Fix IntervalRateLimiter initial state

Summary: When using monotonic clocks the epoch may be reset on system reboot, so `0` may not be enough in the past when requesting a large interval to guarantee that the first check succeeds. Switch to the minimum integer instead.

Reviewed By: satyajanga, terrelln

Differential Revision: D51509394

fbshipit-source-id: 8ff1339184f4cfc9a179e24bcfd96229e39b7c57

folly/logging/RateLimiter.cpp

@@ -33,7 +33,7 @@ bool IntervalRateLimiter::checkSlow() {
     return false;
   }
 
-  if (ts == 0) {
+  if (ts == kInitialTimestamp) {
     // If we initialized timestamp_ for the very first time increment count_ by
     // one instead of setting it to 0.  Our original increment made it roll over
     // to 0, so other threads may have already incremented it again and passed

folly/logging/RateLimiter.h

@@ -19,6 +19,7 @@
 #include <atomic>
 #include <chrono>
 #include <cstdint>
+#include <type_traits>
 
 #include <folly/Chrono.h>
 
@@ -49,6 +50,14 @@ class IntervalRateLimiter {
   }
 
  private:
+  // First check should always succeed, so initial timestamp is at the beginning
+  // of time.
+  static_assert(
+      std::is_signed<clock::rep>::value,
+      "Need signed time point to represent initial time");
+  constexpr static auto kInitialTimestamp =
+      std::numeric_limits<clock::rep>::min();
+
   bool checkSlow();
 
   const uint64_t maxPerInterval_;
@@ -61,7 +70,7 @@ class IntervalRateLimiter {
   // Ideally timestamp_ would be a
   // std::atomic<clock::time_point>, but this does not
   // work since time_point's constructor is not noexcept
-  std::atomic<clock::rep> timestamp_{0};
+  std::atomic<clock::rep> timestamp_{kInitialTimestamp};
 };
 
 } // namespace logging

folly/logging/test/RateLimiterTest.cpp

@@ -119,3 +119,8 @@ TEST(RateLimiter, concurrentThreads) {
   // We should have passed the check exactly maxEvents times
   EXPECT_EQ(maxEvents, count.load(std::memory_order_relaxed));
 }
+
+TEST(RateLimiter, LargeInterval) {
+  IntervalRateLimiter limiter{1, std::chrono::years{1}};
+  EXPECT_TRUE(limiter.check());
+}