pull: abort if --ff-only is given and fast-forwarding is impossible

The warning about pulling without specifying how to reconcile divergent
branches says that after setting pull.rebase to true, --ff-only can
still be passed on the command line to require a fast-forward. Make that
actually work.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
[en: updated tests; note 3 fixes and 1 new failure]
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

advice.c

@@ -286,6 +286,11 @@ void NORETURN die_conclude_merge(void)
 	die(_("Exiting because of unfinished merge."));
 }
 
+void NORETURN die_ff_impossible(void)
+{
+	die(_("Not possible to fast-forward, aborting."));
+}
+
 void advise_on_updating_sparse_paths(struct string_list *pathspec_list)
 {
 	struct string_list_item *item;

advice.h

@@ -95,6 +95,7 @@ void advise_if_enabled(enum advice_type type, const char *advice, ...);
 int error_resolve_conflict(const char *me);
 void NORETURN die_resolve_conflict(const char *me);
 void NORETURN die_conclude_merge(void);
+void NORETURN die_ff_impossible(void);
 void advise_on_updating_sparse_paths(struct string_list *pathspec_list);
 void detach_advice(const char *new_name);
 

builtin/merge.c

@@ -1620,7 +1620,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	}
 
 	if (fast_forward == FF_ONLY)
-		die(_("Not possible to fast-forward, aborting."));
+		die_ff_impossible();
 
 	if (autostash)
 		create_autostash(the_repository,

builtin/pull.c

@@ -1046,9 +1046,14 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 
 	can_ff = get_can_ff(&orig_head, &merge_heads.oid[0]);
 
-	if (rebase_unspecified && !opt_ff && !can_ff) {
-		if (opt_verbosity >= 0)
-			show_advice_pull_non_ff();
+	if (!can_ff) {
+		if (opt_ff) {
+			if (!strcmp(opt_ff, "--ff-only"))
+				die_ff_impossible();
+		} else {
+			if (rebase_unspecified && opt_verbosity >= 0)
+				show_advice_pull_non_ff();
+		}
 	}
 
 	if (opt_rebase) {

t/t7601-merge-pull-config.sh

@@ -209,11 +209,11 @@ test_attempts_fast_forward () {
 # Group 1: Interaction of --ff-only with --[no-]rebase
 # (And related interaction of pull.ff=only with pull.rebase)
 #
-test_expect_failure '--ff-only overrides --rebase' '
+test_expect_success '--ff-only overrides --rebase' '
 	test_attempts_fast_forward pull --rebase --ff-only
 '
 
-test_expect_failure '--ff-only overrides --rebase even if first' '
+test_expect_success '--ff-only overrides --rebase even if first' '
 	test_attempts_fast_forward pull --ff-only --rebase
 '
 
@@ -221,7 +221,7 @@ test_expect_success '--ff-only overrides --no-rebase' '
 	test_attempts_fast_forward pull --ff-only --no-rebase
 '
 
-test_expect_failure 'pull.ff=only overrides pull.rebase=true' '
+test_expect_success 'pull.ff=only overrides pull.rebase=true' '
 	test_attempts_fast_forward -c pull.ff=only -c pull.rebase=true pull
 '
 
@@ -252,7 +252,7 @@ test_expect_success 'pull.rebase=true overrides pull.ff=true' '
 '
 
 # Group 3: command line flags take precedence over config
-test_expect_failure '--ff-only takes precedence over pull.rebase=true' '
+test_expect_success '--ff-only takes precedence over pull.rebase=true' '
 	test_attempts_fast_forward -c pull.rebase=true pull --ff-only
 '
 
@@ -264,7 +264,7 @@ test_expect_failure '--no-rebase takes precedence over pull.ff=only' '
 	test_falls_back_to_full_merge -c pull.ff=only pull --no-rebase
 '
 
-test_expect_success '--rebase takes precedence over pull.ff=only' '
+test_expect_failure '--rebase takes precedence over pull.ff=only' '
 	test_does_rebase -c pull.ff=only pull --rebase
 '