repo: Never delete .commitmeta files

Do not delete a .commitmeta file after removing the last metadata entry.
This way a client will pull the empty .commitmeta file and overwrite old
metadata as expected.

https://bugzilla.gnome.org/750459

src/libostree/ostree-repo-commit.c

@@ -2140,24 +2140,7 @@ ostree_repo_write_commit_detached_metadata (OstreeRepo      *self,
       normalized_size = g_variant_get_size (normalized);
     }
 
-  if (normalized_size == 0)
-    {
-      GError *local_error = NULL;
-
-      (void) g_file_delete (metadata_path, cancellable, &local_error);
-
-      if (g_error_matches (local_error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND))
-        {
-          g_clear_error (&local_error);
-        }
-      else if (local_error != NULL)
-        {
-          g_propagate_error (error, local_error);
-          g_prefix_error (error, "Unable to delete detached metadata: ");
-          goto out;
-        }
-    }
-  else
+  if (normalized_size > 0)
     {
       if (!g_file_replace_contents (metadata_path,
                                     g_variant_get_data (normalized),
@@ -2169,6 +2152,21 @@ ostree_repo_write_commit_detached_metadata (OstreeRepo      *self,
           goto out;
         }
     }
+  else
+    {
+      g_autoptr(GFileOutputStream) output_stream = NULL;
+
+      /* Don't write to the stream, leave the file empty. */
+      output_stream = g_file_replace (metadata_path,
+                                      NULL, FALSE,
+                                      G_FILE_CREATE_NONE,
+                                      cancellable, error);
+      if (output_stream == NULL)
+        {
+          g_prefix_error (error, "Unable to write detached metadata: ");
+          goto out;
+        }
+    }
 
   ret = TRUE;
  out:

src/ostree/ot-builtin-trivial-httpd.c

@@ -273,7 +273,8 @@ do_get (OtTrivialHttpd    *self,
           buffer = soup_buffer_new_with_owner (g_mapped_file_get_contents (mapping),
                                                buffer_length,
                                                mapping, (GDestroyNotify)g_mapped_file_unref);
-          soup_message_body_append_buffer (msg->response_body, buffer);
+          if (buffer->length > 0)
+            soup_message_body_append_buffer (msg->response_body, buffer);
           soup_buffer_free (buffer);
         }
       else /* msg->method == SOUP_METHOD_HEAD */