Merge branch 'jc/detached-head'

* jc/detached-head:
  git-checkout: handle local changes sanely when detaching HEAD
  git-checkout: safety check for detached HEAD checks existing refs
  git-checkout: fix branch name output from the command
  git-checkout: safety when coming back from the detached HEAD state.
  git-checkout: rewording comments regarding detached HEAD.
  git-checkout: do not warn detaching HEAD when it is already detached.
  Detached HEAD (experimental)
  git-branch: show detached HEAD
  git-status: show detached HEAD

builtin-branch.c

@@ -275,7 +275,7 @@ static void print_ref_item(struct ref_item *item, int maxwidth, int verbose,
 	}
 }
 
-static void print_ref_list(int kinds, int verbose, int abbrev)
+static void print_ref_list(int kinds, int detached, int verbose, int abbrev)
 {
 	int i;
 	struct ref_list ref_list;
@@ -286,8 +286,20 @@ static void print_ref_list(int kinds, int verbose, int abbrev)
 
 	qsort(ref_list.list, ref_list.index, sizeof(struct ref_item), ref_cmp);
 
+	detached = (detached && (kinds & REF_LOCAL_BRANCH));
+	if (detached) {
+		struct ref_item item;
+		item.name = "(no branch)";
+		item.kind = REF_LOCAL_BRANCH;
+		hashcpy(item.sha1, head_sha1);
+		if (strlen(item.name) > ref_list.maxwidth)
+			      ref_list.maxwidth = strlen(item.name);
+		print_ref_item(&item, ref_list.maxwidth, verbose, abbrev, 1);
+	}
+
 	for (i = 0; i < ref_list.index; i++) {
-		int current = (ref_list.list[i].kind == REF_LOCAL_BRANCH) &&
+		int current = !detached &&
+			(ref_list.list[i].kind == REF_LOCAL_BRANCH) &&
 			!strcmp(ref_list.list[i].name, head);
 		print_ref_item(&ref_list.list[i], ref_list.maxwidth, verbose,
 			       abbrev, current);
@@ -296,7 +308,8 @@ static void print_ref_list(int kinds, int verbose, int abbrev)
 	free_ref_list(&ref_list);
 }
 
-static void create_branch(const char *name, const char *start,
+static void create_branch(const char *name, const char *start_name,
+			  unsigned char *start_sha1,
 			  int force, int reflog)
 {
 	struct ref_lock *lock;
@@ -315,9 +328,14 @@ static void create_branch(const char *name, const char *start,
 			die("Cannot force update the current branch.");
 	}
 
-	if (get_sha1(start, sha1) ||
-	    (commit = lookup_commit_reference(sha1)) == NULL)
-		die("Not a valid branch point: '%s'.", start);
+	if (start_sha1)
+		/* detached HEAD */
+		hashcpy(sha1, start_sha1);
+	else if (get_sha1(start_name, sha1))
+		die("Not a valid object name: '%s'.", start_name);
+
+	if ((commit = lookup_commit_reference(sha1)) == NULL)
+		die("Not a valid branch point: '%s'.", start_name);
 	hashcpy(sha1, commit->object.sha1);
 
 	lock = lock_any_ref_for_update(ref, NULL);
@@ -326,7 +344,8 @@ static void create_branch(const char *name, const char *start,
 
 	if (reflog) {
 		log_all_ref_updates = 1;
-		snprintf(msg, sizeof msg, "branch: Created from %s", start);
+		snprintf(msg, sizeof msg, "branch: Created from %s",
+			 start_name);
 	}
 
 	if (write_ref_sha1(lock, sha1, msg) < 0)
@@ -338,6 +357,9 @@ static void rename_branch(const char *oldname, const char *newname, int force)
 	char oldref[PATH_MAX], newref[PATH_MAX], logmsg[PATH_MAX*2 + 100];
 	unsigned char sha1[20];
 
+	if (!oldname)
+		die("cannot rename the curren branch while not on any.");
+
 	if (snprintf(oldref, sizeof(oldref), "refs/heads/%s", oldname) > sizeof(oldref))
 		die("Old branchname too long");
 
@@ -367,7 +389,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 {
 	int delete = 0, force_delete = 0, force_create = 0;
 	int rename = 0, force_rename = 0;
-	int verbose = 0, abbrev = DEFAULT_ABBREV;
+	int verbose = 0, abbrev = DEFAULT_ABBREV, detached = 0;
 	int reflog = 0;
 	int kinds = REF_LOCAL_BRANCH;
 	int i;
@@ -444,22 +466,27 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 	head = xstrdup(resolve_ref("HEAD", head_sha1, 0, NULL));
 	if (!head)
 		die("Failed to resolve HEAD as a valid ref.");
+	if (!strcmp(head, "HEAD")) {
+		detached = 1;
+	}
+	else {
 		if (strncmp(head, "refs/heads/", 11))
 			die("HEAD not found below refs/heads!");
 		head += 11;
+	}
 
 	if (delete)
 		return delete_branches(argc - i, argv + i, force_delete, kinds);
 	else if (i == argc)
-		print_ref_list(kinds, verbose, abbrev);
+		print_ref_list(kinds, detached, verbose, abbrev);
 	else if (rename && (i == argc - 1))
 		rename_branch(head, argv[i], force_rename);
 	else if (rename && (i == argc - 2))
 		rename_branch(argv[i], argv[i + 1], force_rename);
 	else if (i == argc - 1)
-		create_branch(argv[i], head, force_create, reflog);
+		create_branch(argv[i], head, head_sha1, force_create, reflog);
 	else if (i == argc - 2)
-		create_branch(argv[i], argv[i + 1], force_create, reflog);
+		create_branch(argv[i], argv[i+1], NULL, force_create, reflog);
 	else
 		usage(builtin_branch_usage);
 

cache.h

@@ -299,7 +299,7 @@ extern char *sha1_to_hex(const unsigned char *sha1);	/* static buffer result! */
 extern int read_ref(const char *filename, unsigned char *sha1);
 extern const char *resolve_ref(const char *path, unsigned char *sha1, int, int *);
 extern int create_symref(const char *ref, const char *refs_heads_master);
-extern int validate_symref(const char *ref);
+extern int validate_headref(const char *ref);
 
 extern int base_name_compare(const char *name1, int len1, int mode1, const char *name2, int len2, int mode2);
 extern int cache_name_compare(const char *name1, int len1, const char *name2, int len2);

git-checkout.sh

@@ -6,6 +6,7 @@ SUBDIRECTORY_OK=Sometimes
 
 old_name=HEAD
 old=$(git-rev-parse --verify $old_name 2>/dev/null)
+oldbranch=$(git-symbolic-ref $old_name 2>/dev/null)
 new=
 new_name=
 force=
@@ -13,6 +14,8 @@ branch=
 newbranch=
 newbranch_log=
 merge=
+LF='
+'
 while [ "$#" != "0" ]; do
     arg="$1"
     shift
@@ -50,7 +53,7 @@ while [ "$#" != "0" ]; do
 				exit 1
 			fi
 			new="$rev"
-			new_name="$arg^0"
+			new_name="$arg"
 			if git-show-ref --verify --quiet -- "refs/heads/$arg"
 			then
 				branch="$arg"
@@ -139,23 +142,49 @@ fi
 
 [ -z "$new" ] && new=$old && new_name="$old_name"
 
-# If we don't have an old branch that we're switching to,
+# If we don't have an existing branch that we're switching to,
 # and we don't have a new branch name for the target we
-# are switching to, then we'd better just be checking out
-# what we already had
+# are switching to, then we are detaching our HEAD from any
+# branch.  However, if "git checkout HEAD" detaches the HEAD
+# from the current branch, even though that may be logically
+# correct, it feels somewhat funny.  More importantly, we do not
+# want "git checkout" nor "git checkout -f" to detach HEAD.
 
-[ -z "$branch$newbranch" ] &&
-	[ "$new" != "$old" ] &&
-	die "git checkout: provided reference cannot be checked out directly
+detached=
+detach_warn=
 
-  You need -b to associate a new branch with the wanted checkout. Example:
-  git checkout -b <new_branch_name> $arg
-"
+if test -z "$branch$newbranch" && test "$new" != "$old"
+then
+	detached="$new"
+	if test -n "$oldbranch"
+	then
+		detach_warn="warning: you are not on ANY branch anymore.
+If you meant to create a new branch from the commit, you need -b to
+associate a new branch with the wanted checkout.  Example:
+  git checkout -b <new_branch_name> $arg"
+	fi
+elif test -z "$oldbranch" && test -n "$branch"
+then
+	# Coming back...
+	if test -z "$force"
+	then
+		git show-ref -d -s | grep "$old" >/dev/null || {
+			echo >&2 \
+"You are not on any branch and switching to branch '$new_name'
+may lose your changes.  At this point, you can do one of two things:
+ (1) Decide it is Ok and say 'git checkout -f $new_name';
+ (2) Start a new branch from the current commit, by saying
+     'git checkout -b <branch-name>'.
+Leaving your HEAD detached; not switching to branch '$new_name'."
+			exit 1;
+		}
+	fi
+fi
 
 if [ "X$old" = X ]
 then
-	echo "warning: You do not appear to currently be on a branch." >&2
-	echo "warning: Forcing checkout of $new_name." >&2
+	echo >&2 "warning: You appear to be on a branch yet to be born."
+	echo >&2 "warning: Forcing checkout of $new_name."
 	force=1
 fi
 
@@ -226,8 +255,25 @@ if [ "$?" -eq 0 ]; then
 		git-update-ref -m "checkout: Created from $new_name" "refs/heads/$newbranch" $new || exit
 		branch="$newbranch"
 	fi
-	[ "$branch" ] &&
+	if test -n "$branch"
+	then
 		GIT_DIR="$GIT_DIR" git-symbolic-ref HEAD "refs/heads/$branch"
+	elif test -n "$detached"
+	then
+		# NEEDSWORK: we would want a command to detach the HEAD
+		# atomically, instead of this handcrafted command sequence.
+		# Perhaps:
+		#	git update-ref --detach HEAD $new
+		# or something like that...
+		#
+		echo "$detached" >"$GIT_DIR/HEAD.new" &&
+		mv "$GIT_DIR/HEAD.new" "$GIT_DIR/HEAD" ||
+			die "Cannot detach HEAD"
+		if test -n "$detach_warn"
+		then
+			echo >&2 "$detach_warn"
+		fi
+	fi
 	rm -f "$GIT_DIR/MERGE_HEAD"
 else
 	exit 1

path.c

@@ -90,10 +90,11 @@ int git_mkstemp(char *path, size_t len, const char *template)
 }
 
 
-int validate_symref(const char *path)
+int validate_headref(const char *path)
 {
 	struct stat st;
 	char *buf, buffer[256];
+	unsigned char sha1[20];
 	int len, fd;
 
 	if (lstat(path, &st) < 0)
@@ -119,14 +120,23 @@ int validate_symref(const char *path)
 	/*
 	 * Is it a symbolic ref?
 	 */
-	if (len < 4 || memcmp("ref:", buffer, 4))
+	if (len < 4)
 		return -1;
+	if (!memcmp("ref:", buffer, 4)) {
 		buf = buffer + 4;
 		len -= 4;
 		while (len && isspace(*buf))
 			buf++, len--;
 		if (len >= 5 && !memcmp("refs/", buf, 5))
 			return 0;
+	}
+
+	/*
+	 * Is this a detached HEAD?
+	 */
+	if (!get_sha1_hex(buffer, sha1))
+		return 0;
+
 	return -1;
 }
 
@@ -241,7 +251,7 @@ char *enter_repo(char *path, int strict)
 		return NULL;
 
 	if (access("objects", X_OK) == 0 && access("refs", X_OK) == 0 &&
-	    validate_symref("HEAD") == 0) {
+	    validate_headref("HEAD") == 0) {
 		putenv("GIT_DIR=.");
 		check_repository_format();
 		return path;

setup.c

@@ -138,7 +138,8 @@ const char **get_pathspec(const char *prefix, const char **pathspec)
  *    GIT_OBJECT_DIRECTORY environment variable
  *  - a refs/ directory
  *  - either a HEAD symlink or a HEAD file that is formatted as
- *    a proper "ref:".
+ *    a proper "ref:", or a regular file HEAD that has a properly
+ *    formatted sha1 object name.
  */
 static int is_git_directory(const char *suspect)
 {
@@ -161,7 +162,7 @@ static int is_git_directory(const char *suspect)
 		return 0;
 
 	strcpy(path + len, "/HEAD");
-	if (validate_symref(path))
+	if (validate_headref(path))
 		return 0;
 
 	return 1;

t/t3200-branch.sh

@@ -48,7 +48,7 @@ test_expect_success \
 	 test ! -f .git/logs/refs/heads/d/e/f'
 
 cat >expect <<EOF
-0000000000000000000000000000000000000000 $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000	checkout: Created from master^0
+0000000000000000000000000000000000000000 $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000	checkout: Created from master
 EOF
 test_expect_success \
     'git checkout -b g/h/i -l should create a branch and a log' \

wt-status.c

@@ -302,9 +302,18 @@ void wt_status_print(struct wt_status *s)
 	unsigned char sha1[20];
 	s->is_initial = get_sha1(s->reference, sha1) ? 1 : 0;
 
-	if (s->branch)
+	if (s->branch) {
+		const char *on_what = "On branch ";
+		const char *branch_name = s->branch;
+		if (!strncmp(branch_name, "refs/heads/", 11))
+			branch_name += 11;
+		else if (!strcmp(branch_name, "HEAD")) {
+			branch_name = "";
+			on_what = "Not currently on any branch.";
+		}
 		color_printf_ln(color(WT_STATUS_HEADER),
-			"# On branch %s", s->branch);
+			"# %s%s", on_what, branch_name);
+	}
 
 	if (s->is_initial) {
 		color_printf_ln(color(WT_STATUS_HEADER), "#");