submodule: teach set-branch subcommand

This teaches git-submodule the set-branch subcommand which allows the
branch of a submodule to be set through a porcelain command without
having to manually manipulate the .gitmodules file.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -14,6 +14,7 @@ SYNOPSIS
 'git submodule' [--quiet] init [--] [<path>...]
 'git submodule' [--quiet] deinit [-f|--force] (--all|[--] <path>...)
 'git submodule' [--quiet] update [<options>] [--] [<path>...]
+'git submodule' [--quiet] set-branch [<options>] [--] <path>
 'git submodule' [--quiet] summary [<options>] [--] [<path>...]
 'git submodule' [--quiet] foreach [--recursive] <command>
 'git submodule' [--quiet] sync [--recursive] [--] [<path>...]
@@ -168,6 +169,12 @@ submodule with the `--init` option.
 If `--recursive` is specified, this command will recurse into the
 registered submodules, and update any nested submodules within.
 --
+set-branch ((-d|--default)|(-b|--branch <branch>)) [--] <path>::
+	Sets the default remote tracking branch for the submodule. The
+	`--branch` option allows the remote branch to be specified. The
+	`--default` option removes the submodule.<name>.branch configuration
+	key, which causes the tracking branch to default to 'master'.
+
 summary [--cached|--files] [(-n|--summary-limit) <n>] [commit] [--] [<path>...]::
 	Show commit summary between the given commit (defaults to HEAD) and
 	working tree/index. For a submodule in question, a series of commits

contrib/completion/git-completion.bash

@@ -2573,7 +2573,7 @@ _git_submodule ()
 {
 	__git_has_doubledash && return
 
-	local subcommands="add status init deinit update summary foreach sync"
+	local subcommands="add status init deinit update set-branch summary foreach sync"
 	local subcommand="$(__git_find_on_cmdline "$subcommands")"
 	if [ -z "$subcommand" ]; then
 		case "$cur" in
@@ -2604,6 +2604,9 @@ _git_submodule ()
 			--force --rebase --merge --reference --depth --recursive --jobs
 		"
 		;;
+	set-branch,--*)
+		__gitcomp "--default --branch"
+		;;
 	summary,--*)
 		__gitcomp "--cached --files --summary-limit"
 		;;

git-submodule.sh

@@ -10,6 +10,7 @@ USAGE="[--quiet] add [-b <branch>] [-f|--force] [--name <name>] [--reference <re
    or: $dashless [--quiet] init [--] [<path>...]
    or: $dashless [--quiet] deinit [-f|--force] (--all| [--] <path>...)
    or: $dashless [--quiet] update [--init] [--remote] [-N|--no-fetch] [-f|--force] [--checkout|--merge|--rebase] [--[no-]recommend-shallow] [--reference <repository>] [--recursive] [--] [<path>...]
+   or: $dashless [--quiet] set-branch (--default|--branch <branch>) [--] <path>
    or: $dashless [--quiet] summary [--cached|--files] [--summary-limit <n>] [commit] [--] [<path>...]
    or: $dashless [--quiet] foreach [--recursive] <command>
    or: $dashless [--quiet] sync [--recursive] [--] [<path>...]
@@ -684,6 +685,72 @@ cmd_update()
 	}
 }
 
+#
+# Configures a submodule's default branch
+#
+# $@ = requested path
+#
+cmd_set_branch() {
+	unset_branch=false
+	branch=
+
+	while test $# -ne 0
+	do
+		case "$1" in
+		-q|--quiet)
+			# we don't do anything with this but we need to accept it
+			;;
+		-d|--default)
+			unset_branch=true
+			;;
+		-b|--branch)
+			case "$2" in '') usage ;; esac
+			branch=$2
+			shift
+			;;
+		--)
+			shift
+			break
+			;;
+		-*)
+			usage
+			;;
+		*)
+			break
+			;;
+		esac
+		shift
+	done
+
+	if test $# -ne 1
+	then
+		usage
+	fi
+
+	# we can't use `git submodule--helper name` here because internally, it
+	# hashes the path so a trailing slash could lead to an unintentional no match
+	name="$(git submodule--helper list "$1" | cut -f2)"
+	if test -z "$name"
+	then
+		exit 1
+	fi
+
+	test -n "$branch"; has_branch=$?
+	test "$unset_branch" = true; has_unset_branch=$?
+
+	if test $((!$has_branch != !$has_unset_branch)) -eq 0
+	then
+		usage
+	fi
+
+	if test $has_branch -eq 0
+	then
+		git submodule--helper config submodule."$name".branch "$branch"
+	else
+		git submodule--helper config --unset submodule."$name".branch
+	fi
+}
+
 #
 # Show commit summary for submodules in index or working tree
 #
@@ -983,7 +1050,7 @@ cmd_absorbgitdirs()
 while test $# != 0 && test -z "$command"
 do
 	case "$1" in
-	add | foreach | init | deinit | update | status | summary | sync | absorbgitdirs)
+	add | foreach | init | deinit | update | set-branch | status | summary | sync | absorbgitdirs)
 		command=$1
 		;;
 	-q|--quiet)
@@ -1024,8 +1091,8 @@ then
     fi
 fi
 
-# "-b branch" is accepted only by "add"
+# "-b branch" is accepted only by "add" and "set-branch"
-if test -n "$branch" && test "$command" != add
+if test -n "$branch" && (test "$command" != add || test "$command" != set-branch)
 then
 	usage
 fi
@@ -1036,4 +1103,4 @@ then
 	usage
 fi
 
-"cmd_$command" "$@"
+"cmd_$(echo $command | sed -e s/-/_/g)" "$@"

t/t7419-submodule-set-branch.sh

@@ -0,0 +1,93 @@
+#!/bin/sh
+#
+# Copyright (c) 2019 Denton Liu
+#
+
+test_description='Test submodules set-branch subcommand
+
+This test verifies that the set-branch subcommand of git-submodule is working
+as expected.
+'
+
+TEST_NO_CREATE_REPO=1
+. ./test-lib.sh
+
+test_expect_success 'submodule config cache setup' '
+	mkdir submodule &&
+	(cd submodule &&
+		git init &&
+		echo a >a &&
+		git add . &&
+		git commit -ma &&
+		git checkout -b topic &&
+		echo b >a &&
+		git add . &&
+		git commit -mb
+	) &&
+	mkdir super &&
+	(cd super &&
+		git init &&
+		git submodule add ../submodule &&
+		git commit -m "add submodule"
+	)
+'
+
+test_expect_success 'ensure submodule branch is unset' '
+	(cd super &&
+		test_must_fail grep branch .gitmodules
+	)
+'
+
+test_expect_success 'test submodule set-branch --branch' '
+	(cd super &&
+		git submodule set-branch --branch topic submodule &&
+		grep "branch = topic" .gitmodules &&
+		git submodule update --remote &&
+		cat <<-\EOF >expect &&
+		b
+		EOF
+		git -C submodule show -s --pretty=%s >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'test submodule set-branch --default' '
+	(cd super &&
+		git submodule set-branch --default submodule &&
+		test_must_fail grep branch .gitmodules &&
+		git submodule update --remote &&
+		cat <<-\EOF >expect &&
+		a
+		EOF
+		git -C submodule show -s --pretty=%s >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'test submodule set-branch -b' '
+	(cd super &&
+		git submodule set-branch -b topic submodule &&
+		grep "branch = topic" .gitmodules &&
+		git submodule update --remote &&
+		cat <<-\EOF >expect &&
+		b
+		EOF
+		git -C submodule show -s --pretty=%s >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'test submodule set-branch -d' '
+	(cd super &&
+		git submodule set-branch -d submodule &&
+		test_must_fail grep branch .gitmodules &&
+		git submodule update --remote &&
+		cat <<-\EOF >expect &&
+		a
+		EOF
+		git -C submodule show -s --pretty=%s >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_done