Perforce, so this is all hear-say and interpretation. :-)
> 
> However, it does seem like an approach that we could
> easily add to git -- not as the default of course.
> (Just think how dead we'd all be if Linus had to manually
> interact with every merge he performed at the tip of the
> Linux Pyramid. :-)
> 
> But for complex or critical merges, a "guided merge"
> strategy seems like it might be a useful tool.  Basically,
> it would offer options to select Stage 1 or Stage 2
> revisions, or step in and offer hunks from Stage 1 and 2,
> revert to "ours" or "theirs", or "revert to 'ours' or 'theirs'
> for all remaining files".  Things like that maybe.

And select which files are which (after renaming, copying, etc.)

> Any thoughts down this line?  Good idea?  Bad idea?

Wouldn't it be better to fallback to graphical/user guided 
merger only on _failed_ merge? Merge helpers like xxdiff, 
Meld or KDiff3 
  http://git.or.cz/gitwiki/InterfacesFrontendsAndToolsWishlist
There was proposal of git script which run xxdiff with correct
extracted files, if I remeber correctly postponed waiting for
more generic version.

-- 
Jakub Narebski
Warsaw, Poland
ShadeHawk on #git

Re: Proposal for new git Merge Strategy

[Junio C Hamano]

Jon Loeliger <jdl@jdl.com> writes:

> But for complex or critical merges, a "guided merge"
> strategy seems like it might be a useful tool.  Basically,
> it would offer options to select Stage 1 or Stage 2
> revisions, or step in and offer hunks from Stage 1 and 2,
> revert to "ours" or "theirs", or "revert to 'ours' or 'theirs'
> for all remaining files".  Things like that maybe.
>
> Any thoughts down this line?  Good idea?  Bad idea?

We had some discussion on this with Catalin in "Unresolved
issues #3" thread, regarding git-xxdiff (did I ever take it?  I
liked it for what it does, but I was not sure about its
odd-man-out-ness) which was proposed by Martin Langhoff.

A merge that results in manual fixups conceptually take these
steps:

 - revs involved in 3-way merge identified with git-merge-base;

 - read-tree is given these three bases;

 - git-merge-index gives three stages as individual temporary
   files to git-merge-one-file for each path that cannot be
   resolved at tree-level.

   - git-merge-one-file calls "merge" (reminds me that I should
     replace this with "diff3").

We should be able to make the part that call "merge/diff3" to
alternatively call xxdiff or its friends (kompare, emerge, pick
your favorites).  Catalin even showed us a code snippet used in
StGIT for this in the thread.

Martin's proposed tool git-xxdiff is meant to be invoked after
all of the above still left conflict markers.  As Catalin
pointed out, using "xxdiff -U" to work on a file with conflict
markers is less powerful than working on three stages directly,
but on the other hand it can be used as the last stage fixup,
independent from what git-merge does internally.  In other
words, it is meant to help solving the same problem but in a
different part of the workflow.

Re: Proposal for new git Merge Strategy

[Martin Langhoff]

On 8/24/06, Junio C Hamano <junkio@cox.net> wrote:
> Jon Loeliger <jdl@jdl.com> writes:
...
> > Any thoughts down this line?  Good idea?  Bad idea?
>
> We had some discussion on this with Catalin in "Unresolved
> issues #3" thread, regarding git-xxdiff (did I ever take it?  I
> liked it for what it does, but I was not sure about its
> odd-man-out-ness) which was proposed by Martin Langhoff.

I've been slack on my reading of the list lately so totally missed out
on that thread. I'll go and read it now...

...

> We should be able to make the part that call "merge/diff3" to
> alternatively call xxdiff or its friends (kompare, emerge, pick
> your favorites).  Catalin even showed us a code snippet used in
> StGIT for this in the thread.

I still think that the default initial behaviour git has is right.
Most conflicts are trivial, and people can deal with conflict markers
just right. It's what we are used to.

Except when it's a mess and it's unclear what goes where and why.
That's when git log --merge and my git-xxdiff help. I've also been
wondering if I can do gitk --merge easily ;-)

> Martin's proposed tool git-xxdiff is meant to be invoked after
> all of the above still left conflict markers.  As Catalin
> pointed out, using "xxdiff -U" to work on a file with conflict
> markers is less powerful than working on three stages directly,
> but on the other hand it can be used as the last stage fixup,
> independent from what git-merge does internally.  In other
> words, it is meant to help solving the same problem but in a
> different part of the workflow.

My implementation doesn't use the 3 stages either, just because I
didn't see xxdiff giving any stage a particular meaning. I should
rework it to have the 3 stages there, and trust users to read the
filename, which should say 'ancestor'.

In terms of the one script or many, if there is concensus on
OneScriptToRuleThemAll, I am not that opposed to reworking it to
something like

  git-mergehelper --tool xxdiff path/to/file.c

with a big switch statement inside the script :-p What bothers me is
that there may be interesting parameters to pass to the invoked tool,
and other than having a stupid '--toolopts' passthrough, we are pretty
fsck'd.

By having separate git-xxdiff, git-meld, etc the git- scripts would
accept all/most of the same params that the tool accepts, therefore
feeling "natural" to users of the tool. A definite advantage, IMHO.

{Ugh, my implementation doesn't get that far. But hey, good intentions!}

cheers,



martin