text: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements. See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership. The ASF licenses this file
   * to you 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 <map>
  #include <memory>
  #include <glog/logging.h>
  #include "folly/ThreadLocal.h"
  #include "folly/RWSpinLock.h"
  
  /**
   * In many cases this header is included as a
   * dependency to libraries which do not need
   * command line flags. GFLAGS is a large binary
   * and thus we do this so that a library which
   * is size sensitive doesn't have to pull in
   * GFLAGS if it doesn't want to.
   */
  #ifndef NO_LIB_GFLAGS
    #include <gflags/gflags.h>
    DECLARE_bool(enable_request_context);
  #endif
  
  namespace folly {
  
  #ifdef NO_LIB_GFLAGS
    extern bool FLAGS_enable_request_context;
  #endif
  
  // Some request context that follows an async request through a process
  // Everything in the context must be thread safe
  
  class RequestData {
   public:
    virtual ~RequestData() {}
  };
  
  class RequestContext;
  
  // If you do not call create() to create a unique request context,
  // this default request context will always be returned, and is never
  // copied between threads.
  extern RequestContext* defaultContext;
  
  class RequestContext {
   public:
    // Create a unique requext context for this request.
    // It will be passed between queues / threads (where implemented),
    // so it should be valid for the lifetime of the request.
    static bool create() {
      if(!FLAGS_enable_request_context) {
        return false;
      }
      bool prev = getStaticContext().get() != nullptr;
      getStaticContext().reset(new std::shared_ptr<RequestContext>(
                       std::make_shared<RequestContext>()));
      return prev;
    }
  
    // Get the current context.
    static RequestContext* get() {
      if (!FLAGS_enable_request_context ||
          getStaticContext().get() == nullptr) {
        if (defaultContext == nullptr) {
          defaultContext = new RequestContext;
        }
        return defaultContext;
      }
      return getStaticContext().get()->get();
    }
  
    // The following API may be used to set per-request data in a thread-safe way.
    // This access is still performance sensitive, so please ask if you need help
    // profiling any use of these functions.
    void setContextData(
      const std::string& val, std::unique_ptr<RequestData> data) {
      if (!FLAGS_enable_request_context) {
        return;
      }
  
      folly::RWSpinLock::WriteHolder guard(lock);
      if (data_.find(val) != data_.end()) {
        LOG_FIRST_N(WARNING, 1) <<
          "Called RequestContext::setContextData with data already set";
  
        data_[val] = nullptr;
      } else {
        data_[val] = std::move(data);
      }
    }
  
    bool hasContextData(const std::string& val) {
      folly::RWSpinLock::ReadHolder guard(lock);
      return data_.find(val) != data_.end();
    }
  
    RequestData* getContextData(const std::string& val) {
      folly::RWSpinLock::ReadHolder guard(lock);
      auto r = data_.find(val);
      if (r == data_.end()) {
        return nullptr;
      } else {
        return r->second.get();
      }
    }
  
    void clearContextData(const std::string& val) {
      folly::RWSpinLock::WriteHolder guard(lock);
      data_.erase(val);
    }
  
    // The following API is used to pass the context through queues / threads.
    // saveContext is called to geta shared_ptr to the context, and
    // setContext is used to reset it on the other side of the queue.
    //
    // A shared_ptr is used, because many request may fan out across
    // multiple threads, or do post-send processing, etc.
  
    static std::shared_ptr<RequestContext>
    setContext(std::shared_ptr<RequestContext> ctx) {
      if (FLAGS_enable_request_context) {
        std::shared_ptr<RequestContext> old_ctx;
        if (getStaticContext().get()) {
          old_ctx = *getStaticContext().get();
        }
        if (ctx == nullptr) {
          getStaticContext().reset(nullptr);
        } else {
          getStaticContext().reset(new std::shared_ptr<RequestContext>(ctx));
        }
        return old_ctx;
      }
      return std::shared_ptr<RequestContext>();
    }
  
    static std::shared_ptr<RequestContext> saveContext() {
      if (!FLAGS_enable_request_context) {
        return std::shared_ptr<RequestContext>();
      }
      if (getStaticContext().get() == nullptr) {
        return std::shared_ptr<RequestContext>();
      } else {
        return *getStaticContext().get();
      }
    }
  
    // Used to solve static destruction ordering issue.  Any static object
    // that uses RequestContext must call this function in its constructor.
    //
    // See below link for more details.
    // http://stackoverflow.com/questions/335369/
    // finding-c-static-initialization-order-problems#335746
    static folly::ThreadLocalPtr<std::shared_ptr<RequestContext>>&
    getStaticContext() {
      static folly::ThreadLocalPtr<std::shared_ptr<RequestContext> > context;
      return context;
    }
  
   private:
    folly::RWSpinLock lock;
    std::map<std::string, std::unique_ptr<RequestData>> data_;
  };
  
  /**
   * Set the request context for a specific scope. For example,
   * if you ran a part of a request in another thread you could
   * use RequestContextGuard to copy apply the request context
   * inside the other therad.
   */
  class RequestContextGuard {
   public:
    explicit RequestContextGuard(std::shared_ptr<RequestContext> ctx) {
      oldctx_ = RequestContext::setContext(std::move(ctx));
    }
  
    ~RequestContextGuard() {
      RequestContext::setContext(std::move(oldctx_));
    }
  
   private:
    std::shared_ptr<RequestContext> oldctx_;
  };
  
  }
  