Fix retryingJitteredExponentialBackoffDur when jitter_param=0

Summary:
`retryingJitteredExponentialBackoffDur()` supports `jitter_param=0` (sanctioned in tests, and widely used to disable jitter based on a runtime parameter), but `std::normal_distribution` does not, so just special-case it.

In practice, libstdc++ already returns 0.

Reviewed By: yfeldblum

Differential Revision: D31722687

fbshipit-source-id: d81b1736379c596c4fc352b0067a4ad5788b0f8c

folly/futures/Retrying.h

@@ -200,12 +200,13 @@ Duration retryingJitteredExponentialBackoffDur(
     Duration backoff_max,
     double jitter_param,
     URNG& rng) {
+  using dist = std::normal_distribution<double>;
+
   // short-circuit to avoid 0 * inf = nan when computing backoff_rep below
   if (UNLIKELY(backoff_min == Duration(0))) {
     return Duration(0);
   }
-  auto dist = std::normal_distribution<double>(0.0, jitter_param);
-  auto jitter = std::exp(dist(rng));
+  auto jitter = jitter_param > 0 ? std::exp(dist{0., jitter_param}(rng)) : 1.;
   auto backoff_rep = jitter * backoff_min.count() * std::pow(2, n - 1);
   if (UNLIKELY(backoff_rep >= static_cast<double>(backoff_max.count()))) {
     return std::max(backoff_min, backoff_max);

folly/futures/test/RetryingTest.cpp

@@ -48,9 +48,9 @@ void multiAttemptExpectDurationWithin(
   }
   sort(durations.begin(), durations.end());
   for (auto d : durations) {
-    EXPECT_GE(d, min_duration);
+    EXPECT_GE(d.count(), min_duration.count());
   }
-  EXPECT_LE(durations[0], max_duration);
+  EXPECT_LE(durations[0].count(), max_duration.count());
 }
 
 TEST(RetryingTest, has_op_call) {
@@ -272,17 +272,17 @@ TEST(RetryingTest, policy_capped_jittered_exponential_backoff_many_retries) {
   Duration max_backoff(10000000);
   Duration backoff = retryingJitteredExponentialBackoffDur(
       80, min_backoff, max_backoff, 0, rng);
-  EXPECT_EQ(backoff, max_backoff);
+  EXPECT_EQ(backoff.count(), max_backoff.count());
 
   max_backoff = Duration(std::numeric_limits<int64_t>::max());
   backoff = retryingJitteredExponentialBackoffDur(
       63, min_backoff, max_backoff, 0, rng);
-  EXPECT_LT(backoff, max_backoff);
+  EXPECT_LT(backoff.count(), max_backoff.count());
 
   max_backoff = Duration(std::numeric_limits<int64_t>::max());
   backoff = retryingJitteredExponentialBackoffDur(
       64, min_backoff, max_backoff, 0, rng);
-  EXPECT_EQ(backoff, max_backoff);
+  EXPECT_EQ(backoff.count(), max_backoff.count());
 }
 
 TEST(RetryingTest, policy_capped_jittered_exponential_backoff_min_zero) {