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

From: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Date: Feb 13, 2012, 11:48 UTC
Message-ID: <1329133736-20817-1-git-send-email-michal.kiedrowicz@gmail.com>

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