Teach 'git merge' and 'git pull' the option --ff-only

For convenience in scripts and aliases, add the option
--ff-only to only allow fast-forwards (and up-to-date,
despite the name).

Disallow combining --ff-only and --no-ff, since they
flatly contradict each other.

Allow all other options to be combined with --ff-only
(i.e. do not add any code to handle them specially),
including the following options:

* --strategy (one or more): As long as the chosen merge
  strategy results in up-to-date or fast-forward, the
  command will succeed.

* --squash: I cannot imagine why anyone would want to
  squash commits only if fast-forward is possible, but I
  also see no reason why it should not be allowed.

* --message: The message will always be ignored, but I see
  no need to explicitly disallow providing a redundant message.

Acknowledgements: I did look at Yuval Kogman's earlier
patch (107768 in gmane), mainly as shortcut to find my
way in the code, but I did not copy anything directly.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index e5b210bc96..57f6d2bae7 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -243,6 +243,16 @@ test_expect_success 'merge c0 with c1' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'merge c0 with c1 with --ff-only' '
+	git reset --hard c0 &&
+	git merge --ff-only c1 &&
+	git merge --ff-only HEAD c0 c1 &&
+	verify_merge file result.1 &&
+	verify_head "$c1"
+'
+
+test_debug 'gitk --all'
+
 test_expect_success 'merge c1 with c2' '
 	git reset --hard c1 &&
 	test_tick &&
@@ -263,6 +273,14 @@ test_expect_success 'merge c1 with c2 and c3' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'failing merges with --ff-only' '
+	git reset --hard c1 &&
+	test_tick &&
+	test_must_fail git merge --ff-only c2 &&
+	test_must_fail git merge --ff-only c3 &&
+	test_must_fail git merge --ff-only c2 c3
+'
+
 test_expect_success 'merge c0 with c1 (no-commit)' '
 	git reset --hard c0 &&
 	git merge --no-commit c1 &&
@@ -303,6 +321,17 @@ test_expect_success 'merge c0 with c1 (squash)' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'merge c0 with c1 (squash, ff-only)' '
+	git reset --hard c0 &&
+	git merge --squash --ff-only c1 &&
+	verify_merge file result.1 &&
+	verify_head $c0 &&
+	verify_no_mergehead &&
+	verify_diff squash.1 .git/SQUASH_MSG "[OOPS] bad squash message"
+'
+
+test_debug 'gitk --all'
+
 test_expect_success 'merge c1 with c2 (squash)' '
 	git reset --hard c1 &&
 	git merge --squash c2 &&
@@ -314,6 +343,13 @@ test_expect_success 'merge c1 with c2 (squash)' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'unsuccesful merge of c1 with c2 (squash, ff-only)' '
+	git reset --hard c1 &&
+	test_must_fail git merge --squash --ff-only c2
+'
+
+test_debug 'gitk --all'
+
 test_expect_success 'merge c1 with c2 and c3 (squash)' '
 	git reset --hard c1 &&
 	git merge --squash c2 c3 &&
@@ -432,6 +468,11 @@ 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 'merge c0 with c1 (ff overrides no-ff)' '
 	git reset --hard c0 &&
 	git config branch.master.mergeoptions "--no-ff" &&