Bug 30173: Add consensus param to disable padding.

Disable padding via limit check and machine condition. Limits cause us to stop
sending padding. Machine conditions cause the machines to be shut down, and
not restarted.

src/core/or/circuitpadding.c

@@ -81,6 +81,7 @@ static void circpad_setup_machine_on_circ(circuit_t *on_circ,
 static double circpad_distribution_sample(circpad_distribution_t dist);
 
 /** Cached consensus params */
+static uint8_t circpad_padding_disabled;
 static uint8_t circpad_global_max_padding_percent;
 static uint16_t circpad_global_allowed_cells;
 static uint16_t circpad_max_circ_queued_cells;
@@ -1081,6 +1082,10 @@ circpad_send_padding_callback(tor_timer_t *timer, void *args,
 void
 circpad_new_consensus_params(const networkstatus_t *ns)
 {
+  circpad_padding_disabled =
+      networkstatus_get_param(ns, "circpad_padding_disabled",
+         0, 0, 1);
+
   circpad_global_allowed_cells =
       networkstatus_get_param(ns, "circpad_global_allowed_cells",
          0, 0, UINT16_MAX-1);
@@ -1112,10 +1117,19 @@ circpad_machine_reached_padding_limit(circpad_machine_runtime_t *mi)
 {
   const circpad_machine_spec_t *machine = CIRCPAD_GET_MACHINE(mi);
 
+  /* If padding has been disabled in the consensus, don't send any more
+   * padding. Technically the machine should be shut down when the next
+   * machine condition check happens, but machine checks only happen on
+   * certain circuit events, and if padding is disabled due to some
+   * network overload or DoS condition, we really want to stop ASAP. */
+  if (circpad_padding_disabled) {
+    return 1;
+  }
+
   /* If machine_padding_pct is non-zero, and we've sent more
    * than the allowed count of padding cells, then check our
    * percent limits for this machine. */
-   if (machine->max_padding_percent &&
+  if (machine->max_padding_percent &&
       mi->padding_sent >= machine->allowed_padding_count) {
     uint32_t total_cells = mi->padding_sent + mi->nonpadding_sent;
 
@@ -1621,6 +1635,11 @@ static inline bool
 circpad_machine_conditions_met(origin_circuit_t *circ,
                                const circpad_machine_spec_t *machine)
 {
+  /* If padding is disabled, no machines should match/apply. This has
+   * the effect of shutting down all machines, and not adding any more. */
+  if (circpad_padding_disabled)
+    return 0;
+
   if (!(circpad_circ_purpose_to_mask(TO_CIRCUIT(circ)->purpose)
       & machine->conditions.purpose_mask))
     return 0;