[BUG] git-merge-octopus creates an empty merge commit with one parent

[BUG] git-merge-octopus creates an empty merge commit with one parent

[Michał Kiedrowicz]

This happens when git merge is run to merge multiple commits that are
descendants of current HEAD (or are HEAD).  We've hit this while updating master
to origin/master but accidentaly we called (while being on master):

	$ git merge master origin/master

Here is a minimal testcase:

	$ git init a

	$ cd a

	$ echo a>a

	$ git commit -minitial

	$ echo b>a

	$ git add a

	$ git commit -msecond

	$ git checkout master^

	$ git merge master master
	Fast-forwarding to: master
	Already up-to-date with master
	Merge made by the 'octopus' strategy.
	 a |    2 +-
	  1 files changed, 1 insertions(+), 1 deletions(-)

	$ git cat-file commit HEAD
	tree eebfed94e75e7760540d1485c740902590a00332
	parent bd679e85202280b263e20a57639a142fa14c2c64
	author Michał Kiedrowicz <michal.kiedrowicz@gmail.com> 1329132996 +0100
	committer Michał Kiedrowicz <michal.kiedrowicz@gmail.com> 1329132996 +0100

	Merge branches 'master' and 'master' into HEAD

... and below is a patch that adds a testcase to Git's testsuite.

I would expect `git merge master master` to just 'Fast forward'.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
---
 t/t6028-merge-up-to-date.sh |   17 ++++++++++++++++-
 1 files changed, 16 insertions(+), 1 deletions(-)

diff --git a/t/t6028-merge-up-to-date.sh b/t/t6028-merge-up-to-date.sh
index a91644e..824fca5 100755
--- a/t/t6028-merge-up-to-date.sh
+++ b/t/t6028-merge-up-to-date.sh
@@ -16,7 +16,12 @@ test_expect_success setup '
 	test_tick &&
 	git commit -m second &&
 	git tag c1 &&
-	git branch test
+	git branch test &&
+	echo third >file &&
+	git add file &&
+	test_tick &&
+	git commit -m third &&
+	git tag c2
 '
 
 test_expect_success 'merge -s recursive up-to-date' '
@@ -74,4 +79,14 @@ test_expect_success 'merge -s subtree up-to-date' '
 
 '
 
+test_expect_failure 'merge fast-forward octopus' '
+
+	git reset --hard c0 &&
+	test_tick &&
+	git merge c1 c2
+	expect=$(git rev-parse c2) &&
+	current=$(git rev-parse HEAD) &&
+	test "$expect" = "$current"
+'
+
 test_done
-- 
1.7.9.rc2.155.g2e96

Re: [BUG] git-merge-octopus creates an empty merge commit with one parent

[Junio C Hamano]

Michał Kiedrowicz <michal.kiedrowicz@gmail.com> writes:

> This happens when git merge is run to merge multiple commits that are
> descendants of current HEAD (or are HEAD).

I am reasonably sure you meant ancestors here.

> to origin/master but accidentaly we called (while being on master):
>
> 	$ git merge master origin/master

I am very tempted to throw this into "don't do it then" category.

Re: [BUG] git-merge-octopus creates an empty merge commit with one parent

[Michał Kiedrowicz]

Junio C Hamano <gitster@pobox.com> wrote:

> Michał Kiedrowicz <michal.kiedrowicz@gmail.com> writes:
> 
> > This happens when git merge is run to merge multiple commits that are
> > descendants of current HEAD (or are HEAD).
> 
> I am reasonably sure you meant ancestors here.

I meant: ... to merge commits whose parent, grand-parent or
grand-grand-...-parent is HEAD. Commits to which HEAD may be
fast-forwarded.

> 
> > to origin/master but accidentaly we called (while being on master):
> >
> > 	$ git merge master origin/master
> 
> I am very tempted to throw this into "don't do it then" category.

I'm all for it. This is not a thing I would want to be doing. At
no time I would call this "a serious bug that needs fixing right
now". Except that I still think this behavior is not correct. git-merge
should not create commits that even aren't merges (they have single
parrent) and have exactly the same tree as its parent, and give it a
message "merge". Isn't this against the normal behavior of Git:
forwarding if possible and refusing to create commits that don't change
any file?

Re: [BUG] git-merge-octopus creates an empty merge commit with one parent

[Junio C Hamano]

Michał Kiedrowicz <michal.kiedrowicz@gmail.com> writes:

> ... Except that I still think this behavior is not correct.

Well, what was missing from my "don't do it then" quote was "Doctor it
HURTS when I do this."; there is no question that it is not correct if it
hurts ;-).

Patches welcome; I suspect it shouldn't be too intrusive or difficult.

Re: [BUG] git-merge-octopus creates an empty merge commit with one parent

[Michał Kiedrowicz]

Junio C Hamano <gitster <at> pobox.com> writes:

> Well, what was missing from my "don't do it then" quote was "Doctor it
> HURTS when I do this."; there is no question that it is not correct if it
> hurts .

All right :)