[PATCH] Add "git-update-ref" to update the HEAD (or other) ref

This is a careful version of the script stuff that currently just
blindly writes HEAD with a new value.

You can use

	git-update-ref HEAD <newhead>

or

	git-update-ref HEAD <newhead> <oldhead>

where the latter version verifies that the old value of HEAD matches
oldhead.

It basically allows a "ref" file to be a symbolic pointer to another ref
file by starting with the four-byte header sequence of "ref:".

More importantly, it allows the update of a ref file to follow these
symbolic pointers, whether they are symlinks or these "regular file
symbolic refs".

NOTE! It follows _real_ symlinks only if they start with "refs/":
otherwise it will just try to read them and update them as a regular file
(ie it will allow the filesystem to follow them, but will overwrite such a
symlink to somewhere else with a regular filename).

In general, using

	git-update-ref HEAD "$head"

should be a _lot_ safer than doing

	echo "$head" > "$GIT_DIR/HEAD"

both from a symlink following standpoint _and_ an error checking
standpoint.  The "refs/" rule for symlinks means that symlinks that point
to "outside" the tree are safe: they'll be followed for reading but not
for writing (so we'll never write through a ref symlink to some other
tree, if you have copied a whole archive by creating a symlink tree).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -116,6 +116,7 @@ PROGRAMS = \
 	git-ssh-upload git-tar-tree git-unpack-file \
 	git-unpack-objects git-update-index git-update-server-info \
 	git-upload-pack git-verify-pack git-write-tree \
+	git-update-ref \
 	$(SIMPLE_PROGRAMS)
 
 # Backward compatibility -- to be removed in 0.99.8

update-ref.c

@@ -0,0 +1,111 @@
+#include "cache.h"
+#include "refs.h"
+
+static const char git_update_ref_usage[] = "git-update-ref <refname> <value> [<oldval>]";
+
+#define MAXDEPTH 5
+
+const char *resolve_ref(const char *path, unsigned char *sha1)
+{
+	int depth = MAXDEPTH, len;
+	char buffer[256];
+
+	for (;;) {
+		struct stat st;
+		int fd;
+
+		if (--depth < 0)
+			return NULL;
+
+		/* Special case: non-existing file */
+		if (lstat(path, &st) < 0) {
+			if (errno != ENOENT)
+				return NULL;
+			memset(sha1, 0, 20);
+			return path;
+		}
+
+		/* Follow "normalized" - ie "refs/.." symlinks by hand */
+		if (S_ISLNK(st.st_mode)) {
+			len = readlink(path, buffer, sizeof(buffer)-1);
+			if (len >= 5 && !memcmp("refs/", buffer, 5)) {
+				path = git_path("%.*s", len, buffer);
+				continue;
+			}
+		}
+
+		/*
+		 * Anything else, just open it and try to use it as
+		 * a ref
+		 */
+		fd = open(path, O_RDONLY);
+		if (fd < 0)
+			return NULL;
+		len = read(fd, buffer, sizeof(buffer)-1);
+		close(fd);
+		break;
+	}
+	if (len < 40 || get_sha1_hex(buffer, sha1))
+		return NULL;
+	return path;
+}
+
+int main(int argc, char **argv)
+{
+	char *hex;
+	const char *refname, *value, *oldval, *path, *lockpath;
+	unsigned char sha1[20], oldsha1[20], currsha1[20];
+	int fd, written;
+
+	setup_git_directory();
+	if (argc < 3 || argc > 4)
+		usage(git_update_ref_usage);
+
+	refname = argv[1];
+	value = argv[2];
+	oldval = argv[3];
+	if (get_sha1(value, sha1) < 0)
+		die("%s: not a valid SHA1", value);
+	memset(oldsha1, 0, 20);
+	if (oldval && get_sha1(oldval, oldsha1) < 0)
+		die("%s: not a valid old SHA1", oldval);
+
+	path = resolve_ref(git_path("%s", refname), currsha1);
+	if (!path)
+		die("No such ref: %s", refname);
+
+	if (oldval) {
+		if (memcmp(currsha1, oldsha1, 20))
+			die("Ref %s changed to %s", refname, sha1_to_hex(currsha1));
+		/* Nothing to do? */
+		if (!memcmp(oldsha1, sha1, 20))
+			exit(0);
+	}
+	path = strdup(path);
+	lockpath = mkpath("%s.lock", path);
+
+	fd = open(lockpath, O_CREAT | O_EXCL | O_WRONLY, 0666);
+	if (fd < 0)
+		die("Unable to create %s", lockpath);
+	hex = sha1_to_hex(sha1);
+	hex[40] = '\n';
+	written = write(fd, hex, 41);
+	close(fd);
+	if (written != 41) {
+		unlink(lockpath);
+		die("Unable to write to %s", lockpath);
+	}
+
+	/*
+	 * FIXME!
+	 *
+	 * We should re-read the old ref here, and re-verify that it
+	 * matches "oldsha1". Otherwise there's a small race.
+	 */
+
+	if (rename(lockpath, path) < 0) {
+		unlink(lockpath);
+		die("Unable to create %s", path);
+	}
+	return 0;
+}