Merge branch 'tk/simple-autosetupmerge' into jch

"git -c branch.autosetupmerge=simple branch $A $B" will set the $B
as $A's upstream only when $A and $B shares the same name, and "git
-c push.default=simple" on branch $A would push to update the
branch $A at the remote $B came from.

* tk/simple-autosetupmerge:
  t3200: tests for new branch.autosetupmerge option "simple"
  merge: new autosetupmerge option 'simple' for matching branches

Documentation/config/branch.txt

@@ -9,7 +9,9 @@ branch.autoSetupMerge::
 	automatic setup is done when the starting point is either a
 	local branch or remote-tracking branch; `inherit` -- if the starting point
 	has a tracking configuration, it is copied to the new
-	branch. This option defaults to true.
+	branch; `simple` -- automatic setup is done only when the starting point
+	is a remote-tracking branch and the new branch has the same name as the
+	remote branch. This option defaults to true.
 
 branch.autoSetupRebase::
 	When a new branch is created with 'git branch', 'git switch' or 'git checkout'

Documentation/git-branch.txt

@@ -221,13 +221,17 @@ The exact upstream branch is chosen depending on the optional argument:
 itself as the upstream; `--track=inherit` means to copy the upstream
 configuration of the start-point branch.
 +
-`--track=direct` is the default when the start point is a remote-tracking branch.
-Set the branch.autoSetupMerge configuration variable to `false` if you
-want `git switch`, `git checkout` and `git branch` to always behave as if `--no-track`
-were given. Set it to `always` if you want this behavior when the
-start-point is either a local or remote-tracking branch. Set it to
-`inherit` if you want to copy the tracking configuration from the
-branch point.
+The branch.autoSetupMerge configuration variable specifies how `git switch`,
+`git checkout` and `git branch` should behave when neither `--track` nor
+`--no-track` are specified:
++
+The default option, `true`, behaves as though `--track=direct`
+were given whenever the start-point is a remote-tracking branch.
+`false` behaves as if `--no-track` were given. `always` behaves as though
+`--track=direct` were given. `inherit` behaves as though `--track=inherit`
+were given. `simple` behaves as though `--track=direct` were given only when
+the start-point is a remote-tracking branch and the new branch has the same
+name as the remote branch.
 +
 See linkgit:git-pull[1] and linkgit:git-config[1] for additional discussion on
 how the `branch.<name>.remote` and `branch.<name>.merge` options are used.

branch.c

@@ -252,10 +252,29 @@ static void setup_tracking(const char *new_ref, const char *orig_ref,
 			goto cleanup;
 		}
 
+	/*
+	 * This check does not apply to the BRANCH_TRACK_INHERIT
+	 * option; you can inherit one or more tracking entries
+	 * and the tracking.matches counter is not incremented.
+	 */
 	if (tracking.matches > 1)
 		die(_("not tracking: ambiguous information for ref %s"),
 		    orig_ref);
 
+	if (track == BRANCH_TRACK_SIMPLE) {
+		/*
+		 * Only track if remote branch name matches.
+		 * Reaching into items[0].string is safe because
+		 * we know there is at least one and not more than
+		 * one entry (because not BRANCH_TRACK_INHERIT).
+		 */
+		const char *tracked_branch;
+		if (!skip_prefix(tracking.srcs->items[0].string,
+				 "refs/heads/", &tracked_branch) ||
+		    strcmp(tracked_branch, new_ref))
+			return;
+	}
+
 	if (tracking.srcs->nr < 1)
 		string_list_append(tracking.srcs, orig_ref);
 	if (install_branch_config_multiple_remotes(config_flags, new_ref,

branch.h

@@ -12,6 +12,7 @@ enum branch_track {
 	BRANCH_TRACK_EXPLICIT,
 	BRANCH_TRACK_OVERRIDE,
 	BRANCH_TRACK_INHERIT,
+	BRANCH_TRACK_SIMPLE,
 };
 
 extern enum branch_track git_branch_track;

config.c

@@ -1686,6 +1686,9 @@ static int git_default_branch_config(const char *var, const char *value)
 		} else if (value && !strcmp(value, "inherit")) {
 			git_branch_track = BRANCH_TRACK_INHERIT;
 			return 0;
+		} else if (value && !strcmp(value, "simple")) {
+			git_branch_track = BRANCH_TRACK_SIMPLE;
+			return 0;
 		}
 		git_branch_track = git_config_bool(var, value);
 		return 0;

t/t3200-branch.sh

@@ -886,6 +886,41 @@ test_expect_success 'branch from tag w/--track causes failure' '
 	test_must_fail git branch --track my11 foobar
 '
 
+test_expect_success 'simple tracking works when remote branch name matches' '
+	test_create_repo otherserver &&
+	test_commit -C otherserver my_commit 1 &&
+	git -C otherserver branch feature &&
+	git config branch.autosetupmerge simple &&
+	git config remote.otherserver.url otherserver &&
+	git config remote.otherserver.fetch refs/heads/*:refs/remotes/otherserver/* &&
+	git fetch otherserver &&
+	git branch feature otherserver/feature &&
+	rm -fr otherserver &&
+	test $(git config branch.feature.remote) = otherserver &&
+	test $(git config branch.feature.merge) = refs/heads/feature
+'
+
+test_expect_success 'simple tracking skips when remote branch name does not match' '
+	git config branch.autosetupmerge simple &&
+	git config remote.local.url . &&
+	git config remote.local.fetch refs/heads/*:refs/remotes/local/* &&
+	(git show-ref -q refs/remotes/local/main || git fetch local) &&
+	git branch my-other local/main &&
+	test -z "$(git config branch.my-other.remote)" &&
+	test -z "$(git config branch.my-other.merge)"
+'
+
+test_expect_success 'simple tracking skips when remote ref is not a branch' '
+	git config branch.autosetupmerge simple &&
+	git tag mytag12 main &&
+	git config remote.localtags.url . &&
+	git config remote.localtags.fetch refs/tags/*:refs/remotes/localtags/* &&
+	(git show-ref -q refs/remotes/localtags/mytag12 || git fetch localtags) &&
+	git branch mytag12 localtags/mytag12 &&
+	test -z "$(git config branch.mytag12.remote)" &&
+	test -z "$(git config branch.mytag12.merge)"
+'
+
 test_expect_success '--set-upstream-to fails on multiple branches' '
 	echo "fatal: too many arguments to set new upstream" >expect &&
 	test_must_fail git branch --set-upstream-to main a b c 2>err &&