+static void
+insert_thread_id (GHashTable *thread_ids, Xapian::Document doc)
+{
+ string value_string;
+ const char *value;
+
+ value_string = doc.get_value (NOTMUCH_VALUE_THREAD);
+ value = value_string.c_str();
+ if (strlen (value))
+ g_hash_table_insert (thread_ids, strdup (value), NULL);
+}
+
+/* Return one or more thread_ids, (as a GPtrArray of strings), for the
+ * given message based on looking into the database for any messages
+ * referenced in parents, and also for any messages in the database
+ * referencing message_id.
+ *
+ * Caller should free all strings in the array and the array itself,
+ * (g_ptr_array_free) when done. */
+static GPtrArray *
+find_thread_ids (Xapian::Database db,
+ GPtrArray *parents,
+ const char *message_id)