sysroot: Write symlinks before calling fsync(), then rename after

There might be a race here in that we create new symlink files *after*
calling `syncfs`, and they are not guaranteed to end up on disk.

Rework the code so that we create symlinks before, and then only
rename them after (and `fsync()` the directory for good measure).

Additional-fixes-by: Giuseppe Scrivano <gscrivan@redhat.com>
Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>

This still needs verification that we're fixing a real bug; but I'm
fairly confident this won't make the fsync situation worse.

https://bugzilla.gnome.org/show_bug.cgi?id=755595

src/libotutil/ot-gio-utils.c

@@ -508,59 +508,3 @@ ot_util_ensure_directory_and_fsync (GFile         *dir,
     (void) close (parentfd);
   return ret;
 }
-
-/**
- * ot_gfile_atomic_symlink_swap:
- * @path: Replace the contents of this symbolic link
- * @target: New symbolic link target
- * @cancellable:
- * @error
- *
- * Create a new temporary symbolic link, then use the Unix rename()
- * function to atomically replace @path with the new symbolic link.
- * Do not use this function inside directories such as /tmp as it uses
- * a predicatable file name.
- */
-gboolean
-ot_gfile_atomic_symlink_swap (GFile          *path,
-                              const char     *target,
-                              GCancellable   *cancellable,
-                              GError        **error)
-{
-  gboolean ret = FALSE;
-  g_autoptr(GFile) parent = g_file_get_parent (path);
-  g_autofree char *tmpname = g_strconcat (gs_file_get_basename_cached (path), ".tmp", NULL);
-  g_autoptr(GFile) tmppath = g_file_get_child (parent, tmpname);
-  int parent_dfd = -1;
-
-  if (!ot_gfile_ensure_unlinked (tmppath, cancellable, error))
-    goto out;
-  
-  if (!g_file_make_symbolic_link (tmppath, target, cancellable, error))
-    goto out;
-
-  if (!gs_file_open_dir_fd (parent, &parent_dfd, cancellable, error))
-    goto out;
-
-  /* Ensure the link has hit disk */
-  if (fsync (parent_dfd) != 0)
-    {
-      gs_set_error_from_errno (error, errno);
-      goto out;
-    }
-
-  if (!gs_file_rename (tmppath, path, cancellable, error))
-    goto out;
-
-  /* And sync again for good measure */
-  if (fsync (parent_dfd) != 0)
-    {
-      gs_set_error_from_errno (error, errno);
-      goto out;
-    }
-
-  ret = TRUE;
- out:
-  if (parent_dfd != -1) (void) close (parent_dfd);
-  return ret;
-}

src/libotutil/ot-gio-utils.h

@@ -85,11 +85,6 @@ gboolean ot_gfile_ensure_unlinked (GFile         *path,
                                    GCancellable  *cancellable,
                                    GError       **error);
 
-gboolean ot_gfile_atomic_symlink_swap (GFile          *path,
-                                       const char     *target,
-                                       GCancellable   *cancellable,
-                                       GError        **error);
-
 gboolean ot_util_ensure_directory_and_fsync (GFile         *dir,
                                              GCancellable  *cancellable,
                                              GError       **error);