Saturation detector. Basic algorithm. Diff 1/7

Summary: Basic FIFO queues observation hooks.

Reviewed By: andriigrynenko

Differential Revision: D20773870

fbshipit-source-id: 3f9cbab1b8b10204139ce153d92e08ffb5f6254c

folly/concurrency/QueueObserver.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/concurrency/QueueObserver.h>
+
+namespace {
+std::unique_ptr<folly::QueueObserverFactory>
+make_queue_observer_factory_fallback() noexcept {
+  return {};
+}
+} // namespace
+
+namespace folly {
+/* static */ std::unique_ptr<QueueObserverFactory>
+QueueObserverFactory::make() {
+  auto f = make_queue_observer_factory ? make_queue_observer_factory
+                                       : make_queue_observer_factory_fallback;
+  return f();
+}
+} // namespace folly

folly/concurrency/QueueObserver.h

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include <memory>
+
+#include <folly/Portability.h>
+
+namespace folly {
+
+class QueueObserver {
+ public:
+  virtual ~QueueObserver() {}
+
+  virtual intptr_t onEnqueued() = 0;
+  virtual void onDequeued(intptr_t) = 0;
+};
+
+class QueueObserverFactory {
+ public:
+  virtual ~QueueObserverFactory() {}
+  virtual std::unique_ptr<QueueObserver> create() = 0;
+
+  static std::unique_ptr<QueueObserverFactory> make();
+};
+
+using MakeQueueObserverFactory = std::unique_ptr<QueueObserverFactory>();
+#if FOLLY_HAVE_WEAK_SYMBOLS
+FOLLY_ATTR_WEAK MakeQueueObserverFactory make_queue_observer_factory;
+#else
+constexpr MakeQueueObserverFactory* make_queue_observer_factory = nullptr;
+#endif
+
+} // namespace folly