[PATCH 6/6] Introduce fast forward option only

[Junio C Hamano <gitster@pobox.com>]

From: Sverre Hvammen Johansen <hvammen@gmail.com>

This commit introduces fast forward option 'only'.  With --ff=only
merge succeeds only if it resolves to fast-forward merge.

This feature is useful for cases where a rebase is desired
instead of a real merge.  This option can then be used to
avoid an accidental merge.

See the documentation for further details.

Signed-off-by: Sverre Hvammen Johansen <hvammen@gmail.com>
---
 Documentation/fast-forward-options.txt |    9 ++
 git-merge.sh                           |   12 +-
 git-pull.sh                            |    2 +-
 t/t7601-merge-ff-options.sh            |  214 ++++++++++++++++++++++++++++++++
 4 files changed, 231 insertions(+), 6 deletions(-)

diff --git a/Documentation/fast-forward-options.txt b/Documentation/fast-forward-options.txt
index 9374aa9..41580ea 100644
--- a/Documentation/fast-forward-options.txt
+++ b/Documentation/fast-forward-options.txt
@@ -12,6 +12,10 @@ never::
 	Generate a merge commit even if the merge resolves as a
 	fast-forward.  This option is equivalent of '--no-ff'.
 
+only::
+	Only allow a fast-forward.  The merge will fail unless HEAD is
+	up to date or the merge resolves as a fast-forward.
+
 If your workflow is always to branch from the special branch
 ("master") when working on a topic and merge that back to "master", if
 you happen to have worked only on a single topic and the "master" was
@@ -42,3 +46,8 @@ The first merge of topicA or the only merge of topicB would have
 resulted in a fast forward without '--ff=never'.  Topic A consist of
 those commits that can be reached from master^2 without passing
 through any of the first-parent ancestries of master.
+
+However, if the workflow require that the branch you are merging with
+is based on the current HEAD you can use "only fast forward" policy to
+enforce fast forward or a failure.  The last merge of topicA in
+the example above would have failed with '--ff=only'.
diff --git a/git-merge.sh b/git-merge.sh
index 775dae7..b87e125 100755
--- a/git-merge.sh
+++ b/git-merge.sh
@@ -168,21 +168,21 @@ parse_config () {
 			no_commit=t ;;
 		--ff)
 			case "$2" in
-			allow|never)
+			allow|never|only)
 				fast_forward=$2; shift ;;
 			-*)
 				fast_forward=allow ;;
 			*)
-				die "Available fast-forward options are: allow and newer" ;;
+				die "Available fast-forward options are: allow, newer, and only" ;;
 			esac
 			;;
 		--ff=*)
 			fast_forward=${1#--ff=}
 			case "$fast_forward" in
-			allow|never)
+			allow|never|only)
 				;;
 			*)
-				die "Available fast-forward options are: allow and newer" ;;
+				die "Available fast-forward options are: allow, newer, and only" ;;
 			esac
 			;;
 		--no-ff)
@@ -209,7 +209,7 @@ parse_config () {
 		shift
 	done
 	test "$fast_forward" = allow -o "$squash" = "" ||
-		die "You cannot combine --squash with --ff=never"
+		die "You cannot combine --squash with --ff=never or --ff=only."
 	args_left=$#
 }
 
@@ -347,6 +347,8 @@ find_reduced_parents "$@"
 # ff_head may be included here or later in actual parents
 if test -n "$reduced_parents"
 then
+	test $fast_forward = only &&
+		die "--ff=only can not handle more than one real parent"
 	test $head = $ff_head ||
 		reduced_parents="$ff_head$LF$reduced_parents"
 fi
diff --git a/git-pull.sh b/git-pull.sh
index 9e91e75..c5fa1ee 100755
--- a/git-pull.sh
+++ b/git-pull.sh
@@ -41,7 +41,7 @@ do
 		no_ff=--ff ;;
 	--no-ff)
 		no_ff=--no-ff ;;
-	--ff=allow|--ff=never)
+	--ff=allow|--ff=only|--ff=never)
 		no_ff=$1 ;;
 	-s=*|--s=*|--st=*|--str=*|--stra=*|--strat=*|--strate=*|\
 		--strateg=*|--strategy=*|\
diff --git a/t/t7601-merge-ff-options.sh b/t/t7601-merge-ff-options.sh
index c7c6d14..56e8370 100755
--- a/t/t7601-merge-ff-options.sh
+++ b/t/t7601-merge-ff-options.sh
@@ -662,4 +662,218 @@ test_expect_success 'merge c1 with new repository (pull --ff=never)' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'merge c0 with c1 (--ff=only overrides --no-ff)' '
+	git reset --hard c0 &&
+	git config branch.master.mergeoptions "--no-ff" &&
+	git merge --ff=only c1 &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c0 with c1 (--ff=only in config)' '
+	git reset --hard c0 &&
+	git config branch.master.mergeoptions "--ff=only" &&
+	git merge c1 &&
+	test_tick &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c0 (--ff=only in config)' '
+	git reset --hard c1 &&
+	git config branch.master.mergeoptions "--ff=only" &&
+	git merge c0 &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c2 (--ff=only in config)' '
+	git reset --hard c1 &&
+	test_tick &&
+	git config branch.master.mergeoptions "--ff=only" &&
+	test_must_fail git merge c2 &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c0 with c1 (--ff=only)' '
+	git reset --hard c0 &&
+	test_tick &&
+	git merge --ff=only c1 &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c0 (--ff=only)' '
+	git reset --hard c1 &&
+	test_tick &&
+	git merge --ff=only c0 &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c0 with c1 and c2 (--ff=only)' '
+	git reset --hard c0 &&
+	test_must_fail git merge --ff=only c1 c2 &&
+	verify_merge file result.0 &&
+	verify_head $c0
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c0 (--ff=only)' '
+	git reset --hard c1 &&
+	test_tick &&
+	git merge --ff=only c0 &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c2 (--ff=only overrides --no-ff)' '
+	git reset --hard c1 &&
+	git config branch.master.mergeoptions "--no-ff" &&
+	test_tick &&
+	test_must_fail git merge c2 --ff=only &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c0 with c1 (--no-ff overrides --ff=only)' '
+	git reset --hard c0 &&
+	git config branch.master.mergeoptions "--ff=only" &&
+	test_tick &&
+	git merge --no-ff c1 &&
+	verify_merge file result.1 &&
+	verify_parents $c0 $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c2 (--ff owerrides --ff=only)' '
+	git reset --hard c1 &&
+	git config branch.master.mergeoptions "--ff=only" &&
+	test_tick &&
+	git merge --ff c2 &&
+	verify_merge file result.1-5 &&
+	verify_parents $c1 $c2
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with x0 (--squash combined with --ff=only)' '
+	git reset --hard c1 &&
+	git config branch.master.mergeoptions "" &&
+	test_tick &&
+	test_must_fail git merge x0 --squash --ff=only &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+
+test_expect_success 'merge x0 with c1 (--squash combined with --ff=only)' '
+	git reset --hard x0 &&
+	git config branch.master.mergeoptions "" &&
+	test_tick &&
+	test_must_fail git merge c1 --squash --ff=only &&
+	verify_merge file result.1-5 &&
+	verify_head $x0
+'
+
+test_debug 'gitk --all'
+
+
+test_expect_success 'merge c1 with c2 (--squash combined with --ff=only)' '
+	git reset --hard c1 &&
+	git config branch.master.mergeoptions "" &&
+	test_tick &&
+	test_must_fail git merge c2 --squash --ff=only &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+
+test_expect_success 'merge c1 with x0 (--no-commit combined with --ff=only)' '
+	git reset --hard c1 &&
+	git config branch.master.mergeoptions "" &&
+	test_tick &&
+	git merge x0 --no-commit --ff=only &&
+	verify_merge file result.1-5 &&
+	verify_head $x0
+'
+
+test_debug 'gitk --all'
+
+
+test_expect_success 'merge x0 with c1 (--no-commit combined with --ff=only)' '
+	git reset --hard x0 &&
+	git config branch.master.mergeoptions "" &&
+	test_tick &&
+	git merge c1 --no-commit --ff=only &&
+	verify_merge file result.1-5 &&
+	verify_head $x0
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c2 (--no-commit combined with --ff=only)' '
+	git reset --hard c1 &&
+	git config branch.master.mergeoptions "" &&
+	test_tick &&
+	test_must_fail git merge c2 --no-commit --ff=only &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with x1 (pull --ff=only)' '
+	git reset --hard c1 &&
+	test_tick &&
+	git pull --ff=only clone refs/heads/master &&
+	verify_merge file result.1-13 &&
+	verify_head $x1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge x2 with x1 (pull --ff=only)' '
+	git reset --hard x2 &&
+	test_tick &&
+	test_must_fail git pull --ff=only clone refs/heads/master &&
+	verify_merge file result.5-13 &&
+	verify_head $x2
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with new repository (pull --ff=only)' '
+	git reset --hard c1 &&
+	test_tick &&
+	test_must_fail git pull --ff=only new refs/heads/master &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
 test_done
-- 
1.5.5.1.499.g878b8