builtin-remote: add set-head subcommand

Provide a porcelain command for setting and deleting
$GIT_DIR/remotes/<remote>/HEAD.

While we're at it, document what $GIT_DIR/remotes/<remote>/HEAD is all
about.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote.txt

@@ -13,6 +13,7 @@ SYNOPSIS
 'git remote add' [-t <branch>] [-m <master>] [-f] [--mirror] <name> <url>
 'git remote rename' <old> <new>
 'git remote rm' <name>
+'git remote set-head' <name> [-a | -d | <branch>]
 'git remote show' [-n] <name>
 'git remote prune' [-n | --dry-run] <name>
 'git remote update' [group]
@@ -53,8 +54,7 @@ is created.  You can give more than one `-t <branch>` to track
 multiple branches without grabbing all branches.
 +
 With `-m <master>` option, `$GIT_DIR/remotes/<name>/HEAD` is set
-up to point at remote's `<master>` branch instead of whatever
-branch the `HEAD` at the remote repository actually points at.
+up to point at remote's `<master>` branch. See also the set-head command.
 +
 In mirror mode, enabled with `\--mirror`, the refs will not be stored
 in the 'refs/remotes/' namespace, but in 'refs/heads/'.  This option
@@ -76,6 +76,30 @@ the configuration file format.
 Remove the remote named <name>. All remote tracking branches and
 configuration settings for the remote are removed.
 
+'set-head'::
+
+Sets or deletes the default branch (`$GIT_DIR/remotes/<name>/HEAD`) for
+the named remote. Having a default branch for a remote is not required,
+but allows the name of the remote to be specified in lieu of a specific
+branch. For example, if the default branch for `origin` is set to
+`master`, then `origin` may be specified wherever you would normally
+specify `origin/master`.
++
+With `-d`, `$GIT_DIR/remotes/<name>/HEAD` is deleted.
++
+With `-a`, the remote is queried to determine its `HEAD`, then
+`$GIT_DIR/remotes/<name>/HEAD` is set to the same branch. e.g., if the remote
+`HEAD` is pointed at `next`, "`git remote set-head origin -a`" will set
+`$GIT_DIR/refs/remotes/origin/HEAD` to `refs/remotes/origin/next`. This will
+only work if `refs/remotes/origin/next` already exists; if not it must be
+fetched first.
++
+Use `<branch>` to set `$GIT_DIR/remotes/<name>/HEAD` explicitly. e.g., "git
+remote set-head origin master" will set `$GIT_DIR/refs/remotes/origin/HEAD` to
+`refs/remotes/origin/master`. This will only work if
+`refs/remotes/origin/master` already exists; if not it must be fetched first.
++
+
 'show'::
 
 Gives some information about the remote <name>.

builtin-remote.c

@@ -12,6 +12,7 @@ static const char * const builtin_remote_usage[] = {
 	"git remote add [-t <branch>] [-m <master>] [-f] [--mirror] <name> <url>",
 	"git remote rename <old> <new>",
 	"git remote rm <name>",
+	"git remote set-head <name> [-a | -d | <branch>]",
 	"git remote show [-n] <name>",
 	"git remote prune [-n | --dry-run] <name>",
 	"git remote [-v | --verbose] update [group]",
@@ -792,6 +793,65 @@ static int show(int argc, const char **argv)
 	return result;
 }
 
+static int set_head(int argc, const char **argv)
+{
+	int i, opt_a = 0, opt_d = 0, result = 0;
+	struct strbuf buf = STRBUF_INIT, buf2 = STRBUF_INIT;
+	char *head_name = NULL;
+
+	struct option options[] = {
+		OPT_GROUP("set-head specific options"),
+		OPT_BOOLEAN('a', "auto", &opt_a,
+			    "set refs/remotes/<name>/HEAD according to remote"),
+		OPT_BOOLEAN('d', "delete", &opt_d,
+			    "delete refs/remotes/<name>/HEAD"),
+		OPT_END()
+	};
+	argc = parse_options(argc, argv, options, builtin_remote_usage, 0);
+	if (argc)
+		strbuf_addf(&buf, "refs/remotes/%s/HEAD", argv[0]);
+
+	if (!opt_a && !opt_d && argc == 2) {
+		head_name = xstrdup(argv[1]);
+	} else if (opt_a && !opt_d && argc == 1) {
+		struct ref_states states;
+		memset(&states, 0, sizeof(states));
+		get_remote_ref_states(argv[0], &states, GET_HEAD_NAMES);
+		if (!states.heads.nr)
+			result |= error("Cannot determine remote HEAD");
+		else if (states.heads.nr > 1) {
+			result |= error("Multiple remote HEAD branches. "
+					"Please choose one explicitly with:");
+			for (i = 0; i < states.heads.nr; i++)
+				fprintf(stderr, "  git remote set-head %s %s\n",
+					argv[0], states.heads.items[i].string);
+		} else
+			head_name = xstrdup(states.heads.items[0].string);
+		free_remote_ref_states(&states);
+	} else if (opt_d && !opt_a && argc == 1) {
+		if (delete_ref(buf.buf, NULL, REF_NODEREF))
+			result |= error("Could not delete %s", buf.buf);
+	} else
+		usage_with_options(builtin_remote_usage, options);
+
+	if (head_name) {
+		unsigned char sha1[20];
+		strbuf_addf(&buf2, "refs/remotes/%s/%s", argv[0], head_name);
+		/* make sure it's valid */
+		if (!resolve_ref(buf2.buf, sha1, 1, NULL))
+			result |= error("Not a valid ref: %s", buf2.buf);
+		else if (create_symref(buf.buf, buf2.buf, "remote set-head"))
+			result |= error("Could not setup %s", buf.buf);
+		if (opt_a)
+			printf("%s/HEAD set to %s\n", argv[0], head_name);
+		free(head_name);
+	}
+
+	strbuf_release(&buf);
+	strbuf_release(&buf2);
+	return result;
+}
+
 static int prune(int argc, const char **argv)
 {
 	int dry_run = 0, result = 0;
@@ -962,6 +1022,8 @@ int cmd_remote(int argc, const char **argv, const char *prefix)
 		result = mv(argc, argv);
 	else if (!strcmp(argv[0], "rm"))
 		result = rm(argc, argv);
+	else if (!strcmp(argv[0], "set-head"))
+		result = set_head(argc, argv);
 	else if (!strcmp(argv[0], "show"))
 		result = show(argc, argv);
 	else if (!strcmp(argv[0], "prune"))

contrib/completion/git-completion.bash

@@ -1443,7 +1443,7 @@ _git_config ()
 
 _git_remote ()
 {
-	local subcommands="add rename rm show prune update"
+	local subcommands="add rename rm show prune update set-head"
 	local subcommand="$(__git_find_subcommand "$subcommands")"
 	if [ -z "$subcommand" ]; then
 		__gitcomp "$subcommands"

t/t5505-remote.sh

@@ -205,6 +205,46 @@ test_expect_success 'prune' '
 	 test_must_fail git rev-parse refs/remotes/origin/side)
 '
 
+test_expect_success 'set-head --delete' '
+	(cd test &&
+	 git symbolic-ref refs/remotes/origin/HEAD &&
+	 git remote set-head --delete origin &&
+	 test_must_fail git symbolic-ref refs/remotes/origin/HEAD)
+'
+
+test_expect_success 'set-head --auto' '
+	(cd test &&
+	 git remote set-head --auto origin &&
+	 echo refs/remotes/origin/master >expect &&
+	 git symbolic-ref refs/remotes/origin/HEAD >output &&
+	 test_cmp expect output
+	)
+'
+
+cat >test/expect <<EOF
+error: Multiple remote HEAD branches. Please choose one explicitly with:
+  git remote set-head two another
+  git remote set-head two master
+EOF
+
+test_expect_success 'set-head --auto fails w/multiple HEADs' '
+	(cd test &&
+	 test_must_fail git remote set-head --auto two >output 2>&1 &&
+	test_cmp expect output)
+'
+
+cat >test/expect <<EOF
+refs/remotes/origin/side2
+EOF
+
+test_expect_success 'set-head explicit' '
+	(cd test &&
+	 git remote set-head origin side2 &&
+	 git symbolic-ref refs/remotes/origin/HEAD >output &&
+	 git remote set-head origin master &&
+	 test_cmp expect output)
+'
+
 cat > test/expect << EOF
 Pruning origin
 URL: $(pwd)/one