One of its callers assumes a non-zero result indicates a permanent failure
(i.e. the current attempt to connect to this HS either has failed or is
doomed). The other caller only requires that this function's result
never equal -2.
Bug reported by Sebastian Hahn.
/** Extend the introduction circuit <b>circ</b> to another valid
* introduction point for the hidden service it is trying to connect
* to, or mark it and launch a new circuit if we can't extend it.
- * Return 0 on success. Return -1 and mark the introduction
- * circuit on failure.
+ * Return 0 on success or possible success. Return -1 and mark the
+ * introduction circuit for close on permanent failure.
*
* On failure, the caller is responsible for marking the associated
* rendezvous circuit for close. */