Fix test prob distr parameters that caused warnings.

They were causing the following warnings in circuitpadding/circuitpadding_sample_distribution:

     src/lib/math/prob_distr.c:1311:17: runtime error: division by zero
     SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/lib/math/prob_distr.c:1311:17 in
     src/lib/math/prob_distr.c:1219:49: runtime error: division by zero
     SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/lib/math/prob_distr.c:1219:49 in

because the distributions were called with erroneous parameters (e.g. geometric
distribution with p=0).

We now defined these test probability distributions with more realistic
parameters.

As far as the circuitpadding_sample_distribution() test is concerned, it
doesn't matter if the distributions return values outside of [0,10] since we
already restrict the values into that interval using min=0 and max=10 (and RTT
estimate is disabled).

src/core/or/circuitpadding.c

@@ -389,11 +389,14 @@ circpad_distribution_sample_iat_delay(const circpad_state_t *state,
   double val = circpad_distribution_sample(state->iat_dist);
   /* These comparisons are safe, because the output is in the range
    * [0, 2**32), and double has a precision of 53 bits. */
+  /* We want a positive sample value */
   val = MAX(0, val);
+  /* Respect the maximum sample setting */
   val = MIN(val, state->dist_max_sample_usec);
 
-  /* This addition is exact: val is at most 2**32-1, min_delay
+  /* Now apply the shift:
-   * is at most 2**32-1, and doubles have a precision of 53 bits. */
+   * This addition is exact: val is at most 2**32-1, delay_shift is at most
+   * 2**32-1, and doubles have a precision of 53 bits. */
   val += delay_shift;
 
   /* Clamp the distribution at infinite delay val */

src/core/or/circuitpadding.h

@@ -198,14 +198,23 @@ typedef enum {
  * These can be used instead of histograms for the inter-packet
  * timing distribution, or to specify a distribution on the number
  * of cells that can be sent while in a specific state of the state
- * machine. */
+ * machine.
+ *
+ * Each distribution takes up to two parameters which are described below. */
 typedef enum {
+  /* No probability distribution is used */
   CIRCPAD_DIST_NONE = 0,
+  /* Uniform distribution: param1 is lower bound and param2 is upper bound */
   CIRCPAD_DIST_UNIFORM = 1,
+  /* Logistic distribution: param1 is Mu, param2 is sigma. */
   CIRCPAD_DIST_LOGISTIC = 2,
+  /* Log-logistic distribution: param1 is Alpha, param2 is 1.0/Beta */
   CIRCPAD_DIST_LOG_LOGISTIC = 3,
+  /* Geometric distribution: param1 is 'p' (success probability) */
   CIRCPAD_DIST_GEOMETRIC = 4,
+  /* Weibull distribution: param1 is k, param2 is Lambda */
   CIRCPAD_DIST_WEIBULL = 5,
+  /* Generalized Pareto distribution: param1 is sigma, param2 is xi */
   CIRCPAD_DIST_PARETO = 6
 } circpad_distribution_type_t;
 

src/test/test_circuitpadding.c

@@ -2096,6 +2096,7 @@ helper_circpad_circ_distribution_machine_setup(int min, int max)
   circpad_state_t *zero_st = &circ_client_machine.states[0];
   zero_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 1;
   zero_st->iat_dist.type = CIRCPAD_DIST_UNIFORM;
+  /* param2 is upper bound, param1 is lower */
   zero_st->iat_dist.param1 = min;
   zero_st->iat_dist.param2 = max;
   zero_st->dist_added_shift_usec = min;
@@ -2104,48 +2105,50 @@ helper_circpad_circ_distribution_machine_setup(int min, int max)
   circpad_state_t *first_st = &circ_client_machine.states[1];
   first_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 2;
   first_st->iat_dist.type = CIRCPAD_DIST_LOGISTIC;
-  first_st->iat_dist.param1 = min;
+  /* param1 is Mu, param2 is sigma. */
-  first_st->iat_dist.param2 = max;
+  first_st->iat_dist.param1 = 9;
+  first_st->iat_dist.param2 = 3;
   first_st->dist_added_shift_usec = min;
   first_st->dist_max_sample_usec = max;
 
   circpad_state_t *second_st = &circ_client_machine.states[2];
   second_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 3;
   second_st->iat_dist.type = CIRCPAD_DIST_LOG_LOGISTIC;
-  second_st->iat_dist.param1 = min;
+  /* param1 is Alpha, param2 is 1.0/Beta */
-  second_st->iat_dist.param2 = max;
+  second_st->iat_dist.param1 = 1;
+  second_st->iat_dist.param2 = 0.5;
   second_st->dist_added_shift_usec = min;
   second_st->dist_max_sample_usec = max;
 
   circpad_state_t *third_st = &circ_client_machine.states[3];
   third_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 4;
   third_st->iat_dist.type = CIRCPAD_DIST_GEOMETRIC;
-  third_st->iat_dist.param1 = min;
+  /* param1 is 'p' (success probability) */
-  third_st->iat_dist.param2 = max;
+  third_st->iat_dist.param1 = 0.2;
   third_st->dist_added_shift_usec = min;
   third_st->dist_max_sample_usec = max;
 
   circpad_state_t *fourth_st = &circ_client_machine.states[4];
   fourth_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 5;
   fourth_st->iat_dist.type = CIRCPAD_DIST_WEIBULL;
-  fourth_st->iat_dist.param1 = min;
+  /* param1 is k, param2 is Lambda */
-  fourth_st->iat_dist.param2 = max;
+  fourth_st->iat_dist.param1 = 1.5;
+  fourth_st->iat_dist.param2 = 1;
   fourth_st->dist_added_shift_usec = min;
   fourth_st->dist_max_sample_usec = max;
 
   circpad_state_t *fifth_st = &circ_client_machine.states[5];
   fifth_st->next_state[CIRCPAD_EVENT_NONPADDING_RECV] = 6;
   fifth_st->iat_dist.type = CIRCPAD_DIST_PARETO;
-  fifth_st->iat_dist.param1 = min;
+  /* param1 is sigma, param2 is xi */
-  fifth_st->iat_dist.param2 = max;
+  fifth_st->iat_dist.param1 = 1;
+  fifth_st->iat_dist.param2 = 5;
   fifth_st->dist_added_shift_usec = min;
   fifth_st->dist_max_sample_usec = max;
 }
 
 /** Simple test that the padding delays sampled from a uniform distribution
  *  actually faill within the uniform distribution range. */
-/* TODO: Upgrade this test so that each state tests a different prob
- * distribution */
 static void
 test_circuitpadding_sample_distribution(void *arg)
 {
@@ -2159,8 +2162,7 @@ test_circuitpadding_sample_distribution(void *arg)
   MOCK(circpad_machine_schedule_padding,
        circpad_machine_schedule_padding_mock);
 
-  /* Initialize a machine with multiple probability distributions that should
+  /* Initialize a machine with multiple probability distributions */
-   * return values between 0 and 5 */
   circpad_machines_init();
   helper_circpad_circ_distribution_machine_setup(0, 10);