lib/repo: Take exclusive lock while generating summary

This ensures that commits aren't deleted and refs aren't added, removed,
or updated while the summary is being generated. This is in preparation
for adding a repo config option that will automatically regenerate the
summary on every ref change.

Closes: #1681
Approved by: jlebon

src/libostree/ostree-repo.c

@@ -5394,6 +5394,8 @@ summary_add_ref_entry (OstreeRepo       *self,
  * file, listed under the %OSTREE_SUMMARY_COLLECTION_MAP key. Collection IDs
  * and refs in %OSTREE_SUMMARY_COLLECTION_MAP are guaranteed to be in
  * lexicographic order.
+ *
+ * Locking: exclusive
  */
 gboolean
 ostree_repo_regenerate_summary (OstreeRepo     *self,
@@ -5401,6 +5403,18 @@ ostree_repo_regenerate_summary (OstreeRepo     *self,
                                 GCancellable   *cancellable,
                                 GError        **error)
 {
+  /* Take an exclusive lock. This makes sure the commits and deltas don't get
+   * deleted while generating the summary. It also means we can be sure refs
+   * won't be created/updated/deleted during the operation, without having to
+   * add exclusive locks to those operations which would prevent concurrent
+   * commits from working.
+   */
+  g_autoptr(OstreeRepoAutoLock) lock = NULL;
+  lock = _ostree_repo_auto_lock_push (self, OSTREE_REPO_LOCK_EXCLUSIVE,
+                                      cancellable, error);
+  if (!lock)
+    return FALSE;
+
   g_auto(GVariantDict) additional_metadata_builder = OT_VARIANT_BUILDER_INITIALIZER;
   g_variant_dict_init (&additional_metadata_builder, additional_metadata);
   g_autoptr(GVariantBuilder) refs_builder = g_variant_builder_new (G_VARIANT_TYPE ("a(s(taya{sv}))"));