Implement KeepAlive in AsyncioLoopController

Summary:
I'm continue to try keeping the implementation simple, instead of copy-pasting EventBase/EventBaseLoopController code.
My understanding is that AsyncioExecutor/AsyncioLoopController will always be single threaded because they are tied to a unique asyncio event_loop, so I don't need an atomic counter.
Then it should be cheap to just always aquire/release the counter in runLoop.

Reviewed By: andriigrynenko

Differential Revision: D7875202

fbshipit-source-id: 900a03e6870c5e4828521a99785cfa976baac54a

folly/fibers/EventBaseLoopController-inl.h

@@ -92,7 +92,8 @@ inline void EventBaseLoopController::scheduleThreadSafe() {
      3) We fulfill the promise from the other thread. */
   assert(eventBaseAttached_);
 
-  eventBase_->runInEventBaseThread([this]() {
+  eventBase_->runInEventBaseThread(
+      [this, eventBaseKeepAlive = getKeepAliveToken(eventBase_)]() {
         if (fm_->shouldRunLoopRemote()) {
           return runLoop();
         }

folly/python/AsyncioExecutor.h

@@ -28,6 +28,13 @@ class AsyncioExecutor : public DrivableExecutor, public SequencedExecutor {
  public:
   using Func = folly::Func;
 
+  ~AsyncioExecutor() override {
+    keepAliveRelease();
+    while (keepAliveCounter_ > 0) {
+      drive();
+    }
+  }
+
   void add(Func func) override {
     queue_.putMessage(std::move(func));
   }
@@ -48,9 +55,24 @@ class AsyncioExecutor : public DrivableExecutor, public SequencedExecutor {
     });
   }
 
+ protected:
+  bool keepAliveAcquire() override {
+    auto keepAliveCounter =
+        keepAliveCounter_.fetch_add(1, std::memory_order_relaxed);
+    // We should never increment from 0
+    DCHECK(keepAliveCounter > 0);
+    return true;
+  }
+
+  void keepAliveRelease() override {
+    auto keepAliveCounter = --keepAliveCounter_;
+    DCHECK(keepAliveCounter >= 0);
+  }
+
  private:
   folly::NotificationQueue<Func> queue_;
   folly::NotificationQueue<Func>::SimpleConsumer consumer_{queue_};
+  std::atomic<size_t> keepAliveCounter_{1};
 }; // AsyncioExecutor
 
 } // namespace python

folly/python/AsyncioLoopController-inl.h

@@ -30,17 +30,28 @@ inline void AsyncioLoopController::setFiberManager(fibers::FiberManager* fm) {
 
 inline void AsyncioLoopController::schedule() {
   // add() is thread-safe, so this isn't properly optimized for addTask()
+  if (!executorKeepAlive_) {
+    executorKeepAlive_ = getKeepAliveToken(executor_);
+  }
   executor_->add([this]() { return runLoop(); });
 }
 
 inline void AsyncioLoopController::runLoop() {
-  if (fm_->hasTasks()) {
+  if (!executorKeepAlive_) {
+    if (!fm_->hasTasks()) {
+      return;
+    }
+    executorKeepAlive_ = getKeepAliveToken(executor_);
+  }
   fm_->loopUntilNoReadyImpl();
+  if (!fm_->hasTasks()) {
+    executorKeepAlive_.reset();
   }
 }
 
 inline void AsyncioLoopController::scheduleThreadSafe() {
-  executor_->add([this]() {
+  executor_->add(
+      [this, executorKeepAlive = getKeepAliveToken(executor_)]() mutable {
         if (fm_->shouldRunLoopRemote()) {
           return runLoop();
         }

folly/python/AsyncioLoopController.h

@@ -31,8 +31,8 @@ class AsyncioLoopController : public fibers::LoopController {
   ~AsyncioLoopController() override;
 
  private:
-  // TODO: KeepAlive token to guarantee lifetime
   AsyncioExecutor* executor_;
+  Executor::KeepAlive<AsyncioExecutor> executorKeepAlive_;
   fibers::FiberManager* fm_{nullptr};
 
   void setFiberManager(fibers::FiberManager* fm) override;