+static int verify_old_ref(const char *name, char *hex_contents)
+{
+ int fd, ret;
+ char buffer[60];
+
+ fd = open(name, O_RDONLY);
+ if (fd < 0)
+ return -1;
+ ret = read(fd, buffer, 40);
+ close(fd);
+ if (ret != 40)
+ return -1;
+ if (memcmp(buffer, hex_contents, 40))
+ return -1;
+ return 0;
+}
+
+static void update(const char *name, unsigned char *old_sha1, unsigned char *new_sha1)
+{
+ char new_hex[60], *old_hex, *lock_name;
+ int newfd, namelen, written;
+
+ namelen = strlen(name);
+ lock_name = xmalloc(namelen + 10);
+ memcpy(lock_name, name, namelen);
+ memcpy(lock_name + namelen, ".lock", 6);
+
+ strcpy(new_hex, sha1_to_hex(new_sha1));
+ new_hex[40] = '\n';
+ new_hex[41] = 0;
+ old_hex = sha1_to_hex(old_sha1);
+ if (!has_sha1_file(new_sha1))
+ die("unpack should have generated %s, but I can't find it!", new_hex);
+
+ newfd = open(lock_name, O_CREAT | O_EXCL | O_WRONLY, 0644);
+ if (newfd < 0)
+ die("unable to create %s (%s)", lock_name, strerror(errno));
+ written = write(newfd, new_hex, 41);
+ close(newfd);
+ if (written != 41) {
+ unlink(lock_name);
+ die("unable to write %s", lock_name);
+ }
+ if (verify_old_ref(name, old_hex) < 0) {
+ unlink(lock_name);
+ die("%s changed during push", name);
+ }
+ if (rename(lock_name, name) < 0) {
+ unlink(lock_name);
+ die("unable to replace %s", name);
+ }
+ fprintf(stderr, "%s: %s -> %s\n", name, old_hex, new_hex);
+}
+
+