Fix lastWakeup_ accounting in ThrottledLifoSem

Summary: `lastWakeup_` should be updated with the timestamp *after* the sleep, otherwise the next sleep will be 0. This means that currently only every other thread in a chain of wake-ups waits for the wakeup interval.

Reviewed By: philippv

Differential Revision: D38684505

fbshipit-source-id: 02f6e033083b58a8d878c73b087395938bd78327

folly/synchronization/ThrottledLifoSem.h

@@ -210,14 +210,18 @@ class ThrottledLifoSem {
             std::is_same<Clock, std::chrono::steady_clock>::value
             ? now
             : std::chrono::steady_clock::now();
-        const auto nextWakeup =
-            std::exchange(lastWakeup_, steadyNow) + options_.wakeUpInterval;
+        const auto nextWakeup = lastWakeup_ + options_.wakeUpInterval;
         const auto sleep = std::min(
             nextWakeup - steadyNow,
             std::chrono::duration_cast<std::chrono::nanoseconds>(
                 deadline - now));
         if (sleep.count() > 0) {
           /* sleep override */ std::this_thread::sleep_for(sleep);
+          // Update with the intended wake-up time instead of the current time,
+          // so if sleep_for oversleeps we'll correct in the next sleep.
+          lastWakeup_ = steadyNow + sleep;
+        } else {
+          lastWakeup_ = steadyNow;
         }
       }