circuit_build_failed: distinguish "first hop chan failed", "CREATE failed"

Roger spotted this on tor-dev in his comments on proposal 221.

(Actually, detect DESTROY vs everything else, since arma likes
network timeout indicating failure but not overload indicating failure.)

src/or/circuituse.c

@@ -1382,10 +1382,11 @@ circuit_build_failed(origin_circuit_t *circ)
     failed_at_last_hop = 1;
   }
   if (circ->cpath &&
-      circ->cpath->state != CPATH_STATE_OPEN) {
-    /* We failed at the first hop. If there's an OR connection
-     * to blame, blame it. Also, avoid this relay for a while, and
-     * fail any one-hop directory fetches destined for it. */
+      circ->cpath->state != CPATH_STATE_OPEN &&
+      ! circ->base_.received_destroy) {
+    /* We failed at the first hop for some reason other than a DESTROY cell.
+     * If there's an OR connection to blame, blame it. Also, avoid this relay
+     * for a while, and fail any one-hop directory fetches destined for it. */
     const char *n_chan_id = circ->cpath->extend_info->identity_digest;
     int already_marked = 0;
     if (circ->base_.n_chan) {

src/or/command.c

@@ -499,6 +499,7 @@ command_process_destroy_cell(cell_t *cell, channel_t *chan)
   log_debug(LD_OR,"Received for circID %u.",(unsigned)cell->circ_id);
 
   reason = (uint8_t)cell->payload[0];
+  circ->received_destroy = 1;
 
   if (!CIRCUIT_IS_ORIGIN(circ) &&
       cell->circ_id == TO_OR_CIRCUIT(circ)->p_circ_id) {

src/or/or.h

@@ -2785,6 +2785,9 @@ typedef struct circuit_t {
    * allowing n_streams to add any more cells. (OR circuit only.) */
   unsigned int streams_blocked_on_p_chan : 1;
 
+  /** True iff this circuit has received a DESTROY cell in either direction */
+  unsigned int received_destroy : 1;
+
   uint8_t state; /**< Current status of this circuit. */
   uint8_t purpose; /**< Why are we creating this circuit? */