Fix for RTT calculation hang during congestion control.

This should allow RTT calculation to resume as soon as one circuit gets a sane
RTT, post clock jump.

src/core/or/congestion_control_common.c

@@ -742,12 +742,10 @@ time_delta_stalled_or_jumped(const congestion_control_t *cc,
     return is_monotime_clock_broken;
   }
 
-  /* If the old_delta is 0, we have no previous values on this circuit.
-   *
-   * So, return the global monotime status from other circuits, and
-   * do not update.
+  /* If we have no previous values on this circuit, return the
+   * global monotime status from other circuits, and do not update.
    */
-  if (old_delta == 0) {
+  if (!cc->got_first_rtt) {
     return is_monotime_clock_broken;
   }
 
@@ -776,8 +774,13 @@ time_delta_stalled_or_jumped(const congestion_control_t *cc,
   }
 
   /* If new_delta is significantly larger than old_delta, then
-   * this means that the monotime clock suddenly jumped forward. */
-  if (new_delta > old_delta * DELTA_DISCREPENCY_RATIO_MAX) {
+   * this means that the monotime clock suddenly jumped forward.
+   *
+   * The old_delta+1 here is because if RTT gets wedged, old_delta
+   * will be stuck at 0 on new circuits, causing this check to always
+   * return cached value and never update.
+   */
+  if (new_delta > (old_delta+1) * DELTA_DISCREPENCY_RATIO_MAX) {
     static ratelim_t dec_notice_limit = RATELIM_INIT(300);
     log_fn_ratelim(&dec_notice_limit, LOG_NOTICE, LD_CIRC,
            "Sudden increase in circuit RTT (%"PRIu64" vs %"PRIu64
@@ -831,9 +834,12 @@ congestion_control_update_circuit_rtt(congestion_control_t *cc,
 
   /* Do not update RTT at all if it looks fishy */
   if (time_delta_stalled_or_jumped(cc, cc->ewma_rtt_usec, rtt)) {
+    cc->got_first_rtt = 1;
     return 0;
   }
 
+  cc->got_first_rtt = 1;
+
   ewma_cnt = n_ewma_count(cc);
 
   cc->ewma_rtt_usec = n_count_ewma(rtt, cc->ewma_rtt_usec, ewma_cnt);

src/core/or/congestion_control_st.h

@@ -159,6 +159,9 @@ struct congestion_control_t {
   /** Are we in slow start? */
   bool in_slow_start;
 
+  /** Did we get at least 1 RTT measurement? */
+  bool got_first_rtt;
+
   /** Is the local channel blocked on us? That's a congestion signal */
   bool blocked_chan;