Fix a regression in per-task allocations in SerialExecutor

Summary: [Folly] Fix a regression in per-task allocations in `SerialExecutor`.

Reviewed By: andriigrynenko

Differential Revision: D15589007

fbshipit-source-id: 4461798435833cb317b41cfea932762eb7ac8662

folly/executors/SerialExecutor.cpp

@@ -19,7 +19,6 @@
 #include <glog/logging.h>
 
 #include <folly/ExceptionString.h>
-#include <folly/io/async/Request.h>
 
 namespace folly {
 
@@ -58,16 +57,12 @@ void SerialExecutor::keepAliveRelease() {
 }
 
 void SerialExecutor::add(Func func) {
-  queue_.enqueue([ctx = folly::RequestContext::saveContext(),
-                  func = std::move(func)]() mutable {
-    folly::RequestContextScopeGuard ctxGuard(ctx);
-    func();
-  });
+  queue_.enqueue(Task{std::move(func), RequestContext::saveContext()});
   parent_->add([keepAlive = getKeepAliveToken(this)] { keepAlive->run(); });
 }
 
 void SerialExecutor::addWithPriority(Func func, int8_t priority) {
-  queue_.enqueue(std::move(func));
+  queue_.enqueue(Task{std::move(func), RequestContext::saveContext()});
   parent_->addWithPriority(
       [keepAlive = getKeepAliveToken(this)] { keepAlive->run(); }, priority);
 }
@@ -80,11 +75,12 @@ void SerialExecutor::run() {
   }
 
   do {
-    Func func;
-    queue_.dequeue(func);
+    Task task;
+    queue_.dequeue(task);
 
     try {
-      func();
+      folly::RequestContextScopeGuard ctxGuard(std::move(task.ctx));
+      task.func();
     } catch (std::exception const& ex) {
       LOG(ERROR) << "SerialExecutor: func threw unhandled exception "
                  << folly::exceptionStr(ex);

folly/executors/SerialExecutor.h

@@ -23,6 +23,7 @@
 #include <folly/concurrency/UnboundedQueue.h>
 #include <folly/executors/GlobalExecutor.h>
 #include <folly/executors/SequencedExecutor.h>
+#include <folly/io/async/Request.h>
 
 namespace folly {
 
@@ -105,6 +106,11 @@ class SerialExecutor : public SequencedExecutor {
   void keepAliveRelease() override;
 
  private:
+  struct Task {
+    Func func;
+    std::shared_ptr<RequestContext> ctx;
+  };
+
   explicit SerialExecutor(KeepAlive<Executor> parent);
   ~SerialExecutor() override;
 
@@ -116,7 +122,7 @@ class SerialExecutor : public SequencedExecutor {
    * Unbounded multi producer single consumer queue where consumers don't block
    * on dequeue.
    */
-  folly::UnboundedQueue<Func, false, true, false> queue_;
+  folly::UnboundedQueue<Task, false, true, false> queue_;
 
   std::atomic<ssize_t> keepAliveCounter_{1};
 };