[PATCH 3/5] checkout --to: no auto-detach if the ref is already checked out

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

Give the user a choice in this case. If they want to detach, they can go
with '--detach --to ...', or they could switch branch of the checkout
that's holding the ref in question. Or they could just create a new
branch with '-b xxx --to yyy'

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Documentation/config.txt |  3 ++
 advice.c                 |  2 ++
 advice.h                 |  1 +
 builtin/checkout.c       | 73 ++++++++++++++++++++++++++++--------------------
 t/t2025-checkout-to.sh   | 16 +++++------
 5 files changed, 56 insertions(+), 39 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 57999fa..4a41202 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -201,6 +201,9 @@ advice.*::
 	rmHints::
 		In case of failure in the output of linkgit:git-rm[1],
 		show directions on how to proceed from the current state.
+	checkoutTo::
+		In case of failure in the output of "git checkout --to",
+		show directions on how to proceed from the current state.
 --
 
 core.fileMode::
diff --git a/advice.c b/advice.c
index 9b42033..b1fb524 100644
--- a/advice.c
+++ b/advice.c
@@ -15,6 +15,7 @@ int advice_detached_head = 1;
 int advice_set_upstream_failure = 1;
 int advice_object_name_warning = 1;
 int advice_rm_hints = 1;
+int advice_checkout_to = 1;
 
 static struct {
 	const char *name;
@@ -35,6 +36,7 @@ static struct {
 	{ "setupstreamfailure", &advice_set_upstream_failure },
 	{ "objectnamewarning", &advice_object_name_warning },
 	{ "rmhints", &advice_rm_hints },
+	{ "checkoutto", &advice_checkout_to },
 
 	/* make this an alias for backward compatibility */
 	{ "pushnonfastforward", &advice_push_update_rejected }
diff --git a/advice.h b/advice.h
index 5ecc6c1..a288219 100644
--- a/advice.h
+++ b/advice.h
@@ -18,6 +18,7 @@ extern int advice_detached_head;
 extern int advice_set_upstream_failure;
 extern int advice_object_name_warning;
 extern int advice_rm_hints;
+extern int advice_checkout_to;
 
 int git_default_advice_config(const char *var, const char *value);
 __attribute__((format (printf, 1, 2)))
diff --git a/builtin/checkout.c b/builtin/checkout.c
index c83f476..d35245a 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -1006,31 +1006,52 @@ static const char *unique_tracking_name(const char *name, unsigned char *sha1)
 	return NULL;
 }
 
-static int check_linked_checkout(struct branch_info *new,
-				  const char *name, const char *path)
+static void check_linked_checkout(struct branch_info *new, const char *id)
 {
 	struct strbuf sb = STRBUF_INIT;
+	struct strbuf path = STRBUF_INIT;
+	struct strbuf gitdir = STRBUF_INIT;
 	const char *start, *end;
-	if (strbuf_read_file(&sb, path, 0) < 0 ||
-	    !skip_prefix(sb.buf, "ref:", &start)) {
-		strbuf_release(&sb);
-		return 0;
-	}
 
+	if (id)
+		strbuf_addf(&path, "%s/repos/%s/HEAD", get_git_common_dir(), id);
+	else
+		strbuf_addf(&path, "%s/HEAD", get_git_common_dir());
+
+	if (strbuf_read_file(&sb, path.buf, 0) <= 0 ||
+	    !skip_prefix(sb.buf, "ref:", &start))
+		goto done;
 	while (isspace(*start))
 		start++;
 	end = start;
 	while (*end && !isspace(*end))
 		end++;
-	if (!strncmp(start, new->path, end - start) &&
-	    new->path[end - start] == '\0') {
-		strbuf_release(&sb);
-		new->path = NULL; /* detach */
-		new->checkout = xstrdup(name); /* reason */
-		return 1;
-	}
+	if (strncmp(start, new->path, end - start) ||
+	    new->path[end - start] != '\0')
+		goto done;
+	if (id) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/repos/%s/gitdir",
+			    get_git_common_dir(), id);
+		if (strbuf_read_file(&gitdir, path.buf, 0) <= 0)
+			goto done;
+		while (gitdir.len && (gitdir.buf[gitdir.len - 1] == '\n' ||
+				      gitdir.buf[gitdir.len - 1] == '\r'))
+			gitdir.buf[--gitdir.len] = '\0';
+	} else
+		strbuf_addstr(&gitdir, get_git_common_dir());
+	if (advice_checkout_to)
+		die(_("%s is already checked out at %s.\n"
+		      "Either use --detach or -b together with --to "
+		      "or switch branch in the the other checkout."),
+		    new->path, gitdir.buf);
+	else
+		die(_("%s is already checked out at %s."),
+		    new->path, gitdir.buf);
+done:
+	strbuf_release(&path);
 	strbuf_release(&sb);
-	return 0;
+	strbuf_release(&gitdir);
 }
 
 static void check_linked_checkouts(struct branch_info *new)
@@ -1045,27 +1066,17 @@ static void check_linked_checkouts(struct branch_info *new)
 		return;
 	}
 
-	strbuf_reset(&path);
-	strbuf_addf(&path, "%s/HEAD", get_git_common_dir());
 	/*
 	 * $GIT_COMMON_DIR/HEAD is practically outside
 	 * $GIT_DIR so resolve_ref_unsafe() won't work (it
 	 * uses git_path). Parse the ref ourselves.
 	 */
-	if (check_linked_checkout(new, "", path.buf)) {
-		strbuf_release(&path);
-		closedir(dir);
-		return;
-	}
+	check_linked_checkout(new, NULL);
 
 	while ((d = readdir(dir)) != NULL) {
 		if (!strcmp(d->d_name, ".") || !strcmp(d->d_name, ".."))
 			continue;
-		strbuf_reset(&path);
-		strbuf_addf(&path, "%s/repos/%s/HEAD",
-			    get_git_common_dir(), d->d_name);
-		if (check_linked_checkout(new, d->d_name, path.buf))
-			break;
+		check_linked_checkout(new, d->d_name);
 	}
 	strbuf_release(&path);
 	closedir(dir);
@@ -1076,7 +1087,8 @@ static int parse_branchname_arg(int argc, const char **argv,
 				struct branch_info *new,
 				struct tree **source_tree,
 				unsigned char rev[20],
-				const char **new_branch)
+				const char **new_branch,
+				int force_detach)
 {
 	int argcount = 0;
 	unsigned char branch_rev[20];
@@ -1198,7 +1210,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 	else
 		new->path = NULL; /* not an existing branch */
 
-	if (new->path) {
+	if (new->path && !force_detach && !*new_branch) {
 		unsigned char sha1[20];
 		int flag;
 		char *head_ref = resolve_refdup("HEAD", sha1, 0, &flag);
@@ -1416,7 +1428,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			!opts.new_branch;
 		int n = parse_branchname_arg(argc, argv, dwim_ok,
 					     &new, &opts.source_tree,
-					     rev, &opts.new_branch);
+					     rev, &opts.new_branch,
+					     opts.force_detach);
 		argv += n;
 		argc -= n;
 	}
diff --git a/t/t2025-checkout-to.sh b/t/t2025-checkout-to.sh
index b0d97a0..c6601a4 100755
--- a/t/t2025-checkout-to.sh
+++ b/t/t2025-checkout-to.sh
@@ -14,7 +14,7 @@ test_expect_success 'checkout --to not updating paths' '
 
 test_expect_success 'checkout --to a new worktree' '
 	git rev-parse HEAD >expect &&
-	git checkout --to here master &&
+	git checkout --detach --to here master &&
 	(
 		cd here &&
 		test_cmp ../init.t init.t &&
@@ -28,7 +28,7 @@ test_expect_success 'checkout --to a new worktree' '
 test_expect_success 'checkout --to from a linked checkout' '
 	(
 		cd here &&
-		git checkout --to nested-here master
+		git checkout --detach --to nested-here master
 		cd nested-here &&
 		git fsck
 	)
@@ -46,19 +46,17 @@ test_expect_success 'checkout --to a new worktree creating new branch' '
 	)
 '
 
-test_expect_success 'detach if the same branch is already checked out' '
+test_expect_success 'die the same branch is already checked out' '
 	(
 		cd here &&
-		git checkout newmaster &&
-		test_must_fail git symbolic-ref HEAD
+		test_must_fail git checkout newmaster
 	)
 '
 
-test_expect_success 'not detach on re-checking out current branch' '
+test_expect_success 'not die on re-checking out current branch' '
 	(
 		cd there &&
-		git checkout newmaster &&
-		git symbolic-ref HEAD
+		git checkout newmaster
 	)
 '
 
@@ -66,7 +64,7 @@ test_expect_success 'checkout --to from a bare repo' '
 	(
 		git clone --bare . bare &&
 		cd bare &&
-		git checkout --to ../there2 master
+		git checkout --to ../there2 -b bare-master master
 	)
 '
 
-- 
1.9.1.346.ga2b5940