checkout: introduce --detach synonym for "git checkout foo^{commit}"

For example, one might use this when making a temporary merge to
test that two topics work well together.

Patch by Junio, with tests from Jeff King.

[jn: with some extra checks for bogus commandline usage]

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -9,6 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git checkout' [-q] [-f] [-m] [<branch>]
+'git checkout' [-q] [-f] [-m] [--detach] [<commit>]
 'git checkout' [-q] [-f] [-m] [[-b|-B|--orphan] <new_branch>] [<start_point>]
 'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <paths>...
 'git checkout' --patch [<tree-ish>] [--] [<paths>...]
@@ -22,9 +23,10 @@ branch.
 
 'git checkout' [<branch>]::
 'git checkout' -b|-B <new_branch> [<start point>]::
+'git checkout' [--detach] [<commit>]::
 
 	This form switches branches by updating the index, working
-	tree, and HEAD to reflect the specified branch.
+	tree, and HEAD to reflect the specified branch or commit.
 +
 If `-b` is given, a new branch is created as if linkgit:git-branch[1]
 were called and then checked out; in this case you can
@@ -115,6 +117,13 @@ explicitly give a name with '-b' in such a case.
 	Create the new branch's reflog; see linkgit:git-branch[1] for
 	details.
 
+--detach::
+	Rather than checking out a branch to work on it, check out a
+	commit for inspection and discardable experiments.
+	This is the default behavior of "git checkout <commit>" when
+	<commit> is not a branch name.  See the "DETACHED HEAD" section
+	below for details.
+
 --orphan::
 	Create a new 'orphan' branch, named <new_branch>, started from
 	<start_point> and switch to it.  The first commit made on this
@@ -204,7 +213,7 @@ leave out at most one of `A` and `B`, in which case it defaults to `HEAD`.
 
 
 
-Detached HEAD
+DETACHED HEAD
 -------------
 
 It is sometimes useful to be able to 'checkout' a commit that is

builtin/checkout.c

@@ -30,6 +30,7 @@ struct checkout_opts {
 	int quiet;
 	int merge;
 	int force;
+	int force_detach;
 	int writeout_stage;
 	int writeout_error;
 
@@ -563,7 +564,7 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 			if (!file_exists(ref_file) && file_exists(log_file))
 				remove_path(log_file);
 		}
-	} else if (strcmp(new->name, "HEAD")) {
+	} else if (opts->force_detach || strcmp(new->name, "HEAD")) {
 		update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
 			   REF_NODEREF, DIE_ON_ERR);
 		if (!opts->quiet) {
@@ -574,7 +575,8 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 	}
 	remove_branch_state();
 	strbuf_release(&msg);
-	if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
+	if (!opts->quiet &&
+	    (new->path || (!opts->force_detach && !strcmp(new->name, "HEAD"))))
 		report_tracking(new);
 }
 
@@ -677,6 +679,7 @@ static const char *unique_tracking_name(const char *name)
 
 static int parse_branchname_arg(int argc, const char **argv,
 				int dwim_new_local_branch_ok,
+				int force_detach,
 				struct branch_info *new,
 				struct tree **source_tree,
 				unsigned char rev[20],
@@ -753,7 +756,8 @@ static int parse_branchname_arg(int argc, const char **argv,
 	new->name = arg;
 	setup_branch_path(new);
 
-	if (check_ref_format(new->path) == CHECK_REF_FORMAT_OK &&
+	if (!force_detach &&
+	    check_ref_format(new->path) == CHECK_REF_FORMAT_OK &&
 	    resolve_ref(new->path, branch_rev, 1, NULL))
 		hashcpy(rev, branch_rev);
 	else
@@ -804,6 +808,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		OPT_STRING('B', NULL, &opts.new_branch_force, "branch",
 			   "create/reset and checkout a branch"),
 		OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "create reflog for new branch"),
+		OPT_BOOLEAN(0, "detach", &opts.force_detach, "detach the HEAD at named commit"),
 		OPT_SET_INT('t', "track",  &opts.track, "set upstream info for new branch",
 			BRANCH_TRACK_EXPLICIT),
 		OPT_STRING(0, "orphan", &opts.new_orphan_branch, "new branch", "new unparented branch"),
@@ -842,9 +847,15 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		opts.new_branch = opts.new_branch_force;
 
 	if (patch_mode && (opts.track > 0 || opts.new_branch
-			   || opts.new_branch_log || opts.merge || opts.force))
+			   || opts.new_branch_log || opts.merge || opts.force
+			   || opts.force_detach))
 		die ("--patch is incompatible with all other options");
 
+	if (opts.force_detach && (opts.new_branch || opts.new_orphan_branch))
+		die("--detach cannot be used with -b/-B/--orphan");
+	if (opts.force_detach && 0 < opts.track)
+		die("--detach cannot be used with -t");
+
 	/* --track without -b should DWIM */
 	if (0 < opts.track && !opts.new_branch) {
 		const char *argv0 = argv[0];
@@ -895,7 +906,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			dwim_new_local_branch &&
 			opts.track == BRANCH_TRACK_UNSPECIFIED &&
 			!opts.new_branch;
-		int n = parse_branchname_arg(argc, argv, dwim_ok,
+		int n = parse_branchname_arg(argc, argv,
+				dwim_ok, opts.force_detach,
 				&new, &source_tree, rev, &opts.new_branch);
 		argv += n;
 		argc -= n;
@@ -922,6 +934,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			}
 		}
 
+		if (opts.force_detach)
+			die("git checkout: --detach does not take a path argument");
+
 		if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
 			die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");
 

t/t2020-checkout-detach.sh

@@ -0,0 +1,95 @@
+#!/bin/sh
+
+test_description='checkout into detached HEAD state'
+. ./test-lib.sh
+
+check_detached () {
+	test_must_fail git symbolic-ref -q HEAD >/dev/null
+}
+
+check_not_detached () {
+	git symbolic-ref -q HEAD >/dev/null
+}
+
+reset () {
+	git checkout master &&
+	check_not_detached
+}
+
+test_expect_success 'setup' '
+	test_commit one &&
+	test_commit two &&
+	git branch branch &&
+	git tag tag
+'
+
+test_expect_success 'checkout branch does not detach' '
+	reset &&
+	git checkout branch &&
+	check_not_detached
+'
+
+test_expect_success 'checkout tag detaches' '
+	reset &&
+	git checkout tag &&
+	check_detached
+'
+
+test_expect_success 'checkout branch by full name detaches' '
+	reset &&
+	git checkout refs/heads/branch &&
+	check_detached
+'
+
+test_expect_success 'checkout non-ref detaches' '
+	reset &&
+	git checkout branch^ &&
+	check_detached
+'
+
+test_expect_success 'checkout ref^0 detaches' '
+	reset &&
+	git checkout branch^0 &&
+	check_detached
+'
+
+test_expect_success 'checkout --detach detaches' '
+	reset &&
+	git checkout --detach branch &&
+	check_detached
+'
+
+test_expect_success 'checkout --detach without branch name' '
+	reset &&
+	git checkout --detach &&
+	check_detached
+'
+
+test_expect_success 'checkout --detach errors out for non-commit' '
+	reset &&
+	test_must_fail git checkout --detach one^{tree} &&
+	check_not_detached
+'
+
+test_expect_success 'checkout --detach errors out for extra argument' '
+	reset &&
+	git checkout master &&
+	test_must_fail git checkout --detach tag one.t &&
+	check_not_detached
+'
+
+test_expect_success 'checkout --detached and -b are incompatible' '
+	reset &&
+	test_must_fail git checkout --detach -b newbranch tag &&
+	check_not_detached
+'
+
+test_expect_success 'checkout --detach moves HEAD' '
+	reset &&
+	git checkout one &&
+	git checkout --detach two &&
+	git diff --exit-code HEAD &&
+	git diff --exit-code two
+'
+
+test_done