+const char *
+notmuch_message_get_thread_id (notmuch_message_t *message)
+{
+ Xapian::TermIterator i;
+
+ if (message->thread_id)
+ return message->thread_id;
+
+ i = message->doc.termlist_begin ();
+ i.skip_to (_find_prefix ("thread"));
+
+ if (i == message->doc.termlist_end ())
+ INTERNAL_ERROR ("Message with document ID of %d has no thread ID.\n",
+ message->doc_id);
+
+ message->thread_id = talloc_strdup (message, (*i).c_str () + 1);
+
+#if DEBUG_DATABASE_SANITY
+ i++;
+
+ if (i != message->doc.termlist_end () &&
+ strncmp ((*i).c_str (), _find_prefix ("thread"),
+ strlen (_find_prefix ("thread"))) == 0)
+ {
+ INTERNAL_ERROR ("Message with document ID of %d has duplicate thread IDs.\n",
+ message->doc_id);
+ }
+#endif
+
+ return message->thread_id;
+}
+