Fix crash in SingletonRelaxedCounter

Summary: [Folly] Fix crash in `SingletonRelaxedCounter` when recreating threads.

Reviewed By: andriigrynenko

Differential Revision: D13645448

fbshipit-source-id: dea09925f1943ebf2141d69b791a9259524c19e1

folly/experimental/SingletonRelaxedCounter.h

@@ -132,6 +132,7 @@ class SingletonRelaxedCounter {
           global.fallback.fetch_add(current, std::memory_order_relaxed);
         }
       }
+      tracking->lifetimes.erase(this);
     }
 
     void track(CounterAndCache& state) {

folly/experimental/test/SingletonRelaxedCounterTest.cpp

@@ -44,7 +44,22 @@ TEST_F(SingletonRelaxedCounterTest, Basic) {
 TEST_F(SingletonRelaxedCounterTest, MultithreadCorrectness) {
   static constexpr size_t const kPerThreadIncrements = 1000000;
   static constexpr size_t const kNumThreads = 24;
+  static constexpr size_t const kNumCounters = 6;
+  static constexpr size_t const kNumIters = 2;
 
+  std::atomic<bool> stop{false};
+  std::vector<std::thread> counters(kNumCounters);
+
+  for (auto& counter : counters) {
+    counter = std::thread([&] {
+      while (!stop.load(std::memory_order_relaxed)) {
+        std::this_thread::yield();
+        Counter::count();
+      }
+    });
+  }
+
+  for (size_t j = 0; j < kNumIters; ++j) {
     std::vector<std::thread> threads(kNumThreads);
 
     boost::barrier barrier{kNumThreads + 1};
@@ -82,5 +97,13 @@ TEST_F(SingletonRelaxedCounterTest, MultithreadCorrectness) {
     for (auto& thread : threads) {
       thread.join();
     }
+  }
+
+  stop.store(true, std::memory_order_relaxed);
+  for (auto& counter : counters) {
+    counter.join();
+  }
+
+  Counter::count();
 }
 } // namespace folly