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>

Documentation/merge-options.txt

@@ -60,6 +60,11 @@
 	a fast-forward, only update the branch pointer. This is
 	the default behavior of git-merge.
 
+--ff-only::
+	Refuse to merge and exit with a non-zero status unless the
+	current `HEAD` is already up-to-date or the merge can be
+	resolved as a fast-forward.
+
 -s <strategy>::
 --strategy=<strategy>::
 	Use the given merge strategy; can be supplied more than

builtin-merge.c

@@ -43,6 +43,7 @@ static const char * const builtin_merge_usage[] = {
 
 static int show_diffstat = 1, option_log, squash;
 static int option_commit = 1, allow_fast_forward = 1;
+static int fast_forward_only;
 static int allow_trivial = 1, have_message;
 static struct strbuf merge_msg;
 static struct commit_list *remoteheads;
@@ -167,6 +168,8 @@ static struct option builtin_merge_options[] = {
 		"perform a commit if the merge succeeds (default)"),
 	OPT_BOOLEAN(0, "ff", &allow_fast_forward,
 		"allow fast forward (default)"),
+	OPT_BOOLEAN(0, "ff-only", &fast_forward_only,
+		"abort if fast forward is not possible"),
 	OPT_CALLBACK('s', "strategy", &use_strategies, "strategy",
 		"merge strategy to use", option_parse_strategy),
 	OPT_CALLBACK('m', "message", &merge_msg, "message",
@@ -874,6 +877,9 @@ 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 (!argc)
 		usage_with_options(builtin_merge_usage,
 			builtin_merge_options);
@@ -1040,7 +1046,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		 * only one common.
 		 */
 		refresh_cache(REFRESH_QUIET);
-		if (allow_trivial) {
+		if (allow_trivial && !fast_forward_only) {
 			/* See if it is really trivial. */
 			git_committer_info(IDENT_ERROR_ON_NO_NAME);
 			printf("Trying really trivial in-index merge...\n");
@@ -1079,6 +1085,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	if (fast_forward_only)
+		die("Not possible to fast forward, aborting.");
+
 	/* We are going to make a new commit. */
 	git_committer_info(IDENT_ERROR_ON_NO_NAME);
 

git-pull.sh

@@ -16,7 +16,8 @@ cd_to_toplevel
 test -z "$(git ls-files -u)" ||
 	die "You are in the middle of a conflicted merge."
 
-strategy_args= diffstat= no_commit= squash= no_ff= log_arg= verbosity=
+strategy_args= diffstat= no_commit= squash= no_ff= ff_only=
+log_arg= verbosity=
 curr_branch=$(git symbolic-ref -q HEAD)
 curr_branch_short=$(echo "$curr_branch" | sed "s|refs/heads/||")
 rebase=$(git config --bool branch.$curr_branch_short.rebase)
@@ -45,6 +46,8 @@ do
 		no_ff=--ff ;;
 	--no-ff)
 		no_ff=--no-ff ;;
+	--ff-only)
+		ff_only=--ff-only ;;
 	-s=*|--s=*|--st=*|--str=*|--stra=*|--strat=*|--strate=*|\
 		--strateg=*|--strategy=*|\
 	-s|--s|--st|--str|--stra|--strat|--strate|--strateg|--strategy)
@@ -215,5 +218,5 @@ merge_name=$(git fmt-merge-msg $log_arg <"$GIT_DIR/FETCH_HEAD") || exit
 test true = "$rebase" &&
 	exec git-rebase $diffstat $strategy_args --onto $merge_head \
 	${oldremoteref:-$merge_head}
-exec git-merge $diffstat $no_commit $squash $no_ff $log_arg $strategy_args \
+exec git-merge $diffstat $no_commit $squash $no_ff $ff_only $log_arg $strategy_args \
 	"$merge_name" HEAD $merge_head $verbosity

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" &&