show all merge conflicts

show all merge conflicts

[Michael Spiegel]

Hi folks,

I'm trying to determine whether a merge required a conflict to resolve
after the merge has occurred. The git book has some advice
(https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging) to use
`git show` on the merge commit or use `git log --cc -p -1`. These
strategies work when the merge conflict was resolved with a change
that is different from either parent. When the conflict is resolved
with a change that is the same as one of the parents, then these
commands are indistinguishable from a merge that did not conflict. Is
it possible to distinguish between a conflict-free merge and a merge
conflict that is resolved by with the changes from one the parents?

Thanks,
--Michael

Re: show all merge conflicts

[Jeff King]

On Fri, Jan 27, 2017 at 11:56:08AM -0500, Michael Spiegel wrote:

> I'm trying to determine whether a merge required a conflict to resolve
> after the merge has occurred. The git book has some advice
> (https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging) to use
> `git show` on the merge commit or use `git log --cc -p -1`. These
> strategies work when the merge conflict was resolved with a change
> that is different from either parent. When the conflict is resolved
> with a change that is the same as one of the parents, then these
> commands are indistinguishable from a merge that did not conflict. Is
> it possible to distinguish between a conflict-free merge and a merge
> conflict that is resolved by with the changes from one the parents?

No. You'd have to replay the merge to know if it would have had
conflicts.

There was a patch series a few years ago that added a new diff-mode to
do exactly that, and show the diff against what was resolved. It had a
few issues (I don't remember exactly what) and never got merged.

Certainly one complication is that you don't know exactly _how_ the
merge was done in the first place (e.g., which merge strategy, which
custom merge drivers were in effect, etc). But in general, replaying
with a standard merge-recursive would get you most of what you want to
know.

I've done this manually sometimes when digging into erroneous merges
(e.g., somebody accidentally runs "git reset -- <paths>" in the middle
of a merge and throws away some changes.

You should be able to do:

  git checkout $merge^1
  git merge $merge^2
  git diff -R $merge

to see what the original resolution did.

-Peff

Re: show all merge conflicts

[G. Sylvie Davies]

On Fri, Jan 27, 2017 at 9:51 AM, Jeff King <peff@peff.net> wrote:
> On Fri, Jan 27, 2017 at 11:56:08AM -0500, Michael Spiegel wrote:
>
>> I'm trying to determine whether a merge required a conflict to resolve
>> after the merge has occurred. The git book has some advice
>> (https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging) to use
>> `git show` on the merge commit or use `git log --cc -p -1`. These
>> strategies work when the merge conflict was resolved with a change
>> that is different from either parent. When the conflict is resolved
>> with a change that is the same as one of the parents, then these
>> commands are indistinguishable from a merge that did not conflict. Is
>> it possible to distinguish between a conflict-free merge and a merge
>> conflict that is resolved by with the changes from one the parents?
>
> No. You'd have to replay the merge to know if it would have had
> conflicts.
>


Aside from the usual "git log -cc", I think this should work (replace
HEAD with whichever commit you are analyzing):

git diff --name-only HEAD^2...HEAD^1 > m1
git diff --name-only HEAD^1...HEAD^2 > b1
git diff --name-only HEAD^1..HEAD    > m2
git diff --name-only HEAD^2..HEAD    > b2

If files listed between m1 and b2 differ, then the merge is dirty.
Similarly for m2 and b1.

More information here:

http://stackoverflow.com/questions/27683077/how-do-you-detect-an-evil-merge-in-git/41356308#41356308


- Sylvie



> There was a patch series a few years ago that added a new diff-mode to
> do exactly that, and show the diff against what was resolved. It had a
> few issues (I don't remember exactly what) and never got merged.
>
> Certainly one complication is that you don't know exactly _how_ the
> merge was done in the first place (e.g., which merge strategy, which
> custom merge drivers were in effect, etc). But in general, replaying
> with a standard merge-recursive would get you most of what you want to
> know.
>
> I've done this manually sometimes when digging into erroneous merges
> (e.g., somebody accidentally runs "git reset -- <paths>" in the middle
> of a merge and throws away some changes.
>
> You should be able to do:
>
>   git checkout $merge^1
>   git merge $merge^2
>   git diff -R $merge
>
> to see what the original resolution did.
>
> -Peff

Re: show all merge conflicts

[Philip Oakley]

From: "G. Sylvie Davies" <sylvie@bit-booster.com>
> On Fri, Jan 27, 2017 at 9:51 AM, Jeff King <peff@peff.net> wrote:
>> On Fri, Jan 27, 2017 at 11:56:08AM -0500, Michael Spiegel wrote:
>>
>>> I'm trying to determine whether a merge required a conflict to resolve
>>> after the merge has occurred. The git book has some advice
>>> (https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging) to use
>>> `git show` on the merge commit or use `git log --cc -p -1`. These
>>> strategies work when the merge conflict was resolved with a change
>>> that is different from either parent. When the conflict is resolved
>>> with a change that is the same as one of the parents, then these
>>> commands are indistinguishable from a merge that did not conflict. Is
>>> it possible to distinguish between a conflict-free merge and a merge
>>> conflict that is resolved by with the changes from one the parents?
>>
>> No. You'd have to replay the merge to know if it would have had
>> conflicts.
>>
>
>
> Aside from the usual "git log -cc", I think this should work (replace
> HEAD with whichever commit you are analyzing):
>
> git diff --name-only HEAD^2...HEAD^1 > m1
> git diff --name-only HEAD^1...HEAD^2 > b1
> git diff --name-only HEAD^1..HEAD    > m2
> git diff --name-only HEAD^2..HEAD    > b2
>
> If files listed between m1 and b2 differ, then the merge is dirty.
> Similarly for m2 and b1.
>
> More information here:
>
> http://stackoverflow.com/questions/27683077/how-do-you-detect-an-evil-merge-in-git/41356308#41356308
>
>
> - Sylvie

This feels as though there ought to be some sort of --left-right option to 
get an indication of which side various changes came from

>
>> There was a patch series a few years ago that added a new diff-mode to
>> do exactly that, and show the diff against what was resolved. It had a
>> few issues (I don't remember exactly what) and never got merged.
>>
>> Certainly one complication is that you don't know exactly _how_ the
>> merge was done in the first place (e.g., which merge strategy, which
>> custom merge drivers were in effect, etc). But in general, replaying
>> with a standard merge-recursive would get you most of what you want to
>> know.
>>
>> I've done this manually sometimes when digging into erroneous merges
>> (e.g., somebody accidentally runs "git reset -- <paths>" in the middle
>> of a merge and throws away some changes.
>>
>> You should be able to do:
>>
>>   git checkout $merge^1
>>   git merge $merge^2
>>   git diff -R $merge
>>
>> to see what the original resolution did.
>>
>> -Peff
> 

Re: show all merge conflicts

[Jeff King]

On Fri, Jan 27, 2017 at 09:42:41PM -0800, G. Sylvie Davies wrote:

> Aside from the usual "git log -cc", I think this should work (replace
> HEAD with whichever commit you are analyzing):
> 
> git diff --name-only HEAD^2...HEAD^1 > m1
> git diff --name-only HEAD^1...HEAD^2 > b1
> git diff --name-only HEAD^1..HEAD    > m2
> git diff --name-only HEAD^2..HEAD    > b2
> 
> If files listed between m1 and b2 differ, then the merge is dirty.
> Similarly for m2 and b1.
> 
> More information here:
> 
> http://stackoverflow.com/questions/27683077/how-do-you-detect-an-evil-merge-in-git/41356308#41356308

I don't think that can reliably find evil merges, since it looks at the
file level. If you had one hunk resolved for "theirs" and one hunk for
"ours" in a given file, then the file will be listed in each diff,
whether it has evil hunks or not.

I don't think this is just about evil merges, though. For instance,
try:

  seq 1 10 >file
  git add file
  git commit -m base

  sed s/4/master/ <file >tmp && mv tmp file
  git commit -am master

  git checkout -b other HEAD^
  sed s/4/other/ <file >tmp && mv tmp file
  git commit -am other

  git merge master
  git checkout --ours file
  git commit -am merged

  merge=$(git rev-parse HEAD)

The question is: were there conflicts in $merge, and how were they
resolved?

That isn't an evil merge, but there's still something interesting to
show that "git log --cc" won't display.

Replaying the merge like:

  git checkout $merge^1
  git merge $merge^2
  git diff -R $merge

shows you the patch to go from the conflict state to the final one.

-Peff

Re: show all merge conflicts

[G. Sylvie Davies]

On Sat, Jan 28, 2017 at 6:28 AM, Jeff King <peff@peff.net> wrote:
> On Fri, Jan 27, 2017 at 09:42:41PM -0800, G. Sylvie Davies wrote:
>
>> Aside from the usual "git log -cc", I think this should work (replace
>> HEAD with whichever commit you are analyzing):
>>
>> git diff --name-only HEAD^2...HEAD^1 > m1
>> git diff --name-only HEAD^1...HEAD^2 > b1
>> git diff --name-only HEAD^1..HEAD    > m2
>> git diff --name-only HEAD^2..HEAD    > b2
>>
>> If files listed between m1 and b2 differ, then the merge is dirty.
>> Similarly for m2 and b1.
>>
>> More information here:
>>
>> http://stackoverflow.com/questions/27683077/how-do-you-detect-an-evil-merge-in-git/41356308#41356308
>
> I don't think that can reliably find evil merges, since it looks at the
> file level. If you had one hunk resolved for "theirs" and one hunk for
> "ours" in a given file, then the file will be listed in each diff,
> whether it has evil hunks or not.
>

Well, you have to do both.  Do "git show -c" to catch that one
("theirs" for one hunk, "ours" for the other, same file).

And then do that sequence of the 4 "git diff" commands to identify
dirty merges where "theirs" or "ours" was applied to entire files, and
thus not showing up in the "git show -c".

> I don't think this is just about evil merges, though. For instance,
> try:
>
>   seq 1 10 >file
>   git add file
>   git commit -m base
>
>   sed s/4/master/ <file >tmp && mv tmp file
>   git commit -am master
>
>   git checkout -b other HEAD^
>   sed s/4/other/ <file >tmp && mv tmp file
>   git commit -am other
>
>   git merge master
>   git checkout --ours file
>   git commit -am merged
>
>   merge=$(git rev-parse HEAD)
>
> The question is: were there conflicts in $merge, and how were they
> resolved?
>
> That isn't an evil merge, but there's still something interesting to
> show that "git log --cc" won't display.
>
> Replaying the merge like:
>
>   git checkout $merge^1
>   git merge $merge^2
>   git diff -R $merge
>
> shows you the patch to go from the conflict state to the final one.
>

I know the stackoverflow question asks "how to detect evil merges",
and I go along with that in my answer.  But honestly I prefer to call
them dirty rather than evil, and by "dirty" I just mean merges that
did not resolve cleanly via "git merge", and had some form of user
intervention, be it conflict resolution, or other strange things.

The trick I propose with the sequence of 4 "git diff" commands
identifies that merge from your example as dirty:

$ cat b1 m2
file

$ cat b2 m1
file
file

The trick doesn't really tell you much except that the merge is dirty.
If you notice that the "m2" file is empty, I think that's one way to
realize that master's edit was dropped, and therefore "other" won.

Maybe it even merged cleanly but someone did a "git commit --amend" to
make it the merge dirty after the fact.

I do like your approach, it's very simple and reliable.  But in my
situation I'm writing pre-receive hooks for bare repos, so I don't
think I can actually do "git merge"!

I think my suggestion would work for OP, as long as they also run "git
show -c" alongside it.   (And your suggestion would work, too, of
course).



- Sylvie

Re: show all merge conflicts

[Michael J Gruber]

G. Sylvie Davies venit, vidit, dixit 29.01.2017 07:45:
> On Sat, Jan 28, 2017 at 6:28 AM, Jeff King <peff@peff.net> wrote:
>> On Fri, Jan 27, 2017 at 09:42:41PM -0800, G. Sylvie Davies wrote:
>>
>>> Aside from the usual "git log -cc", I think this should work (replace
>>> HEAD with whichever commit you are analyzing):
>>>
>>> git diff --name-only HEAD^2...HEAD^1 > m1
>>> git diff --name-only HEAD^1...HEAD^2 > b1
>>> git diff --name-only HEAD^1..HEAD    > m2
>>> git diff --name-only HEAD^2..HEAD    > b2
>>>
>>> If files listed between m1 and b2 differ, then the merge is dirty.
>>> Similarly for m2 and b1.
>>>
>>> More information here:
>>>
>>> http://stackoverflow.com/questions/27683077/how-do-you-detect-an-evil-merge-in-git/41356308#41356308
>>
>> I don't think that can reliably find evil merges, since it looks at the
>> file level. If you had one hunk resolved for "theirs" and one hunk for
>> "ours" in a given file, then the file will be listed in each diff,
>> whether it has evil hunks or not.
>>
> 
> Well, you have to do both.  Do "git show -c" to catch that one
> ("theirs" for one hunk, "ours" for the other, same file).
> 
> And then do that sequence of the 4 "git diff" commands to identify
> dirty merges where "theirs" or "ours" was applied to entire files, and
> thus not showing up in the "git show -c".
> 
>> I don't think this is just about evil merges, though. For instance,
>> try:
>>
>>   seq 1 10 >file
>>   git add file
>>   git commit -m base
>>
>>   sed s/4/master/ <file >tmp && mv tmp file
>>   git commit -am master
>>
>>   git checkout -b other HEAD^
>>   sed s/4/other/ <file >tmp && mv tmp file
>>   git commit -am other
>>
>>   git merge master
>>   git checkout --ours file
>>   git commit -am merged
>>
>>   merge=$(git rev-parse HEAD)
>>
>> The question is: were there conflicts in $merge, and how were they
>> resolved?
>>
>> That isn't an evil merge, but there's still something interesting to
>> show that "git log --cc" won't display.
>>
>> Replaying the merge like:
>>
>>   git checkout $merge^1
>>   git merge $merge^2
>>   git diff -R $merge
>>
>> shows you the patch to go from the conflict state to the final one.
>>
> 
> I know the stackoverflow question asks "how to detect evil merges",
> and I go along with that in my answer.  But honestly I prefer to call
> them dirty rather than evil, and by "dirty" I just mean merges that
> did not resolve cleanly via "git merge", and had some form of user
> intervention, be it conflict resolution, or other strange things.
> 
> The trick I propose with the sequence of 4 "git diff" commands
> identifies that merge from your example as dirty:
> 
> $ cat b1 m2
> file
> 
> $ cat b2 m1
> file
> file
> 
> The trick doesn't really tell you much except that the merge is dirty.
> If you notice that the "m2" file is empty, I think that's one way to
> realize that master's edit was dropped, and therefore "other" won.
> 
> Maybe it even merged cleanly but someone did a "git commit --amend" to
> make it the merge dirty after the fact.
> 
> I do like your approach, it's very simple and reliable.  But in my
> situation I'm writing pre-receive hooks for bare repos, so I don't
> think I can actually do "git merge"!
> 
> I think my suggestion would work for OP, as long as they also run "git
> show -c" alongside it.   (And your suggestion would work, too, of
> course).

If you're curious, I kept rebasing Thomas' remerge-diff (on top of our
next) so far. You can find it at

https://github.com/mjg/git/tree/remerge-diff

if you're interested. I don't know what problems were found back then,
or what it would take to get this in-tree now.

Michael

Re: show all merge conflicts

[Junio C Hamano]

Michael J Gruber <git@drmicha.warpmail.net> writes:

> If you're curious, I kept rebasing Thomas' remerge-diff (on top of our
> next) so far. You can find it at
>
> https://github.com/mjg/git/tree/remerge-diff

;-).
Yes, this was a good one.  


> if you're interested. I don't know what problems were found back then,
> or what it would take to get this in-tree now.

If I recall correctly, everybody was in favor of what it does (or at
least attempted to do), but was leaky and not ready for "log -p" to
be used on a long stretch of history or something?

Re: show all merge conflicts

[Jeff King]

On Mon, Feb 27, 2017 at 11:45:31AM -0800, Junio C Hamano wrote:

> Michael J Gruber <git@drmicha.warpmail.net> writes:
> 
> > If you're curious, I kept rebasing Thomas' remerge-diff (on top of our
> > next) so far. You can find it at
> >
> > https://github.com/mjg/git/tree/remerge-diff
> 
> ;-).
> Yes, this was a good one.

FWIW, I have also been carrying it forward. It's not a tool I reach for
often, but a couple of times it has come in very handy (mostly helping
somebody to track down a mistake that somebody made in a merge, like
accidentally using "checkout --ours" on top of a conflict).

> > if you're interested. I don't know what problems were found back then,
> > or what it would take to get this in-tree now.
> 
> If I recall correctly, everybody was in favor of what it does (or at
> least attempted to do), but was leaky and not ready for "log -p" to
> be used on a long stretch of history or something?

The last round was at:

  http://public-inbox.org/git/cover.1409860234.git.tr@thomasrast.ch/

I think. I think the leakiness was dealt with by rebasing onto the
name_hash refactoring. But it looks like there are a lot of little
issues, and maybe one bigger one: it turns "log" from a read-only
operation into that writes into the object database.

-Peff