pull: --ff-only should make it a noop when already-up-to-date

Earlier, we made sure that "git pull --ff-only" (and "git -c
pull.ff=only pull") errors out when our current HEAD is not an
ancestor of the tip of the history we are merging, but the condition
to trigger the error was implemented incorrectly.

Imagine you forked from a remote branch, built your history on top
of it, and then attempted to pull from them again.  If they have not
made any update in the meantime, our current HEAD is obviously not
their ancestor, and this new error triggers.

Without the --ff-only option, we just report that there is no need
to pull; we did the same historically with --ff-only, too.

Make sure we do not fail with the recently added check to restore
the historical behaviour.

Reported-by: Kenneth Arnold <ka37@calvin.edu>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pull.c

@@ -933,6 +933,33 @@ static int get_can_ff(struct object_id *orig_head,
 	return ret;
 }
 
+/*
+ * Is orig_head a descendant of _all_ merge_heads?
+ * Unfortunately is_descendant_of() cannot be used as it asks
+ * if orig_head is a descendant of at least one of them.
+ */
+static int already_up_to_date(struct object_id *orig_head,
+			      struct oid_array *merge_heads)
+{
+	int i;
+	struct commit *ours;
+
+	ours = lookup_commit_reference(the_repository, orig_head);
+	for (i = 0; i < merge_heads->nr; i++) {
+		struct commit_list *list = NULL;
+		struct commit *theirs;
+		int ok;
+
+		theirs = lookup_commit_reference(the_repository, &merge_heads->oid[i]);
+		commit_list_insert(theirs, &list);
+		ok = repo_is_descendant_of(the_repository, ours, list);
+		free_commit_list(list);
+		if (!ok)
+			return 0;
+	}
+	return 1;
+}
+
 static void show_advice_pull_non_ff(void)
 {
 	advise(_("You have divergent branches and need to specify how to reconcile them.\n"
@@ -1074,7 +1101,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 
 	/* ff-only takes precedence over rebase */
 	if (opt_ff && !strcmp(opt_ff, "--ff-only")) {
-		if (!can_ff)
+		if (!can_ff && !already_up_to_date(&orig_head, &merge_heads))
 			die_ff_impossible();
 		opt_rebase = REBASE_FALSE;
 	}

t/t7601-merge-pull-config.sh

@@ -2,7 +2,7 @@
 
 test_description='git merge
 
-Testing pull.* configuration parsing.'
+Testing pull.* configuration parsing and other things.'
 
 . ./test-lib.sh
 
@@ -387,6 +387,20 @@ test_expect_success 'pull prevents non-fast-forward with "only" in pull.ff' '
 	test_must_fail git pull . c3
 '
 
+test_expect_success 'already-up-to-date pull succeeds with "only" in pull.ff' '
+	git reset --hard c1 &&
+	test_config pull.ff only &&
+	git pull . c0 &&
+	test "$(git rev-parse HEAD)" = "$(git rev-parse c1)"
+'
+
+test_expect_success 'already-up-to-date pull/rebase succeeds with "only" in pull.ff' '
+	git reset --hard c1 &&
+	test_config pull.ff only &&
+	git -c pull.rebase=true pull . c0 &&
+	test "$(git rev-parse HEAD)" = "$(git rev-parse c1)"
+'
+
 test_expect_success 'merge c1 with c2 (ours in pull.twohead)' '
 	git reset --hard c1 &&
 	git config pull.twohead ours &&