Detached HEAD (experimental)

This allows "git checkout v1.4.3" to dissociate the HEAD of
repository from any branch.  After this point, "git branch"
starts reporting that you are not on any branch.  You can go
back to an existing branch by saying "git checkout master", for
example.

This is still experimental.  While I think it makes sense to
allow commits on top of detached HEAD, it is rather dangerous
unless you are careful in the current form.  Next "git checkout
master" will obviously lose what you have done, so we might want
to require "git checkout -f" out of a detached HEAD if we find
that the HEAD commit is not an ancestor of any other branches.
There is no such safety valve implemented right now.

On the other hand, the reason the user did not start the ad-hoc
work on a new branch with "git checkout -b" was probably because
the work was of a throw-away nature, so the convenience of not
having that safety valve might be even better.  The user, after
accumulating some commits on top of a detached HEAD, can always
create a new branch with "git checkout -b" not to lose useful
work done while the HEAD was detached.

We'll see.

Signed-off-by: Junio C Hamano <junkio@cox.net>

5 files changed, 50 insertions, 23 deletions

diff --git a/builtin-branch.c b/builtin-branch.c
index 487965b540..c760e188ea 100644
--- a/builtin-branch.c
+++ b/builtin-branch.c
@@ -308,7 +308,8 @@ static void print_ref_list(int kinds, int detached, 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;
@@ -327,9 +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);
@@ -338,7 +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)
@@ -350,6 +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");
 
@@ -474,9 +484,9 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 	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);
 
diff --git a/cache.h b/cache.h
index 36be64e386..5218548ccf 100644
--- a/cache.h
+++ b/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);
diff --git a/git-checkout.sh b/git-checkout.sh
index 92ec069a3a..8e11ca4bc8 100755
--- a/git-checkout.sh
+++ b/git-checkout.sh
@@ -144,13 +144,19 @@ fi
 # are switching to, then we'd better just be checking out
 # what we already had
 
-[ -z "$branch$newbranch" ] &&
-	[ "$new" != "$old" ] &&
-	die "git checkout: provided reference cannot be checked out directly
-
-  You need -b to associate a new branch with the wanted checkout. Example:
+if test -z "$branch$newbranch" && test "$new" != "$old"
+then
+	# NEEDSWORK: we would want to have this command here
+	# that allows us to detach the HEAD atomically.
+	# git update-ref --detach HEAD "$new"
+	rm -f "$GIT_DIR/HEAD"
+	echo "$new" >"$GIT_DIR/HEAD"
+	echo >&2 "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
 
 if [ "X$old" = X ]
 then
diff --git a/path.c b/path.c
index 066f621955..94ddd7eafc 100644
--- a/path.c
+++ b/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;
-	buf = buffer + 4;
-	len -= 4;
-	while (len && isspace(*buf))
-		buf++, len--;
-	if (len >= 5 && !memcmp("refs/", buf, 5))
+	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;
diff --git a/setup.c b/setup.c
index 2ae57f7c94..cc97f9f5c1 100644
--- a/setup.c
+++ b/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;