Fix UndefinedBehaviorSanitizer error in `retryingPolicyCappedJitteredExponentialBackoff`

Summary:
While running some unit tests which happen to call into `folly::futures::retryingPolicyCappedJitteredExponentialBackoff`, I noticed that  `retryingJitteredExponentialBackoffDur` was triggering a "float-cast-overflow" error from UndefinedBehaviorSanitizer.

It turns out that this only happened if `backoff_min` was 0 and there were at least 1025 retries. The large retry count caused the exponential term in the calculation to become `Infinity`. In floating point, Infinity multiplied by 0 was `NaN`, causing the sanitizer error.

Although `backoff_min` should typically not be set to 0 because this effectively disables all backoff, I think there may be cases, such as unit tests, where setting `backoff_min` to 0 does make sense. This commit adds an additional check to handle this unusual case.

Reviewed By: yfeldblum

Differential Revision: D24122470

fbshipit-source-id: 48a8aede0d22e73cf001dab5eb184e841f3c23aa

folly/futures/Retrying.h

@@ -178,6 +178,10 @@ Duration retryingJitteredExponentialBackoffDur(
     Duration backoff_max,
     double jitter_param,
     URNG& rng) {
+  // 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 backoff_rep = jitter * backoff_min.count() * std::pow(2, n - 1);

folly/futures/test/RetryingTest.cpp

@@ -207,6 +207,30 @@ TEST(RetryingTest, policy_capped_jittered_exponential_backoff_many_retries) {
   EXPECT_EQ(backoff, max_backoff);
 }
 
+TEST(RetryingTest, policy_capped_jittered_exponential_backoff_min_zero) {
+  using namespace futures::detail;
+  mt19937_64 rng(0);
+
+  Duration min_backoff(0);
+  Duration max_backoff(2000);
+
+  EXPECT_EQ(
+      retryingJitteredExponentialBackoffDur(5, min_backoff, max_backoff, 0, rng)
+          .count(),
+      0);
+
+  EXPECT_EQ(
+      retryingJitteredExponentialBackoffDur(5, min_backoff, max_backoff, 1, rng)
+          .count(),
+      0);
+
+  EXPECT_EQ(
+      retryingJitteredExponentialBackoffDur(
+          1025, min_backoff, max_backoff, 0, rng)
+          .count(),
+      0);
+}
+
 TEST(RetryingTest, policy_sleep_defaults) {
   multiAttemptExpectDurationWithin(5, milliseconds(200), milliseconds(400), [] {
     //  To ensure that this compiles with default params.