/*
 * 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 <chrono>
#include <memory>

#include <folly/Optional.h>
#include <folly/io/IOBuf.h>
#include <folly/io/async/AsyncSocketBase.h>
#include <folly/io/async/AsyncTransportCertificate.h>
#include <folly/io/async/DelayedDestruction.h>
#include <folly/io/async/EventBase.h>
#include <folly/portability/OpenSSL.h>
#include <folly/portability/SysUio.h>
#include <folly/ssl/OpenSSLPtrTypes.h>

namespace folly {

class AsyncSocketException;
class EventBase;
class SocketAddress;

/*
 * flags given by the application for write* calls
 */
enum class WriteFlags : uint32_t {
  NONE = 0x00,
  /*
   * Whether to delay the output until a subsequent non-corked write.
   * (Note: may not be supported in all subclasses or on all platforms.)
   */
  CORK = 0x01,
  /*
   * Set MSG_EOR flag when writing the last byte of the buffer to the socket.
   *
   * EOR tracking may need to be enabled to ensure that the MSG_EOR flag is only
   * set when the final byte is being written.
   *
   *  - If the MSG_EOR flag is set, it is marked in the corresponding
   *    tcp_skb_cb; this can be useful when debugging.
   *  - The kernel uses it to decide whether socket buffers can be collapsed
   *    together (see tcp_skb_can_collapse_to).
   */
  EOR = 0x02,
  /*
   * this indicates that only the write side of socket should be shutdown
   */
  WRITE_SHUTDOWN = 0x04,
  /*
   * use msg zerocopy if allowed
   */
  WRITE_MSG_ZEROCOPY = 0x08,
  /*
   * Request timestamp when entire buffer transmitted by the NIC.
   *
   * How timestamping is performed is implementation specific and may rely on
   * software or hardware timestamps
   */
  TIMESTAMP_TX = 0x10,
  /*
   * Request timestamp when entire buffer ACKed by remote endpoint.
   *
   * How timestamping is performed is implementation specific and may rely on
   * software or hardware timestamps
   */
  TIMESTAMP_ACK = 0x20,
  /*
   * Request timestamp when entire buffer has entered packet scheduler.
   */
  TIMESTAMP_SCHED = 0x40,
  /*
   * Request timestamp when entire buffer has been written to system socket.
   */
  TIMESTAMP_WRITE = 0x80,
};

/*
 * union operator
 */
constexpr WriteFlags operator|(WriteFlags a, WriteFlags b) {
  return static_cast<WriteFlags>(
      static_cast<uint32_t>(a) | static_cast<uint32_t>(b));
}

/*
 * compound assignment union operator
 */
constexpr WriteFlags& operator|=(WriteFlags& a, WriteFlags b) {
  a = a | b;
  return a;
}

/*
 * intersection operator
 */
constexpr WriteFlags operator&(WriteFlags a, WriteFlags b) {
  return static_cast<WriteFlags>(
      static_cast<uint32_t>(a) & static_cast<uint32_t>(b));
}

/*
 * compound assignment intersection operator
 */
constexpr WriteFlags& operator&=(WriteFlags& a, WriteFlags b) {
  a = a & b;
  return a;
}

/*
 * exclusion parameter
 */
constexpr WriteFlags operator~(WriteFlags a) {
  return static_cast<WriteFlags>(~static_cast<uint32_t>(a));
}

/*
 * unset operator
 */
constexpr WriteFlags unSet(WriteFlags a, WriteFlags b) {
  return a & ~b;
}

/*
 * inclusion operator
 */
constexpr bool isSet(WriteFlags a, WriteFlags b) {
  return (a & b) == b;
}

/**
 * Write flags that are related to timestamping.
 */
constexpr WriteFlags kWriteFlagsForTimestamping = WriteFlags::TIMESTAMP_SCHED |
    WriteFlags::TIMESTAMP_TX | WriteFlags::TIMESTAMP_ACK;

class AsyncReader {
 public:
  class ReadCallback {
   public:
    enum class ReadMode : uint8_t {
      ReadBuffer = 0,
      ReadVec = 1,
    };

    virtual ~ReadCallback() = default;

    ReadMode getReadMode() const noexcept { return readMode_; }

    void setReadMode(ReadMode readMode) noexcept { readMode_ = readMode; }

    /**
     * When data becomes available, getReadBuffer()/getReadBuffers() will be
     * invoked to get the buffer/buffers into which data should be read.
     *
     * These methods allows the ReadCallback to delay buffer allocation until
     * data becomes available.  This allows applications to manage large
     * numbers of idle connections, without having to maintain a separate read
     * buffer for each idle connection.
     */

    /**
     * It is possible that in some cases, getReadBuffer() may be called
     * multiple times before readDataAvailable() is invoked.  In this case, the
     * data will be written to the buffer returned from the most recent call to
     * readDataAvailable().  If the previous calls to readDataAvailable()
     * returned different buffers, the ReadCallback is responsible for ensuring
     * that they are not leaked.
     *
     * If getReadBuffer() throws an exception, returns a nullptr buffer, or
     * returns a 0 length, the ReadCallback will be uninstalled and its
     * readError() method will be invoked.
     *
     * getReadBuffer() is not allowed to change the transport state before it
     * returns.  (For example, it should never uninstall the read callback, or
     * set a different read callback.)
     *
     * @param bufReturn getReadBuffer() should update *bufReturn to contain the
     *                  address of the read buffer.  This parameter will never
     *                  be nullptr.
     * @param lenReturn getReadBuffer() should update *lenReturn to contain the
     *                  maximum number of bytes that may be written to the read
     *                  buffer.  This parameter will never be nullptr.
     */
    virtual void getReadBuffer(void** bufReturn, size_t* lenReturn) = 0;

    /**
     * It is possible that in some cases, getReadBuffers() may be called
     * multiple times before readDataAvailable() is invoked.  In this case, the
     * data will be written to the buffer returned from the most recent call to
     * readDataAvailable().  If the previous calls to readDataAvailable()
     * returned different buffers, the ReadCallback is responsible for ensuring
     * that they are not leaked.
     *
     * If getReadBuffera() throws an exception or returns a zero length array
     * the ReadCallback will be uninstalled and its readError() method will be
     * invoked.
     *
     * getReadBuffers() is not allowed to change the transport state before it
     * returns.  (For example, it should never uninstall the read callback, or
     * set a different read callback.)
     *
     * @param iovs      getReadBuffers() will copy up to num iovec entries into
     *                  iovs. iovs cannot be nullptr unless num is 0
     * @param num       number of iovec entries in the iovs array
     * @return          number of entried copied to the iovs array
     *                  this is less than or equal to num
     */
    virtual size_t getReadBuffers(
        FOLLY_MAYBE_UNUSED struct iovec* iovs, FOLLY_MAYBE_UNUSED size_t num) {
      return 0;
    }

    /**
     * readDataAvailable() will be invoked when data has been successfully read
     * into the buffer(s) returned by the last call to
     * getReadBuffer()/getReadBuffers()
     *
     * The read callback remains installed after readDataAvailable() returns.
     * It must be explicitly uninstalled to stop receiving read events.
     * getReadBuffer() will be called at least once before each call to
     * readDataAvailable().  getReadBuffer() will also be called before any
     * call to readEOF().
     *
     * @param len       The number of bytes placed in the buffer.
     */

    virtual void readDataAvailable(size_t len) noexcept = 0;

    /**
     * When data becomes available, isBufferMovable() will be invoked to figure
     * out which API will be used, readBufferAvailable() or
     * readDataAvailable(). If isBufferMovable() returns true, that means
     * ReadCallback supports the IOBuf ownership transfer and
     * readBufferAvailable() will be used.  Otherwise, not.

     * By default, isBufferMovable() always return false. If
     * readBufferAvailable() is implemented and to be invoked, You should
     * overwrite isBufferMovable() and return true in the inherited class.
     *
     * This method allows the AsyncSocket/AsyncSSLSocket do buffer allocation by
     * itself until data becomes available.  Compared with the pre/post buffer
     * allocation in getReadBuffer()/readDataAvailabe(), readBufferAvailable()
     * has two advantages.  First, this can avoid memcpy. E.g., in
     * AsyncSSLSocket, the decrypted data was copied from the openssl internal
     * buffer to the readbuf buffer.  With the buffer ownership transfer, the
     * internal buffer can be directly "moved" to ReadCallback. Second, the
     * memory allocation can be more precise.  The reason is
     * AsyncSocket/AsyncSSLSocket can allocate the memory of precise size
     * because they have more context about the available data than
     * ReadCallback.  Think about the getReadBuffer() pre-allocate 4072 bytes
     * buffer, but the available data is always 16KB (max OpenSSL record size).
     */

    virtual bool isBufferMovable() noexcept { return false; }

    /**
     * Suggested buffer size, allocated for read operations,
     * if callback is movable and supports folly::IOBuf
     */

    virtual size_t maxBufferSize() const {
      return 64 * 1024; // 64K
    }

    /**
     * readBufferAvailable() will be invoked when data has been successfully
     * read.
     *
     * Note that only either readBufferAvailable() or readDataAvailable() will
     * be invoked according to the return value of isBufferMovable(). The timing
     * and aftereffect of readBufferAvailable() are the same as
     * readDataAvailable()
     *
     * @param readBuf The unique pointer of read buffer.
     */

    virtual void readBufferAvailable(
        std::unique_ptr<IOBuf> /*readBuf*/) noexcept {}

    /**
     * readEOF() will be invoked when the transport is closed.
     *
     * The read callback will be automatically uninstalled immediately before
     * readEOF() is invoked.
     */
    virtual void readEOF() noexcept = 0;

    /**
     * readError() will be invoked if an error occurs reading from the
     * transport.
     *
     * The read callback will be automatically uninstalled immediately before
     * readError() is invoked.
     *
     * @param ex        An exception describing the error that occurred.
     */
    virtual void readErr(const AsyncSocketException& ex) noexcept = 0;

   protected:
    ReadMode readMode_{ReadMode::ReadBuffer};
  };

  // Read methods that aren't part of AsyncTransport.
  virtual void setReadCB(ReadCallback* callback) = 0;
  virtual ReadCallback* getReadCallback() const = 0;
  virtual void setEventCallback(EventRecvmsgCallback* /*cb*/) {}

 protected:
  virtual ~AsyncReader() = default;
};

class AsyncWriter {
 public:
  class ReleaseIOBufCallback {
   public:
    virtual ~ReleaseIOBufCallback() = default;

    virtual void releaseIOBuf(std::unique_ptr<folly::IOBuf>) noexcept = 0;
  };

  class WriteCallback {
   public:
    virtual ~WriteCallback() = default;

    /**
     * writeSuccess() will be invoked when all of the data has been
     * successfully written.
     *
     * Note that this mainly signals that the buffer containing the data to
     * write is no longer needed and may be freed or re-used.  It does not
     * guarantee that the data has been fully transmitted to the remote
     * endpoint.  For example, on socket-based transports, writeSuccess() only
     * indicates that the data has been given to the kernel for eventual
     * transmission.
     */
    virtual void writeSuccess() noexcept = 0;

    /**
     * writeError() will be invoked if an error occurs writing the data.
     *
     * @param bytesWritten      The number of bytes that were successfull
     * @param ex                An exception describing the error that occurred.
     */
    virtual void writeErr(
        size_t bytesWritten, const AsyncSocketException& ex) noexcept = 0;

    virtual ReleaseIOBufCallback* getReleaseIOBufCallback() noexcept {
      return nullptr;
    }
  };

  /**
   * If you supply a non-null WriteCallback, exactly one of writeSuccess()
   * or writeErr() will be invoked when the write completes. If you supply
   * the same WriteCallback object for multiple write() calls, it will be
   * invoked exactly once per call. The only way to cancel outstanding
   * write requests is to close the socket (e.g., with closeNow() or
   * shutdownWriteNow()). When closing the socket this way, writeErr() will
   * still be invoked once for each outstanding write operation.
   */
  virtual void write(
      WriteCallback* callback,
      const void* buf,
      size_t bytes,
      WriteFlags flags = WriteFlags::NONE) = 0;

  /**
   * If you supply a non-null WriteCallback, exactly one of writeSuccess()
   * or writeErr() will be invoked when the write completes. If you supply
   * the same WriteCallback object for multiple write() calls, it will be
   * invoked exactly once per call. The only way to cancel outstanding
   * write requests is to close the socket (e.g., with closeNow() or
   * shutdownWriteNow()). When closing the socket this way, writeErr() will
   * still be invoked once for each outstanding write operation.
   */
  virtual void writev(
      WriteCallback* callback,
      const iovec* vec,
      size_t count,
      WriteFlags flags = WriteFlags::NONE) = 0;

  /**
   * If you supply a non-null WriteCallback, exactly one of writeSuccess()
   * or writeErr() will be invoked when the write completes. If you supply
   * the same WriteCallback object for multiple write() calls, it will be
   * invoked exactly once per call. The only way to cancel outstanding
   * write requests is to close the socket (e.g., with closeNow() or
   * shutdownWriteNow()). When closing the socket this way, writeErr() will
   * still be invoked once for each outstanding write operation.
   */
  virtual void writeChain(
      WriteCallback* callback,
      std::unique_ptr<IOBuf>&& buf,
      WriteFlags flags = WriteFlags::NONE) = 0;

  /** zero copy related
   * */
  virtual bool setZeroCopy(bool /*enable*/) { return false; }

  virtual bool getZeroCopy() const { return false; }

  using ZeroCopyEnableFunc =
      std::function<bool(const std::unique_ptr<folly::IOBuf>& buf)>;

  virtual void setZeroCopyEnableFunc(ZeroCopyEnableFunc /*func*/) {}

 protected:
  virtual ~AsyncWriter() = default;
};

/**
 * AsyncTransport defines an asynchronous API for bidirectional streaming I/O.
 *
 * This class provides an API to for asynchronously waiting for data
 * on a streaming transport, and for asynchronously sending data.
 *
 * The APIs for reading and writing are intentionally asymmetric.  Waiting for
 * data to read is a persistent API: a callback is installed, and is notified
 * whenever new data is available.  It continues to be notified of new events
 * until it is uninstalled.
 *
 * AsyncTransport does not provide read timeout functionality, because it
 * typically cannot determine when the timeout should be active.  Generally, a
 * timeout should only be enabled when processing is blocked waiting on data
 * from the remote endpoint.  For server-side applications, the timeout should
 * not be active if the server is currently processing one or more outstanding
 * requests on this transport.  For client-side applications, the timeout
 * should not be active if there are no requests pending on the transport.
 * Additionally, if a client has multiple pending requests, it will ususally
 * want a separate timeout for each request, rather than a single read timeout.
 *
 * The write API is fairly intuitive: a user can request to send a block of
 * data, and a callback will be informed once the entire block has been
 * transferred to the kernel, or on error.  AsyncTransport does provide a send
 * timeout, since most callers want to give up if the remote end stops
 * responding and no further progress can be made sending the data.
 */
class AsyncTransport : public DelayedDestruction,
                       public AsyncSocketBase,
                       public AsyncReader,
                       public AsyncWriter {
 public:
  typedef std::unique_ptr<AsyncTransport, Destructor> UniquePtr;

  /**
   * Close the transport.
   *
   * This gracefully closes the transport, waiting for all pending write
   * requests to complete before actually closing the underlying transport.
   *
   * If a read callback is set, readEOF() will be called immediately.  If there
   * are outstanding write requests, the close will be delayed until all
   * remaining writes have completed.  No new writes may be started after
   * close() has been called.
   */
  virtual void close() = 0;

  /**
   * Close the transport immediately.
   *
   * This closes the transport immediately, dropping any outstanding data
   * waiting to be written.
   *
   * If a read callback is set, readEOF() will be called immediately.
   * If there are outstanding write requests, these requests will be aborted
   * and writeError() will be invoked immediately on all outstanding write
   * callbacks.
   */
  virtual void closeNow() = 0;

  /**
   * Reset the transport immediately.
   *
   * This closes the transport immediately, sending a reset to the remote peer
   * if possible to indicate abnormal shutdown.
   *
   * Note that not all subclasses implement this reset functionality: some
   * subclasses may treat reset() the same as closeNow().  Subclasses that use
   * TCP transports should terminate the connection with a TCP reset.
   */
  virtual void closeWithReset() { closeNow(); }

  /**
   * Perform a half-shutdown of the write side of the transport.
   *
   * The caller should not make any more calls to write() or writev() after
   * shutdownWrite() is called.  Any future write attempts will fail
   * immediately.
   *
   * Not all transport types support half-shutdown.  If the underlying
   * transport does not support half-shutdown, it will fully shutdown both the
   * read and write sides of the transport.  (Fully shutting down the socket is
   * better than doing nothing at all, since the caller may rely on the
   * shutdownWrite() call to notify the other end of the connection that no
   * more data can be read.)
   *
   * If there is pending data still waiting to be written on the transport,
   * the actual shutdown will be delayed until the pending data has been
   * written.
   *
   * Note: There is no corresponding shutdownRead() equivalent.  Simply
   * uninstall the read callback if you wish to stop reading.  (On TCP sockets
   * at least, shutting down the read side of the socket is a no-op anyway.)
   */
  virtual void shutdownWrite() = 0;

  /**
   * Perform a half-shutdown of the write side of the transport.
   *
   * shutdownWriteNow() is identical to shutdownWrite(), except that it
   * immediately performs the shutdown, rather than waiting for pending writes
   * to complete.  Any pending write requests will be immediately failed when
   * shutdownWriteNow() is called.
   */
  virtual void shutdownWriteNow() = 0;

  /**
   * Determine if transport is open and ready to read or write.
   *
   * Note that this function returns false on EOF; you must also call error()
   * to distinguish between an EOF and an error.
   *
   * @return  true iff the transport is open and ready, false otherwise.
   */
  virtual bool good() const = 0;

  /**
   * Determine if the transport is readable or not.
   *
   * @return  true iff the transport is readable, false otherwise.
   */
  virtual bool readable() const = 0;

  /**
   * Determine if the transport is writable or not.
   *
   * @return  true iff the transport is writable, false otherwise.
   */
  virtual bool writable() const {
    // By default return good() - leave it to implementers to override.
    return good();
  }

  /**
   * Determine if the there is pending data on the transport.
   *
   * @return  true iff the if the there is pending data, false otherwise.
   */
  virtual bool isPending() const { return readable(); }

  /**
   * Determine if transport is connected to the endpoint
   *
   * @return  false iff the transport is connected, otherwise true
   */
  virtual bool connecting() const = 0;

  /**
   * Determine if an error has occurred with this transport.
   *
   * @return  true iff an error has occurred (not EOF).
   */
  virtual bool error() const = 0;

  /**
   * Attach the transport to a EventBase.
   *
   * This may only be called if the transport is not currently attached to a
   * EventBase (by an earlier call to detachEventBase()).
   *
   * This method must be invoked in the EventBase's thread.
   */
  virtual void attachEventBase(EventBase* eventBase) = 0;

  /**
   * Detach the transport from its EventBase.
   *
   * This may only be called when the transport is idle and has no reads or
   * writes pending.  Once detached, the transport may not be used again until
   * it is re-attached to a EventBase by calling attachEventBase().
   *
   * This method must be called from the current EventBase's thread.
   */
  virtual void detachEventBase() = 0;

  /**
   * Determine if the transport can be detached.
   *
   * This method must be called from the current EventBase's thread.
   */
  virtual bool isDetachable() const = 0;

  /**
   * Set the send timeout.
   *
   * If write requests do not make any progress for more than the specified
   * number of milliseconds, fail all pending writes and close the transport.
   *
   * If write requests are currently pending when setSendTimeout() is called,
   * the timeout interval is immediately restarted using the new value.
   *
   * @param milliseconds  The timeout duration, in milliseconds.  If 0, no
   *                      timeout will be used.
   */
  virtual void setSendTimeout(uint32_t milliseconds) = 0;

  /**
   * Get the send timeout.
   *
   * @return Returns the current send timeout, in milliseconds.  A return value
   *         of 0 indicates that no timeout is set.
   */
  virtual uint32_t getSendTimeout() const = 0;

  /**
   * Get the address of the local endpoint of this transport.
   *
   * This function may throw AsyncSocketException on error.
   *
   * @param address  The local address will be stored in the specified
   *                 SocketAddress.
   */
  virtual void getLocalAddress(SocketAddress* address) const = 0;

  /**
   * Get the address of the remote endpoint to which this transport is
   * connected.
   *
   * This function may throw AsyncSocketException on error.
   *
   * @return         Return the local address
   */
  SocketAddress getLocalAddress() const {
    SocketAddress addr;
    getLocalAddress(&addr);
    return addr;
  }

  void getAddress(SocketAddress* address) const override {
    getLocalAddress(address);
  }

  /**
   * Get the address of the remote endpoint to which this transport is
   * connected.
   *
   * This function may throw AsyncSocketException on error.
   *
   * @param address  The remote endpoint's address will be stored in the
   *                 specified SocketAddress.
   */
  virtual void getPeerAddress(SocketAddress* address) const = 0;

  /**
   * Get the address of the remote endpoint to which this transport is
   * connected.
   *
   * This function may throw AsyncSocketException on error.
   *
   * @return         Return the remote endpoint's address
   */
  SocketAddress getPeerAddress() const {
    SocketAddress addr;
    getPeerAddress(&addr);
    return addr;
  }

  /**
   * Get the peer certificate information if any
   */
  virtual const AsyncTransportCertificate* getPeerCertificate() const {
    return nullptr;
  }

  /**
   * Hints to transport implementations that the associated certificate is no
   * longer required by the application. The transport implementation may
   * choose to free up resources associated with the peer certificate.
   *
   * After this call, `getPeerCertificate()` may return nullptr, even if it
   * previously returned non-null
   */
  virtual void dropPeerCertificate() noexcept {}

  /**
   * Hints to transport implementations that the associated certificate is no
   * longer required by the application. The transport implementation may
   * choose to free up resources associated with the self certificate.
   *
   * After this call, `getPeerCertificate()` may return nullptr, even if it
   * previously returned non-null
   */
  virtual void dropSelfCertificate() noexcept {}

  /**
   * Get the certificate information of this transport, if any
   */
  virtual const AsyncTransportCertificate* getSelfCertificate() const {
    return nullptr;
  }

  /**
   * Return the application protocol being used by the underlying transport
   * protocol. This is useful for transports which are used to tunnel other
   * protocols.
   */
  virtual std::string getApplicationProtocol() const noexcept { return ""; }

  /**
   * Returns the name of the security protocol being used.
   */
  virtual std::string getSecurityProtocol() const { return ""; }

  /**
   * @return True iff end of record tracking is enabled
   */
  virtual bool isEorTrackingEnabled() const = 0;

  virtual void setEorTracking(bool track) = 0;

  virtual size_t getAppBytesWritten() const = 0;
  virtual size_t getRawBytesWritten() const = 0;
  virtual size_t getAppBytesReceived() const = 0;
  virtual size_t getRawBytesReceived() const = 0;

  /**
   * Calculates the total number of bytes that are currently buffered in the
   * transport to be written later.
   */
  virtual size_t getAppBytesBuffered() const { return 0; }
  virtual size_t getRawBytesBuffered() const { return 0; }

  /**
   * Callback class to signal changes in the transport's internal buffers.
   */
  class BufferCallback {
   public:
    virtual ~BufferCallback() = default;

    /**
     * onEgressBuffered() will be invoked when there's a partial write and it
     * is necessary to buffer the remaining data.
     */
    virtual void onEgressBuffered() = 0;

    /**
     * onEgressBufferCleared() will be invoked when whatever was buffered is
     * written, or when it errors out.
     */
    virtual void onEgressBufferCleared() = 0;
  };

  /**
   * Callback class to signal when a transport that did not have replay
   * protection gains replay protection. This is needed for 0-RTT security
   * protocols.
   */
  class ReplaySafetyCallback {
   public:
    virtual ~ReplaySafetyCallback() = default;

    /**
     * Called when the transport becomes replay safe.
     */
    virtual void onReplaySafe() = 0;
  };

  /**
   * False if the transport does not have replay protection, but will in the
   * future.
   */
  virtual bool isReplaySafe() const { return true; }

  /**
   * Set the ReplaySafeCallback on this transport.
   *
   * This should only be called if isReplaySafe() returns false.
   */
  virtual void setReplaySafetyCallback(ReplaySafetyCallback* callback) {
    if (callback) {
      CHECK(false) << "setReplaySafetyCallback() not supported";
    }
  }

  /**
   * Structure used to communicate ByteEvents, such as TX and ACK timestamps.
   */
  struct ByteEvent {
    enum Type : uint8_t { WRITE = 1, SCHED = 2, TX = 3, ACK = 4 };
    // type
    Type type;

    // offset of corresponding byte in raw byte stream
    uint64_t offset{0};

    // transport timestamp, as recorded by AsyncTransport implementation
    std::chrono::steady_clock::time_point ts = {
        std::chrono::steady_clock::now()};

    // kernel software timestamp; for Linux this is CLOCK_REALTIME
    // see https://www.kernel.org/doc/Documentation/networking/timestamping.txt
    folly::Optional<std::chrono::nanoseconds> maybeSoftwareTs;

    // hardware timestamp; see kernel documentation
    // see https://www.kernel.org/doc/Documentation/networking/timestamping.txt
    folly::Optional<std::chrono::nanoseconds> maybeHardwareTs;

    // for WRITE ByteEvents, additional WriteFlags passed
    folly::Optional<WriteFlags> maybeWriteFlags;

    /**
     * For WRITE events, returns if SCHED timestamp requested.
     */
    bool schedTimestampRequested() const {
      CHECK_EQ(Type::WRITE, type);
      CHECK(maybeWriteFlags.has_value());
      return isSet(*maybeWriteFlags, WriteFlags::TIMESTAMP_SCHED);
    }

    /**
     * For WRITE events, returns if TX timestamp requested.
     */
    bool txTimestampRequested() const {
      CHECK_EQ(Type::WRITE, type);
      CHECK(maybeWriteFlags.has_value());
      return isSet(*maybeWriteFlags, WriteFlags::TIMESTAMP_TX);
    }

    /**
     * For WRITE events, returns if ACK timestamp requested.
     */
    bool ackTimestampRequested() const {
      CHECK_EQ(Type::WRITE, type);
      CHECK(maybeWriteFlags.has_value());
      return isSet(*maybeWriteFlags, WriteFlags::TIMESTAMP_ACK);
    }
  };

  /**
   * Observer of transport events.
   */
  class LifecycleObserver {
   public:
    /**
     * Observer configuration.
     *
     * Specifies events observer wants to receive. Cannot be changed post
     * initialization because the transport may turn on / off instrumentation
     * when observers are added / removed, based on the observer configuration.
     */
    struct Config {
      // enables full support for ByteEvents
      bool byteEvents{false};
    };

    /**
     * Constructor for observer, uses default config (instrumentation disabled).
     */
    LifecycleObserver() : LifecycleObserver(Config()) {}

    /**
     * Constructor for observer.
     *
     * @param config      Config, defaults to auxilary instrumentaton disabled.
     */
    explicit LifecycleObserver(const Config& observerConfig)
        : observerConfig_(observerConfig) {}

    virtual ~LifecycleObserver() = default;

    /**
     * Returns observers configuration.
     */
    const Config& getConfig() { return observerConfig_; }

    /**
     * observerAttach() will be invoked when an observer is added.
     *
     * @param transport   Transport where observer was installed.
     */
    virtual void observerAttach(AsyncTransport* /* transport */) noexcept = 0;

    /**
     * observerDetached() will be invoked if the observer is uninstalled prior
     * to transport destruction.
     *
     * No further events will be invoked after observerDetach().
     *
     * @param transport   Transport where observer was uninstalled.
     */
    virtual void observerDetach(AsyncTransport* /* transport */) noexcept = 0;

    /**
     * destroy() will be invoked when the transport's destructor is invoked.
     *
     * No further events will be invoked after destroy().
     *
     * @param transport   Transport being destroyed.
     */
    virtual void destroy(AsyncTransport* /* transport */) noexcept = 0;

    /**
     * close() will be invoked when the transport is being closed.
     *
     * Can be called multiple times during shutdown / destruction for the same
     * transport. Observers may detach after first call or track if event
     * previously observed.
     *
     * @param transport   Transport being closed.
     */
    virtual void close(AsyncTransport* /* transport */) noexcept = 0;

    /**
     * connect() will be invoked when connect() returns successfully.
     *
     * Triggered before any application connection callback.
     *
     * @param transport   Transport that has connected.
     */
    virtual void connect(AsyncTransport* /* transport */) noexcept = 0;

    /**
     * Invoked when the transport is being attached to an EventBase.
     *
     * Called from within the EventBase thread being attached.
     *
     * @param transport   Transport with EventBase change.
     * @param evb         The EventBase being attached.
     */
    virtual void evbAttach(
        AsyncTransport* /* transport */, EventBase* /* evb */) {}

    /**
     * Invoked when the transport is being detached from an EventBase.
     *
     * Called from within the EventBase thread being detached.
     *
     * @param transport   Transport with EventBase change.
     * @param evb         The EventBase that is being detached.
     */
    virtual void evbDetach(
        AsyncTransport* /* transport */, EventBase* /* evb */) {}

    /**
     * Invoked each time a ByteEvent is available.
     *
     * Multiple ByteEvent may be generated for the same byte offset and event.
     * For instance, kernel software and hardware TX timestamps for the same
     * are delivered in separate CMsg, and thus will result in separate
     * ByteEvent.
     *
     * @param transport   Transport that ByteEvent is available for.
     * @param event       ByteEvent (WRITE, SCHED, TX, ACK).
     */
    virtual void byteEvent(
        AsyncTransport* /* transport */,
        const ByteEvent& /* event */) noexcept {}

    /**
     * Invoked if ByteEvents are enabled.
     *
     * Only called if the observer's configuration requested ByteEvents. May
     * be invoked multiple times if ByteEvent configuration changes (i.e., if
     * ByteEvents are enabled without hardware timestamps, and then enabled
     * with them).
     *
     * @param transport    Transport that ByteEvents are enabled for.
     */
    virtual void byteEventsEnabled(AsyncTransport* /* transport */) noexcept {}

    /**
     * Invoked if ByteEvents could not be enabled, or if an error occurred that
     * will prevent further delivery of ByteEvents.
     *
     * An observer may be waiting to receive a ByteEvent, such as an ACK event
     * confirming delivery of the last byte of a payload, before closing the
     * transport. If the transport has become unhealthy then this ByteEvent may
     * never occur, yet the handler may be unaware that the transport is
     * unhealthy if reads have been shutdown and no writes are occurring; this
     * observer signal breaks this 'deadlock'.
     *
     * @param transport   Transport that ByteEvents are now unavailable for.
     * @param ex          Details on why ByteEvents are now unavailable.
     */
    virtual void byteEventsUnavailable(
        AsyncTransport* /* transport */,
        const AsyncSocketException& /* ex */) noexcept {}

   protected:
    // observer configuration; cannot be changed post instantiation
    const Config observerConfig_;
  };

  /**
   * Adds a lifecycle observer.
   *
   * Observers can tie their lifetime to aspects of this socket's lifecycle /
   * lifetime and perform inspection at various states.
   *
   * This enables instrumentation to be added without changing / interfering
   * with how the application uses the socket.
   *
   * @param observer     Observer to add (implements LifecycleObserver).
   */
  virtual void addLifecycleObserver(LifecycleObserver* /* observer */) {
    folly::terminate_with<std::runtime_error>(
        "addLifecycleObserver() not supported");
  }

  /**
   * Removes a lifecycle observer.
   *
   * @param observer     Observer to remove.
   * @return             Whether observer found and removed from list.
   */
  virtual bool removeLifecycleObserver(LifecycleObserver* /* observer */) {
    folly::terminate_with<std::runtime_error>(
        "removeLifecycleObserver() not supported");
  }

  /**
   * Returns installed lifecycle observers.
   *
   * @return             Vector with installed observers.
   */
  FOLLY_NODISCARD virtual std::vector<LifecycleObserver*>
  getLifecycleObservers() const {
    folly::terminate_with<std::runtime_error>(
        "getLifecycleObservers() not supported");
  }

  /**
   * AsyncTransports may wrap other AsyncTransport. This returns the
   * transport that is wrapped. It returns nullptr if there is no wrapped
   * transport.
   */
  virtual const AsyncTransport* getWrappedTransport() const { return nullptr; }

  /**
   * In many cases when we need to set socket properties or otherwise access the
   * underlying transport from a wrapped transport. This method allows access to
   * the derived classes of the underlying transport.
   */
  template <class T>
  const T* getUnderlyingTransport() const {
    const AsyncTransport* current = this;
    while (current) {
      auto sock = dynamic_cast<const T*>(current);
      if (sock) {
        return sock;
      }
      current = current->getWrappedTransport();
    }
    return nullptr;
  }

  template <class T>
  T* getUnderlyingTransport() {
    return const_cast<T*>(
        static_cast<const AsyncTransport*>(this)->getUnderlyingTransport<T>());
  }

 protected:
  ~AsyncTransport() override = default;
};

using AsyncTransportWrapper = AsyncTransport;
} // namespace folly
