Feature request: a merge strategy that makes any file difference a merge conflict

Feature request: a merge strategy that makes any file difference a merge conflict

[Michael Ernst]

Git's built-in merge strategies, such as ort, sometimes create a
clean-but-incorrect merge.  A merge driver or a mergetool cannot be
used to correct such problems, because a merge driver or mergetool is
only called when the strategy resulted in a conflict (so far as I
understand).  It is challenging to write a merge strategy, but it is
much easier to write a merge driver or a mergetool.

If git had a strategy that always caused a merge conflict (even when
the differences are mergeable), then a merge driver or mergetool would
be called on every file difference.  This effectively permits a user
to completely replace git's merge strategy, without the difficulty of
writing a merge strategy.

I realize that the proposed strategy would likely only create merge
conflicts when no two of {parent1,parent2,base} are the same -- that
is, only when the low-level merge driver `git merge-file` is called.
That is fine with me.

What do you think of this feature request?

Thanks in advance,
-Mike

Re: Feature request: a merge strategy that makes any file difference a merge conflict

[Junio C Hamano]

Michael Ernst <mernst@cs.washington.edu> writes:

> Git's built-in merge strategies, such as ort, sometimes create a
> clean-but-incorrect merge.  A merge driver or a mergetool cannot be
> used to correct such problems, because a merge driver or mergetool is
> only called when the strategy resulted in a conflict (so far as I
> understand).

A custom low-level merge driver is always called when selected via
the attribute mechansism (see how merge-ll.c:ll_merge() calls
find_ll_merge_driver()) and participates in a content-level 3-way
merge.

If you are trying to interfere with cases that a content-level 3-way
merge does not kick in (e.g., your side did not change anything in
the file since their history forked, and they modified the file; the
tree level 3-way merge will resolve it to take their version), then
it is true that the low-level merge driver is not invoked, but I
somehow get an impression from the above description that it is not
what you are trying to do.

> It is challenging to write a merge strategy, but it is
> much easier to write a merge driver or a mergetool.

A merge strategy is about performing three-way merge at the tree
level, figuring out which three variants of contents to hand to a
merge driver that handles the content-level three-way merge.  They
serve totally different purposes and comparing them is like
comparing apples and oranges.

Re: Feature request: a merge strategy that makes any file difference a merge conflict

[Michael Ernst]

Junio-

Thank you for the clarification.  There is a lot of misleading
information about this on the Internet.

I was mistaken about when a merge driver is called.  Now I see that
the merge driver is called anytime that no two of
{base,parent1,parent2} are the same.  This means that a merge driver
*can* prevent a clean-but-incorrect merge.  I was right that a
mergetool cannot be used to correct a clean-but-incorrect merge.

-Mike

Re: Feature request: a merge strategy that makes any file difference a merge conflict

[Thomas Braun]

On 29.03.2024 20:20, Michael Ernst wrote:

[...]

> What do you think of this feature request?

I wanted to propose to mark the relevant files as binary as that should 
prevent the merge to happen. But this is not working.

git init

echo "*.txt binary" > .gitattributes
git add .gitattributes
git commit -m ".gitattributes: Add it" .gitattributes

echo "a" > test.txt
git add test.txt
git commit -m "test.txt: main" test.txt

git checkout -b feature
echo "ab" > test.txt
git commit -m "test.txt: feature" test.txt

git checkout main
git merge --no-ff --stat --no-edit  feature

Merge made by the 'ort' strategy.
  test.txt | Bin 22 -> 3 bytes
  1 file changed, 0 insertions(+), 0 deletions(-)

git log -p test.txt
commit ec7cab2fa30c26738c6254202a399a616959a661 (feature)
Author: Thomas Braun <thomas.braun@byte-physics.de>
Date:   Mon Apr 1 12:56:07 2024 +0200

     test.txt: feature

diff --git a/test.txt b/test.txt
index 346a56a..81bf396 100644
Binary files a/test.txt and b/test.txt differ

commit 61e195554351dada7494e4eb4935d96120680420
Author: Thomas Braun <thomas.braun@byte-physics.de>
Date:   Mon Apr 1 12:50:29 2024 +0200

     test.txt: main

diff --git a/test.txt b/test.txt
new file mode 100644
index 0000000..346a56a
Binary files /dev/null and b/test.txt differ

I would have expected to see a merge conflict as git can't know how to 
merge binaries. Or am I misunderstanding something here?