+/* Fetch and clear the stored thread_id for message, or NULL if none. */
+static char *
+_consume_metadata_thread_id (void *ctx, notmuch_database_t *notmuch,
+ notmuch_message_t *message)
+{
+ const char *message_id;
+ string stored_id;
+ char *metadata_key;
+
+ message_id = notmuch_message_get_message_id (message);
+ metadata_key = _get_metadata_thread_id_key (ctx, message_id);
+
+ /* Check if we have already seen related messages to this one.
+ * If we have then use the thread_id that we stored at that time.
+ */
+ stored_id = notmuch->xapian_db->get_metadata (metadata_key);
+ if (stored_id.empty ()) {
+ return NULL;
+ } else {
+ Xapian::WritableDatabase *db;
+
+ db = static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db);
+
+ /* Clear the metadata for this message ID. We don't need it
+ * anymore. */
+ db->set_metadata (metadata_key, "");
+
+ return talloc_strdup (ctx, stored_id.c_str ());
+ }
+}
+