[PATCH] merge: allow using --no-ff and --ff-only at the same time

[Miklos Vajna <vmiklos@suse.cz>]

1347483 (Teach 'git merge' and 'git pull' the option --ff-only,
2009-10-29) says this is not allowed, as they contradict each other.

However, --ff-only is about asserting the input of the merge, and
--no-ff is about instructing merge to always create a merge commit, i.e.
it makes sense to use these options together in some workflow, e.g. when
branches are integrated by rebasing then merging, and the maintainer
wants to be sure the branch is rebased.

Signed-off-by: Miklos Vajna <vmiklos@suse.cz>
---
 builtin/merge.c  | 12 ++++++++----
 t/t7600-merge.sh | 11 ++++++++---
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/builtin/merge.c b/builtin/merge.c
index 2ebe732..7026ce0 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -1162,9 +1162,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		option_commit = 0;
 	}
 
-	if (!allow_fast_forward && fast_forward_only)
-		die(_("You cannot combine --no-ff with --ff-only."));
-
 	if (!abort_current_merge) {
 		if (!argc) {
 			if (default_to_upstream)
@@ -1433,7 +1430,14 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	if (fast_forward_only)
+	/*
+	 * If --ff-only was used without --no-ff, or: --ff-only and --no-ff was
+	 * used at the same time, and this is not a fast-forward.
+	 */
+	if (fast_forward_only && (allow_fast_forward || remoteheads->next ||
+				common->next ||
+				hashcmp(common->item->object.sha1,
+					head_commit->object.sha1)))
 		die(_("Not possible to fast-forward, aborting."));
 
 	/* We are going to make a new commit. */
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 460d8eb..bf3d9b2 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -497,9 +497,14 @@ test_expect_success 'combining --squash and --no-ff is refused' '
 	test_must_fail git merge --no-ff --squash c1
 '
 
-test_expect_success 'combining --ff-only and --no-ff is refused' '
-	test_must_fail git merge --ff-only --no-ff c1 &&
-	test_must_fail git merge --no-ff --ff-only c1
+test_expect_success 'combining --ff-only and --no-ff (ff is possible)' '
+	git reset --hard c0 &&
+	git merge --ff-only --no-ff c1
+'
+
+test_expect_success 'combining --ff-only and --no-ff (ff is not possible)' '
+	git reset --hard c1 &&
+	test_must_fail git merge --ff-only --no-ff c2
 '
 
 test_expect_success 'merge c0 with c1 (ff overrides no-ff)' '
-- 
1.8.1.4