allow /dev/fd/N as a mountpoint (#166)

sshfs.c

@@ -3471,7 +3471,19 @@ static int sshfs_opt_proc(void *data, const char *arg, int key,
  }
  }
 #else
- sshfs.mountpoint = realpath(arg, NULL);
+ int fd, len;
+ if (sscanf(arg, "/dev/fd/%u%n", &fd, &len) == 1 &&
+ len == strlen(arg)) {
+ /*
+ * Allow /dev/fd/N unchanged; it can be
+ * use for pre-mounting a generic fuse
+ * mountpoint to later be completely
+ * unprivileged with libfuse >= 3.3.0.
+ */
+ sshfs.mountpoint = arg;
+ } else {
+ sshfs.mountpoint = realpath(arg, NULL);
+ }
 #endif
  if (!sshfs.mountpoint) {
  fprintf(stderr, "sshfs: bad mount point `%s': %s\n",