Add logging for SemiFutures that are discarded with potential

Summary: Add tracing to SemiFuture destruction that is potentially work-dropping.

Reviewed By: yfeldblum

Differential Revision: D21671142

fbshipit-source-id: 8e3e733d9a0845b00fe7ccf465e9c817a10fee59

folly/detail/AsyncTrace.cpp

@@ -29,5 +29,7 @@ FOLLY_ATTR_WEAK void logGetImmutableIOExecutor(IOExecutor*) noexcept {}
 FOLLY_ATTR_WEAK void logSemiFutureVia(Executor*, Executor*) noexcept {}
 FOLLY_ATTR_WEAK void logFutureVia(Executor*, Executor*) noexcept {}
 FOLLY_ATTR_WEAK void logBlockingOperation(std::chrono::milliseconds) noexcept {}
+FOLLY_ATTR_WEAK void logSemiFutureDiscard(
+    DiscardHasDeferred /* hasDeferredExecutor */) noexcept {}
 } // namespace async_tracing
 } // namespace folly

folly/detail/AsyncTrace.h

@@ -24,6 +24,10 @@ namespace folly {
 class Executor;
 class IOExecutor;
 namespace async_tracing {
+enum class DiscardHasDeferred {
+  NO_EXECUTOR,
+  DEFERRED_EXECUTOR,
+};
 void logSetGlobalCPUExecutor(Executor*) noexcept;
 void logSetGlobalCPUExecutorToImmutable() noexcept;
 void logGetGlobalCPUExecutor(Executor*) noexcept;
@@ -34,5 +38,7 @@ void logGetImmutableIOExecutor(IOExecutor*) noexcept;
 void logSemiFutureVia(Executor*, Executor*) noexcept;
 void logFutureVia(Executor*, Executor*) noexcept;
 void logBlockingOperation(std::chrono::milliseconds) noexcept;
+void logSemiFutureDiscard(
+    DiscardHasDeferred /* hasDeferredExecutor */) noexcept;
 } // namespace async_tracing
 } // namespace folly

folly/detail/test/AsyncTraceTest.cpp

@@ -36,4 +36,7 @@ TEST(FollyCountersTest, Trivial) {
   folly::async_tracing::logFutureVia(lastExec, exec);
 
   folly::async_tracing::logBlockingOperation(std::chrono::milliseconds{100});
+
+  folly::async_tracing::logSemiFutureDiscard(
+      folly::async_tracing::DiscardHasDeferred::NO_EXECUTOR);
 }

folly/futures/Future-inl.h

@@ -678,7 +678,11 @@ void SemiFuture<T>::releaseDeferredExecutor(Core* core) {
   if (!core || core->hasCallback()) {
     return;
   }
-  if (auto executor = core->stealDeferredExecutor()) {
+  auto executor = core->stealDeferredExecutor();
+  async_tracing::logSemiFutureDiscard(
+      executor.get() ? async_tracing::DiscardHasDeferred::DEFERRED_EXECUTOR
+                     : async_tracing::DiscardHasDeferred::NO_EXECUTOR);
+  if (executor) {
     executor.get()->detach();
   }
 }