[PATCH v4 21/27] checkout: support checking out into a new working directory

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

"git checkout --to" sets up a new working directory with a .git file
pointing to $GIT_DIR/repos/<id>. It then executes "git checkout" again
on the new worktree with the same arguments except "--to" is taken
out. The second checkout execution, which is not contaminated with any
info from the current repository, will actually check out and
everything that normal "git checkout" does.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Documentation/git-checkout.txt         | 34 +++++++++++++
 Documentation/git.txt                  |  3 +-
 Documentation/gitrepository-layout.txt |  7 +++
 builtin/checkout.c                     | 93 +++++++++++++++++++++++++++++++++-
 path.c                                 |  2 +-
 t/t2025-checkout-to.sh (new +x)        | 48 ++++++++++++++++++
 6 files changed, 183 insertions(+), 4 deletions(-)
 create mode 100755 t/t2025-checkout-to.sh

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 33ad2ad..fcf73b2 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -225,6 +225,13 @@ This means that you can use `git checkout -p` to selectively discard
 edits from your current working tree. See the ``Interactive Mode''
 section of linkgit:git-add[1] to learn how to operate the `--patch` mode.
 
+--to=<path>::
+	Check out a new branch in a separate working directory at
+	`<path>`. A new working directory is linked to the current
+	repository, sharing everything except working directory
+	specific files such as HEAD, index... See "MULTIPLE CHECKOUT
+	MODE" section for more information.
+
 <branch>::
 	Branch to checkout; if it refers to a branch (i.e., a name that,
 	when prepended with "refs/heads/", is a valid ref), then that
@@ -388,6 +395,33 @@ $ git reflog -2 HEAD # or
 $ git log -g -2 HEAD
 ------------
 
+MULTIPLE CHECKOUT MODE
+-------------------------------
+Normally a working directory is attached to repository. When "git
+checkout --to" is used, a new working directory is attached to the
+current repository. This new working directory is called "linked
+checkout" as compared to the "main checkout" prepared by "git init" or
+"git clone". A repository has one main checkout and zero or more
+linked checkouts.
+
+All checkouts share the same repository. Linked checkouts see the
+repository a bit different from the main checkout. When the checkout
+"new" reads the path $GIT_DIR/HEAD for example, the actual path
+returned could be $GIT_DIR/repos/new/HEAD. This ensures checkouts
+won't step on each other.
+
+Each linked checkout has a private space in $GIT_DIR/repos, usually
+named after the base name of the working directory with a number added
+to make it unique. The linked checkout's $GIT_DIR points to this
+private space while $GIT_COMMON_DIR points to the main checkout's
+$GIT_DIR. These settings are done by "git checkout --to".
+
+Because in this mode $GIT_DIR becomes a lightweight virtual file
+system where a path could be rewritten to some place else, accessing
+$GIT_DIR from scripts should use `git rev-parse --git-path` to resolve
+a path instead of using it directly unless the path is known to be
+private to the working directory.
+
 EXAMPLES
 --------
 
diff --git a/Documentation/git.txt b/Documentation/git.txt
index b094b1f..bdb9b0f 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -777,7 +777,8 @@ Git so take care if using Cogito etc.
 	If this variable is set to a path, non-worktree files that are
 	normally in $GIT_DIR will be taken from this path
 	instead. Worktree-specific files such as HEAD or index are
-	taken from $GIT_DIR. See linkgit:gitrepository-layout[5] for
+	taken from $GIT_DIR. See linkgit:gitrepository-layout[5] and
+	the section 'MULTIPLE CHECKOUT MODE' in linkgit:checkout[1]
 	details. This variable has lower precedence than other path
 	variables such as GIT_INDEX_FILE, GIT_OBJECT_DIRECTORY...
 
diff --git a/Documentation/gitrepository-layout.txt b/Documentation/gitrepository-layout.txt
index f72ce75..418e5c8 100644
--- a/Documentation/gitrepository-layout.txt
+++ b/Documentation/gitrepository-layout.txt
@@ -245,6 +245,13 @@ modules::
 	directory is ignored $GIT_COMMON_DIR is set and
 	"$GIT_COMMON_DIR/modules" will be used instead.
 
+repos::
+	Contains worktree specific information of linked
+	checkouts. Each subdirectory contains the worktree-related
+	part of a linked checkout. This directory is ignored
+	$GIT_COMMON_DIR is set and "$GIT_COMMON_DIR/repos" will be
+	used instead.
+
 SEE ALSO
 --------
 linkgit:git-init[1],
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 0570e41..fa7b54a 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -48,6 +48,10 @@ struct checkout_opts {
 	const char *prefix;
 	struct pathspec pathspec;
 	struct tree *source_tree;
+
+	const char *new_worktree;
+	const char **saved_argv;
+	int new_worktree_mode;
 };
 
 static int post_checkout_hook(struct commit *old, struct commit *new,
@@ -250,6 +254,9 @@ static int checkout_paths(const struct checkout_opts *opts,
 		die(_("Cannot update paths and switch to branch '%s' at the same time."),
 		    opts->new_branch);
 
+	if (opts->new_worktree)
+		die(_("'%s' cannot be used with updating paths"), "--to");
+
 	if (opts->patch_mode)
 		return run_add_interactive(revision, "--patch=checkout",
 					   &opts->pathspec);
@@ -486,7 +493,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			topts.dir->flags |= DIR_SHOW_IGNORED;
 			setup_standard_excludes(topts.dir);
 		}
-		tree = parse_tree_indirect(old->commit ?
+		tree = parse_tree_indirect(old->commit && !opts->new_worktree_mode ?
 					   old->commit->object.sha1 :
 					   EMPTY_TREE_SHA1_BIN);
 		init_tree_desc(&trees[0], tree->buffer, tree->size);
@@ -796,7 +803,8 @@ static int switch_branches(const struct checkout_opts *opts,
 		return ret;
 	}
 
-	if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
+	if (!opts->quiet && !old.path && old.commit &&
+	    new->commit != old.commit && !opts->new_worktree_mode)
 		orphaned_commit_warning(old.commit, new->commit);
 
 	update_refs_for_switch(opts, &old, new);
@@ -806,6 +814,74 @@ static int switch_branches(const struct checkout_opts *opts,
 	return ret || writeout_error;
 }
 
+static int prepare_linked_checkout(const struct checkout_opts *opts,
+				   struct branch_info *new)
+{
+	struct strbuf sb_git = STRBUF_INIT, sb_repo = STRBUF_INIT;
+	struct strbuf sb = STRBUF_INIT;
+	const char *path = opts->new_worktree, *name;
+	struct stat st;
+	struct child_process cp;
+	int counter = 0, len;
+
+	if (!new->commit)
+		die(_("no branch specified"));
+
+	len = strlen(path);
+	while (len && is_dir_sep(path[len - 1]))
+		len--;
+
+	for (name = path + len - 1; name > path; name--)
+		if (is_dir_sep(*name)) {
+			name++;
+			break;
+		}
+	strbuf_addstr(&sb_repo,
+		      git_path("repos/%.*s", (int)(path + len - name), name));
+	len = sb_repo.len;
+	if (safe_create_leading_directories_const(sb_repo.buf))
+		die_errno(_("could not create leading directories of '%s'"),
+			  sb_repo.buf);
+	while (!stat(sb_repo.buf, &st)) {
+		counter++;
+		strbuf_setlen(&sb_repo, len);
+		strbuf_addf(&sb_repo, "%d", counter);
+	}
+	name = strrchr(sb_repo.buf, '/') + 1;
+	if (mkdir(sb_repo.buf, 0777))
+		die_errno(_("could not create directory of '%s'"), sb_repo.buf);
+
+	strbuf_addf(&sb_git, "%s/.git", path);
+	if (safe_create_leading_directories_const(sb_git.buf))
+		die_errno(_("could not create leading directories of '%s'"),
+			  sb_git.buf);
+
+	write_file(sb_git.buf, 1, "gitdir: %s/repos/%s\n",
+		   real_path(get_git_common_dir()), name);
+	/*
+	 * This is to keep resolve_ref() happy. We need a valid HEAD
+	 * or is_git_directory() will reject the directory. Any valid
+	 * value would do because this value will be ignored and
+	 * replaced at the next (real) checkout.
+	 */
+	strbuf_addf(&sb, "%s/HEAD", sb_repo.buf);
+	write_file(sb.buf, 1, "%s\n", sha1_to_hex(new->commit->object.sha1));
+	strbuf_reset(&sb);
+	strbuf_addf(&sb, "%s/commondir", sb_repo.buf);
+	write_file(sb.buf, 1, "../..\n");
+
+	if (!opts->quiet)
+		fprintf_ln(stderr, _("Enter %s (identifier %s)"), path, name);
+
+	setenv("GIT_CHECKOUT_NEW_WORKTREE", "1", 1);
+	setenv(GIT_DIR_ENVIRONMENT, sb_git.buf, 1);
+	setenv(GIT_WORK_TREE_ENVIRONMENT, path, 1);
+	memset(&cp, 0, sizeof(cp));
+	cp.git_cmd = 1;
+	cp.argv = opts->saved_argv;
+	return run_command(&cp);
+}
+
 static int git_checkout_config(const char *var, const char *value, void *cb)
 {
 	if (!strcmp(var, "diff.ignoresubmodules")) {
@@ -1067,6 +1143,9 @@ static int checkout_branch(struct checkout_opts *opts,
 		die(_("Cannot switch branch to a non-commit '%s'"),
 		    new->name);
 
+	if (opts->new_worktree)
+		return prepare_linked_checkout(opts, new);
+
 	if (!new->commit && opts->new_branch) {
 		unsigned char rev[20];
 		int flag;
@@ -1109,6 +1188,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			 N_("do not limit pathspecs to sparse entries only")),
 		OPT_HIDDEN_BOOL(0, "guess", &dwim_new_local_branch,
 				N_("second guess 'git checkout no-such-branch'")),
+		OPT_STRING(0, "to", &opts.new_worktree, N_("path"),
+			   N_("check a branch out in a separate working directory")),
 		OPT_END(),
 	};
 
@@ -1117,6 +1198,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	opts.overwrite_ignore = 1;
 	opts.prefix = prefix;
 
+	opts.saved_argv = xmalloc(sizeof(const char *) * (argc + 2));
+	memcpy(opts.saved_argv, argv, sizeof(const char *) * (argc + 1));
+
 	gitmodules_config();
 	git_config(git_checkout_config, &opts);
 
@@ -1125,6 +1209,11 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, options, checkout_usage,
 			     PARSE_OPT_KEEP_DASHDASH);
 
+	/* recursive execution from checkout_new_worktree() */
+	opts.new_worktree_mode = getenv("GIT_CHECKOUT_NEW_WORKTREE") != NULL;
+	if (opts.new_worktree_mode)
+		opts.new_worktree = NULL;
+
 	if (conflict_style) {
 		opts.merge = 1; /* implied */
 		git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
diff --git a/path.c b/path.c
index 6129026..47383ff 100644
--- a/path.c
+++ b/path.c
@@ -94,7 +94,7 @@ static void update_common_dir(struct strbuf *buf, int git_dir_len)
 {
 	const char *common_dir_list[] = {
 		"branches", "hooks", "info", "logs", "lost-found", "modules",
-		"objects", "refs", "remotes", "rr-cache", "svn",
+		"objects", "refs", "remotes", "repos", "rr-cache", "svn",
 		NULL
 	};
 	const char *common_top_file_list[] = {
diff --git a/t/t2025-checkout-to.sh b/t/t2025-checkout-to.sh
new file mode 100755
index 0000000..5ec49e2
--- /dev/null
+++ b/t/t2025-checkout-to.sh
@@ -0,0 +1,48 @@
+#!/bin/sh
+
+test_description='test git checkout --to'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit init
+'
+
+test_expect_success 'checkout --to not updating paths' '
+	test_must_fail git checkout --to -- init.t
+'
+
+test_expect_success 'checkout --to a new worktree' '
+	git checkout --to here master &&
+	(
+		cd here &&
+		test_cmp ../init.t init.t &&
+		git symbolic-ref HEAD >actual &&
+		echo refs/heads/master >expect &&
+		test_cmp expect actual &&
+		git fsck
+	)
+'
+
+test_expect_success 'checkout --to from a linked checkout' '
+	(
+		cd here &&
+		git checkout --to nested-here master
+		cd nested-here &&
+		git fsck
+	)
+'
+
+test_expect_success 'checkout --to a new worktree creating new branch' '
+	git checkout --to there -b newmaster master &&
+	(
+		cd there &&
+		test_cmp ../init.t init.t &&
+		git symbolic-ref HEAD >actual &&
+		echo refs/heads/newmaster >expect &&
+		test_cmp expect actual &&
+		git fsck
+	)
+'
+
+test_done
-- 
1.9.0.40.gaa8c3ea