1 files changed, 85 insertions, 0 deletions
diff --git a/update-ref.c b/update-ref.c
new file mode 100644
index 0000000000..ba4bf5153e
--- /dev/null
+++ b/update-ref.c
@@ -0,0 +1,85 @@
+#include "cache.h"
+#include "refs.h"
+
+static const char git_update_ref_usage[] = "git-update-ref <refname> <value> [<oldval>]";
+
+static int re_verify(const char *path, unsigned char *oldsha1, unsigned char *currsha1)
+{
+	char buf[40];
+	int fd = open(path, O_RDONLY), nr;
+	if (fd < 0)
+		return -1;
+	nr = read(fd, buf, 40);
+	close(fd);
+	if (nr != 40 || get_sha1_hex(buf, currsha1) < 0)
+		return -1;
+	return memcmp(oldsha1, currsha1, 20) ? -1 : 0;
+}
+
+int main(int argc, char **argv)
+{
+	char *hex;
+	const char *refname, *value, *oldval, *path;
+	char *lockpath;
+	unsigned char sha1[20], oldsha1[20], currsha1[20];
+	int fd, written;
+
+	setup_git_directory();
+	git_config(git_default_config);
+	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, !!oldval);
+	if (!path)
+		die("No such ref: %s", refname);
+
+	if (oldval) {
+		if (memcmp(currsha1, oldsha1, 20))
+			die("Ref %s is at %s but expected %s", refname, sha1_to_hex(currsha1), sha1_to_hex(oldsha1));
+		/* Nothing to do? */
+		if (!memcmp(oldsha1, sha1, 20))
+			exit(0);
+	}
+	path = strdup(path);
+	lockpath = mkpath("%s.lock", path);
+	if (safe_create_leading_directories(lockpath) < 0)
+		die("Unable to create all of %s", lockpath);
+
+	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);
+	}
+
+	/*
+	 * Re-read the ref after getting the lock to verify
+	 */
+	if (oldval && re_verify(path, oldsha1, currsha1) < 0) {
+		unlink(lockpath);
+		die("Ref lock failed");
+	}
+
+	/*
+	 * Finally, replace the old ref with the new one
+	 */
+	if (rename(lockpath, path) < 0) {
+		unlink(lockpath);
+		die("Unable to create %s", path);
+	}
+	return 0;
+}