/*
 * Copyright 2016 Facebook, Inc.
 *
 * 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/io/async/AsyncSSLSocket.h>

#include <folly/io/async/EventBase.h>
#include <folly/portability/Sockets.h>

#include <boost/noncopyable.hpp>
#include <errno.h>
#include <fcntl.h>
#include <openssl/err.h>
#include <openssl/asn1.h>
#include <openssl/ssl.h>
#include <sys/types.h>
#include <chrono>

#include <folly/Bits.h>
#include <folly/SocketAddress.h>
#include <folly/SpinLock.h>
#include <folly/io/IOBuf.h>
#include <folly/io/Cursor.h>
#include <folly/portability/Unistd.h>

using folly::SocketAddress;
using folly::SSLContext;
using std::string;
using std::shared_ptr;

using folly::Endian;
using folly::IOBuf;
using folly::SpinLock;
using folly::SpinLockGuard;
using folly::io::Cursor;
using std::unique_ptr;
using std::bind;

namespace {
using folly::AsyncSocket;
using folly::AsyncSocketException;
using folly::AsyncSSLSocket;
using folly::Optional;
using folly::SSLContext;
using folly::ssl::OpenSSLUtils;

// We have one single dummy SSL context so that we can implement attach
// and detach methods in a thread safe fashion without modifying opnessl.
static SSLContext *dummyCtx = nullptr;
static SpinLock dummyCtxLock;

// If given min write size is less than this, buffer will be allocated on
// stack, otherwise it is allocated on heap
const size_t MAX_STACK_BUF_SIZE = 2048;

// This converts "illegal" shutdowns into ZERO_RETURN
inline bool zero_return(int error, int rc) {
  return (error == SSL_ERROR_ZERO_RETURN || (rc == 0 && errno == 0));
}

class AsyncSSLSocketConnector: public AsyncSocket::ConnectCallback,
                                public AsyncSSLSocket::HandshakeCB {

 private:
  AsyncSSLSocket *sslSocket_;
  AsyncSSLSocket::ConnectCallback *callback_;
  int timeout_;
  int64_t startTime_;

 protected:
  ~AsyncSSLSocketConnector() override {}

 public:
  AsyncSSLSocketConnector(AsyncSSLSocket *sslSocket,
                           AsyncSocket::ConnectCallback *callback,
                           int timeout) :
      sslSocket_(sslSocket),
      callback_(callback),
      timeout_(timeout),
      startTime_(std::chrono::duration_cast<std::chrono::milliseconds>(
                   std::chrono::steady_clock::now().time_since_epoch()).count()) {
  }

  void connectSuccess() noexcept override {
    VLOG(7) << "client socket connected";

    int64_t timeoutLeft = 0;
    if (timeout_ > 0) {
      auto curTime = std::chrono::duration_cast<std::chrono::milliseconds>(
        std::chrono::steady_clock::now().time_since_epoch()).count();

      timeoutLeft = timeout_ - (curTime - startTime_);
      if (timeoutLeft <= 0) {
        AsyncSocketException ex(AsyncSocketException::TIMED_OUT,
                                "SSL connect timed out");
        fail(ex);
        delete this;
        return;
      }
    }
    sslSocket_->sslConn(this, timeoutLeft);
  }

  void connectErr(const AsyncSocketException& ex) noexcept override {
    LOG(ERROR) << "TCP connect failed: " <<  ex.what();
    fail(ex);
    delete this;
  }

  void handshakeSuc(AsyncSSLSocket* /* sock */) noexcept override {
    VLOG(7) << "client handshake success";
    if (callback_) {
      callback_->connectSuccess();
    }
    delete this;
  }

  void handshakeErr(AsyncSSLSocket* /* socket */,
                    const AsyncSocketException& ex) noexcept override {
    LOG(ERROR) << "client handshakeErr: " << ex.what();
    fail(ex);
    delete this;
  }

  void fail(const AsyncSocketException &ex) {
    // fail is a noop if called twice
    if (callback_) {
      AsyncSSLSocket::ConnectCallback *cb = callback_;
      callback_ = nullptr;

      cb->connectErr(ex);
      sslSocket_->closeNow();
      // closeNow can call handshakeErr if it hasn't been called already.
      // So this may have been deleted, no member variable access beyond this
      // point
      // Note that closeNow may invoke writeError callbacks if the socket had
      // write data pending connection completion.
    }
  }
};

// XXX: implement an equivalent to corking for platforms with TCP_NOPUSH?
#ifdef TCP_CORK // Linux-only
/**
 * Utility class that corks a TCP socket upon construction or uncorks
 * the socket upon destruction
 */
class CorkGuard : private boost::noncopyable {
 public:
  CorkGuard(int fd, bool multipleWrites, bool haveMore, bool* corked):
    fd_(fd), haveMore_(haveMore), corked_(corked) {
    if (*corked_) {
      // socket is already corked; nothing to do
      return;
    }
    if (multipleWrites || haveMore) {
      // We are performing multiple writes in this performWrite() call,
      // and/or there are more calls to performWrite() that will be invoked
      // later, so enable corking
      int flag = 1;
      setsockopt(fd_, IPPROTO_TCP, TCP_CORK, &flag, sizeof(flag));
      *corked_ = true;
    }
  }

  ~CorkGuard() {
    if (haveMore_) {
      // more data to come; don't uncork yet
      return;
    }
    if (!*corked_) {
      // socket isn't corked; nothing to do
      return;
    }

    int flag = 0;
    setsockopt(fd_, IPPROTO_TCP, TCP_CORK, &flag, sizeof(flag));
    *corked_ = false;
  }

 private:
  int fd_;
  bool haveMore_;
  bool* corked_;
};
#else
class CorkGuard : private boost::noncopyable {
 public:
  CorkGuard(int, bool, bool, bool*) {}
};
#endif

void setup_SSL_CTX(SSL_CTX *ctx) {
#ifdef SSL_MODE_RELEASE_BUFFERS
  SSL_CTX_set_mode(ctx,
                   SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER |
                   SSL_MODE_ENABLE_PARTIAL_WRITE
                   | SSL_MODE_RELEASE_BUFFERS
                   );
#else
  SSL_CTX_set_mode(ctx,
                   SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER |
                   SSL_MODE_ENABLE_PARTIAL_WRITE
                   );
#endif
// SSL_CTX_set_mode is a Macro
#ifdef SSL_MODE_WRITE_IOVEC
  SSL_CTX_set_mode(ctx,
                   SSL_CTX_get_mode(ctx)
                   | SSL_MODE_WRITE_IOVEC);
#endif

}

BIO_METHOD sslWriteBioMethod;

void* initsslWriteBioMethod(void) {
  memcpy(&sslWriteBioMethod, BIO_s_socket(), sizeof(sslWriteBioMethod));
  // override the bwrite method for MSG_EOR support
  OpenSSLUtils::setCustomBioWriteMethod(
      &sslWriteBioMethod, AsyncSSLSocket::bioWrite);

  // Note that the sslWriteBioMethod.type and sslWriteBioMethod.name are not
  // set here. openssl code seems to be checking ".type == BIO_TYPE_SOCKET" and
  // then have specific handlings. The sslWriteBioWrite should be compatible
  // with the one in openssl.

  // Return something here to enable AsyncSSLSocket to call this method using
  // a function-scoped static.
  return nullptr;
}

} // anonymous namespace

namespace folly {

/**
 * Create a client AsyncSSLSocket
 */
AsyncSSLSocket::AsyncSSLSocket(const shared_ptr<SSLContext> &ctx,
                               EventBase* evb, bool deferSecurityNegotiation) :
    AsyncSocket(evb),
    ctx_(ctx),
    handshakeTimeout_(this, evb) {
  init();
  if (deferSecurityNegotiation) {
    sslState_ = STATE_UNENCRYPTED;
  }
}

/**
 * Create a server/client AsyncSSLSocket
 */
AsyncSSLSocket::AsyncSSLSocket(const shared_ptr<SSLContext>& ctx,
                               EventBase* evb, int fd, bool server,
                               bool deferSecurityNegotiation) :
    AsyncSocket(evb, fd),
    server_(server),
    ctx_(ctx),
    handshakeTimeout_(this, evb) {
  init();
  if (server) {
    SSL_CTX_set_info_callback(ctx_->getSSLCtx(),
                              AsyncSSLSocket::sslInfoCallback);
  }
  if (deferSecurityNegotiation) {
    sslState_ = STATE_UNENCRYPTED;
  }
}

#if OPENSSL_VERSION_NUMBER >= 0x1000105fL && !defined(OPENSSL_NO_TLSEXT)
/**
 * Create a client AsyncSSLSocket and allow tlsext_hostname
 * to be sent in Client Hello.
 */
AsyncSSLSocket::AsyncSSLSocket(const shared_ptr<SSLContext> &ctx,
                                 EventBase* evb,
                               const std::string& serverName,
                               bool deferSecurityNegotiation) :
    AsyncSSLSocket(ctx, evb, deferSecurityNegotiation) {
  tlsextHostname_ = serverName;
}

/**
 * Create a client AsyncSSLSocket from an already connected fd
 * and allow tlsext_hostname to be sent in Client Hello.
 */
AsyncSSLSocket::AsyncSSLSocket(const shared_ptr<SSLContext>& ctx,
                                 EventBase* evb, int fd,
                               const std::string& serverName,
                               bool deferSecurityNegotiation) :
    AsyncSSLSocket(ctx, evb, fd, false, deferSecurityNegotiation) {
  tlsextHostname_ = serverName;
}
#endif

AsyncSSLSocket::~AsyncSSLSocket() {
  VLOG(3) << "actual destruction of AsyncSSLSocket(this=" << this
          << ", evb=" << eventBase_ << ", fd=" << fd_
          << ", state=" << int(state_) << ", sslState="
          << sslState_ << ", events=" << eventFlags_ << ")";
}

void AsyncSSLSocket::init() {
  // Do this here to ensure we initialize this once before any use of
  // AsyncSSLSocket instances and not as part of library load.
  static const auto sslWriteBioMethodInitializer = initsslWriteBioMethod();
  (void)sslWriteBioMethodInitializer;

  setup_SSL_CTX(ctx_->getSSLCtx());
}

void AsyncSSLSocket::closeNow() {
  // Close the SSL connection.
  if (ssl_ != nullptr && fd_ != -1) {
    int rc = SSL_shutdown(ssl_);
    if (rc == 0) {
      rc = SSL_shutdown(ssl_);
    }
    if (rc < 0) {
      ERR_clear_error();
    }
  }

  if (sslSession_ != nullptr) {
    SSL_SESSION_free(sslSession_);
    sslSession_ = nullptr;
  }

  sslState_ = STATE_CLOSED;

  if (handshakeTimeout_.isScheduled()) {
    handshakeTimeout_.cancelTimeout();
  }

  DestructorGuard dg(this);

  invokeHandshakeErr(
      AsyncSocketException(
        AsyncSocketException::END_OF_FILE,
        "SSL connection closed locally"));

  if (ssl_ != nullptr) {
    SSL_free(ssl_);
    ssl_ = nullptr;
  }

  // Close the socket.
  AsyncSocket::closeNow();
}

void AsyncSSLSocket::shutdownWrite() {
  // SSL sockets do not support half-shutdown, so just perform a full shutdown.
  //
  // (Performing a full shutdown here is more desirable than doing nothing at
  // all.  The purpose of shutdownWrite() is normally to notify the other end
  // of the connection that no more data will be sent.  If we do nothing, the
  // other end will never know that no more data is coming, and this may result
  // in protocol deadlock.)
  close();
}

void AsyncSSLSocket::shutdownWriteNow() {
  closeNow();
}

bool AsyncSSLSocket::good() const {
  return (AsyncSocket::good() &&
          (sslState_ == STATE_ACCEPTING || sslState_ == STATE_CONNECTING ||
           sslState_ == STATE_ESTABLISHED || sslState_ == STATE_UNENCRYPTED));
}

// The TAsyncTransport definition of 'good' states that the transport is
// ready to perform reads and writes, so sslState_ == UNINIT must report !good.
// connecting can be true when the sslState_ == UNINIT because the AsyncSocket
// is connected but we haven't initiated the call to SSL_connect.
bool AsyncSSLSocket::connecting() const {
  return (!server_ &&
          (AsyncSocket::connecting() ||
           (AsyncSocket::good() && (sslState_ == STATE_UNINIT ||
                                     sslState_ == STATE_CONNECTING))));
}

std::string AsyncSSLSocket::getApplicationProtocol() noexcept {
  const unsigned char* protoName = nullptr;
  unsigned protoLength;
  if (getSelectedNextProtocolNoThrow(&protoName, &protoLength)) {
    return std::string(reinterpret_cast<const char*>(protoName), protoLength);
  }
  return "";
}

bool AsyncSSLSocket::isEorTrackingEnabled() const {
  return trackEor_;
}

void AsyncSSLSocket::setEorTracking(bool track) {
  if (trackEor_ != track) {
    trackEor_ = track;
    appEorByteNo_ = 0;
    minEorRawByteNo_ = 0;
  }
}

size_t AsyncSSLSocket::getRawBytesWritten() const {
  BIO *b;
  if (!ssl_ || !(b = SSL_get_wbio(ssl_))) {
    return 0;
  }

  return BIO_number_written(b);
}

size_t AsyncSSLSocket::getRawBytesReceived() const {
  BIO *b;
  if (!ssl_ || !(b = SSL_get_rbio(ssl_))) {
    return 0;
  }

  return BIO_number_read(b);
}


void AsyncSSLSocket::invalidState(HandshakeCB* callback) {
  LOG(ERROR) << "AsyncSSLSocket(this=" << this << ", fd=" << fd_
             << ", state=" << int(state_) << ", sslState=" << sslState_ << ", "
             << "events=" << eventFlags_ << ", server=" << short(server_)
             << "): " << "sslAccept/Connect() called in invalid "
             << "state, handshake callback " << handshakeCallback_
             << ", new callback " << callback;
  assert(!handshakeTimeout_.isScheduled());
  sslState_ = STATE_ERROR;

  AsyncSocketException ex(AsyncSocketException::INVALID_STATE,
                         "sslAccept() called with socket in invalid state");

  handshakeEndTime_ = std::chrono::steady_clock::now();
  if (callback) {
    callback->handshakeErr(this, ex);
  }

  // Check the socket state not the ssl state here.
  if (state_ != StateEnum::CLOSED || state_ != StateEnum::ERROR) {
    failHandshake(__func__, ex);
  }
}

void AsyncSSLSocket::sslAccept(HandshakeCB* callback, uint32_t timeout,
      const SSLContext::SSLVerifyPeerEnum& verifyPeer) {
  DestructorGuard dg(this);
  assert(eventBase_->isInEventBaseThread());
  verifyPeer_ = verifyPeer;

  // Make sure we're in the uninitialized state
  if (!server_ || (sslState_ != STATE_UNINIT &&
                   sslState_ != STATE_UNENCRYPTED) ||
      handshakeCallback_ != nullptr) {
    return invalidState(callback);
  }

  // Cache local and remote socket addresses to keep them available
  // after socket file descriptor is closed.
  if (cacheAddrOnFailure_ && -1 != getFd()) {
    cacheLocalPeerAddr();
  }

  handshakeStartTime_ = std::chrono::steady_clock::now();
  // Make end time at least >= start time.
  handshakeEndTime_ = handshakeStartTime_;

  sslState_ = STATE_ACCEPTING;
  handshakeCallback_ = callback;

  if (timeout > 0) {
    handshakeTimeout_.scheduleTimeout(timeout);
  }

  /* register for a read operation (waiting for CLIENT HELLO) */
  updateEventRegistration(EventHandler::READ, EventHandler::WRITE);
}

#if OPENSSL_VERSION_NUMBER >= 0x009080bfL
void AsyncSSLSocket::attachSSLContext(
  const std::shared_ptr<SSLContext>& ctx) {

  // Check to ensure we are in client mode. Changing a server's ssl
  // context doesn't make sense since clients of that server would likely
  // become confused when the server's context changes.
  DCHECK(!server_);
  DCHECK(!ctx_);
  DCHECK(ctx);
  DCHECK(ctx->getSSLCtx());
  ctx_ = ctx;

  // In order to call attachSSLContext, detachSSLContext must have been
  // previously called which sets the socket's context to the dummy
  // context. Thus we must acquire this lock.
  SpinLockGuard guard(dummyCtxLock);
  SSL_set_SSL_CTX(ssl_, ctx->getSSLCtx());
}

void AsyncSSLSocket::detachSSLContext() {
  DCHECK(ctx_);
  ctx_.reset();
  // We aren't using the initial_ctx for now, and it can introduce race
  // conditions in the destructor of the SSL object.
#ifndef OPENSSL_NO_TLSEXT
  if (ssl_->initial_ctx) {
    SSL_CTX_free(ssl_->initial_ctx);
    ssl_->initial_ctx = nullptr;
  }
#endif
  SpinLockGuard guard(dummyCtxLock);
  if (nullptr == dummyCtx) {
    // We need to lazily initialize the dummy context so we don't
    // accidentally override any programmatic settings to openssl
    dummyCtx = new SSLContext;
  }
  // We must remove this socket's references to its context right now
  // since this socket could get passed to any thread. If the context has
  // had its locking disabled, just doing a set in attachSSLContext()
  // would not be thread safe.
  SSL_set_SSL_CTX(ssl_, dummyCtx->getSSLCtx());
}
#endif

#if OPENSSL_VERSION_NUMBER >= 0x1000105fL && !defined(OPENSSL_NO_TLSEXT)
void AsyncSSLSocket::switchServerSSLContext(
  const std::shared_ptr<SSLContext>& handshakeCtx) {
  CHECK(server_);
  if (sslState_ != STATE_ACCEPTING) {
    // We log it here and allow the switch.
    // It should not affect our re-negotiation support (which
    // is not supported now).
    VLOG(6) << "fd=" << getFd()
            << " renegotation detected when switching SSL_CTX";
  }

  setup_SSL_CTX(handshakeCtx->getSSLCtx());
  SSL_CTX_set_info_callback(handshakeCtx->getSSLCtx(),
                            AsyncSSLSocket::sslInfoCallback);
  handshakeCtx_ = handshakeCtx;
  SSL_set_SSL_CTX(ssl_, handshakeCtx->getSSLCtx());
}

bool AsyncSSLSocket::isServerNameMatch() const {
  CHECK(!server_);

  if (!ssl_) {
    return false;
  }

  SSL_SESSION *ss = SSL_get_session(ssl_);
  if (!ss) {
    return false;
  }

  if(!ss->tlsext_hostname) {
    return false;
  }
  return (tlsextHostname_.compare(ss->tlsext_hostname) ? false : true);
}

void AsyncSSLSocket::setServerName(std::string serverName) noexcept {
  tlsextHostname_ = std::move(serverName);
}

#endif

void AsyncSSLSocket::timeoutExpired() noexcept {
  if (state_ == StateEnum::ESTABLISHED &&
      (sslState_ == STATE_CACHE_LOOKUP ||
       sslState_ == STATE_ASYNC_PENDING)) {
    sslState_ = STATE_ERROR;
    // We are expecting a callback in restartSSLAccept.  The cache lookup
    // and rsa-call necessarily have pointers to this ssl socket, so delay
    // the cleanup until he calls us back.
  } else {
    assert(state_ == StateEnum::ESTABLISHED &&
           (sslState_ == STATE_CONNECTING || sslState_ == STATE_ACCEPTING));
    DestructorGuard dg(this);
    AsyncSocketException ex(AsyncSocketException::TIMED_OUT,
                           (sslState_ == STATE_CONNECTING) ?
                           "SSL connect timed out" : "SSL accept timed out");
    failHandshake(__func__, ex);
  }
}

int AsyncSSLSocket::getSSLExDataIndex() {
  static auto index = SSL_get_ex_new_index(
      0, (void*)"AsyncSSLSocket data index", nullptr, nullptr, nullptr);
  return index;
}

AsyncSSLSocket* AsyncSSLSocket::getFromSSL(const SSL *ssl) {
  return static_cast<AsyncSSLSocket *>(SSL_get_ex_data(ssl,
      getSSLExDataIndex()));
}

void AsyncSSLSocket::failHandshake(const char* /* fn */,
                                   const AsyncSocketException& ex) {
  startFail();
  if (handshakeTimeout_.isScheduled()) {
    handshakeTimeout_.cancelTimeout();
  }
  invokeHandshakeErr(ex);
  finishFail();
}

void AsyncSSLSocket::invokeHandshakeErr(const AsyncSocketException& ex) {
  handshakeEndTime_ = std::chrono::steady_clock::now();
  if (handshakeCallback_ != nullptr) {
    HandshakeCB* callback = handshakeCallback_;
    handshakeCallback_ = nullptr;
    callback->handshakeErr(this, ex);
  }
}

void AsyncSSLSocket::invokeHandshakeCB() {
  handshakeEndTime_ = std::chrono::steady_clock::now();
  if (handshakeTimeout_.isScheduled()) {
    handshakeTimeout_.cancelTimeout();
  }
  if (handshakeCallback_) {
    HandshakeCB* callback = handshakeCallback_;
    handshakeCallback_ = nullptr;
    callback->handshakeSuc(this);
  }
}

void AsyncSSLSocket::cacheLocalPeerAddr() {
  SocketAddress address;
  try {
    getLocalAddress(&address);
    getPeerAddress(&address);
  } catch (const std::system_error& e) {
    // The handle can be still valid while the connection is already closed.
    if (e.code() != std::error_code(ENOTCONN, std::system_category())) {
      throw;
    }
  }
}

void AsyncSSLSocket::connect(ConnectCallback* callback,
                              const folly::SocketAddress& address,
                              int timeout,
                              const OptionMap &options,
                              const folly::SocketAddress& bindAddr)
                              noexcept {
  assert(!server_);
  assert(state_ == StateEnum::UNINIT);
  assert(sslState_ == STATE_UNINIT);
  AsyncSSLSocketConnector *connector =
    new AsyncSSLSocketConnector(this, callback, timeout);
  AsyncSocket::connect(connector, address, timeout, options, bindAddr);
}

void AsyncSSLSocket::applyVerificationOptions(SSL * ssl) {
  // apply the settings specified in verifyPeer_
  if (verifyPeer_ == SSLContext::SSLVerifyPeerEnum::USE_CTX) {
    if(ctx_->needsPeerVerification()) {
      SSL_set_verify(ssl, ctx_->getVerificationMode(),
        AsyncSSLSocket::sslVerifyCallback);
    }
  } else {
    if (verifyPeer_ == SSLContext::SSLVerifyPeerEnum::VERIFY ||
        verifyPeer_ == SSLContext::SSLVerifyPeerEnum::VERIFY_REQ_CLIENT_CERT) {
      SSL_set_verify(ssl, SSLContext::getVerificationMode(verifyPeer_),
        AsyncSSLSocket::sslVerifyCallback);
    }
  }
}

bool AsyncSSLSocket::setupSSLBio() {
  auto wb = BIO_new(&sslWriteBioMethod);

  if (!wb) {
    return false;
  }

  OpenSSLUtils::setBioAppData(wb, this);
  BIO_set_fd(wb, fd_, BIO_NOCLOSE);
  SSL_set_bio(ssl_, wb, wb);
  return true;
}

void AsyncSSLSocket::sslConn(HandshakeCB* callback, uint64_t timeout,
        const SSLContext::SSLVerifyPeerEnum& verifyPeer) {
  DestructorGuard dg(this);
  assert(eventBase_->isInEventBaseThread());

  // Cache local and remote socket addresses to keep them available
  // after socket file descriptor is closed.
  if (cacheAddrOnFailure_ && -1 != getFd()) {
    cacheLocalPeerAddr();
  }

  verifyPeer_ = verifyPeer;

  // Make sure we're in the uninitialized state
  if (server_ || (sslState_ != STATE_UNINIT && sslState_ !=
                  STATE_UNENCRYPTED) ||
      handshakeCallback_ != nullptr) {
    return invalidState(callback);
  }

  handshakeStartTime_ = std::chrono::steady_clock::now();
  // Make end time at least >= start time.
  handshakeEndTime_ = handshakeStartTime_;

  sslState_ = STATE_CONNECTING;
  handshakeCallback_ = callback;

  try {
    ssl_ = ctx_->createSSL();
  } catch (std::exception &e) {
    sslState_ = STATE_ERROR;
    AsyncSocketException ex(AsyncSocketException::INTERNAL_ERROR,
                           "error calling SSLContext::createSSL()");
    LOG(ERROR) << "AsyncSSLSocket::sslConn(this=" << this << ", fd="
            << fd_ << "): " << e.what();
    return failHandshake(__func__, ex);
  }

  if (!setupSSLBio()) {
    sslState_ = STATE_ERROR;
    AsyncSocketException ex(
        AsyncSocketException::INTERNAL_ERROR, "error creating SSL bio");
    return failHandshake(__func__, ex);
  }

  applyVerificationOptions(ssl_);

  if (sslSession_ != nullptr) {
    SSL_set_session(ssl_, sslSession_);
    SSL_SESSION_free(sslSession_);
    sslSession_ = nullptr;
  }
#if OPENSSL_VERSION_NUMBER >= 0x1000105fL && !defined(OPENSSL_NO_TLSEXT)
  if (tlsextHostname_.size()) {
    SSL_set_tlsext_host_name(ssl_, tlsextHostname_.c_str());
  }
#endif

  SSL_set_ex_data(ssl_, getSSLExDataIndex(), this);

  if (timeout > 0) {
    handshakeTimeout_.scheduleTimeout(timeout);
  }

  handleConnect();
}

SSL_SESSION *AsyncSSLSocket::getSSLSession() {
  if (ssl_ != nullptr && sslState_ == STATE_ESTABLISHED) {
    return SSL_get1_session(ssl_);
  }

  return sslSession_;
}

const SSL* AsyncSSLSocket::getSSL() const {
  return ssl_;
}

void AsyncSSLSocket::setSSLSession(SSL_SESSION *session, bool takeOwnership) {
  sslSession_ = session;
  if (!takeOwnership && session != nullptr) {
    // Increment the reference count
    CRYPTO_add(&session->references, 1, CRYPTO_LOCK_SSL_SESSION);
  }
}

void AsyncSSLSocket::getSelectedNextProtocol(
    const unsigned char** protoName,
    unsigned* protoLen,
    SSLContext::NextProtocolType* protoType) const {
  if (!getSelectedNextProtocolNoThrow(protoName, protoLen, protoType)) {
    throw AsyncSocketException(AsyncSocketException::NOT_SUPPORTED,
                              "NPN not supported");
  }
}

bool AsyncSSLSocket::getSelectedNextProtocolNoThrow(
    const unsigned char** protoName,
    unsigned* protoLen,
    SSLContext::NextProtocolType* protoType) const {
  *protoName = nullptr;
  *protoLen = 0;
#if OPENSSL_VERSION_NUMBER >= 0x1000200fL && !defined(OPENSSL_NO_TLSEXT)
  SSL_get0_alpn_selected(ssl_, protoName, protoLen);
  if (*protoLen > 0) {
    if (protoType) {
      *protoType = SSLContext::NextProtocolType::ALPN;
    }
    return true;
  }
#endif
#ifdef OPENSSL_NPN_NEGOTIATED
  SSL_get0_next_proto_negotiated(ssl_, protoName, protoLen);
  if (protoType) {
    *protoType = SSLContext::NextProtocolType::NPN;
  }
  return true;
#else
  (void)protoType;
  return false;
#endif
}

bool AsyncSSLSocket::getSSLSessionReused() const {
  if (ssl_ != nullptr && sslState_ == STATE_ESTABLISHED) {
    return SSL_session_reused(ssl_);
  }
  return false;
}

const char *AsyncSSLSocket::getNegotiatedCipherName() const {
  return (ssl_ != nullptr) ? SSL_get_cipher_name(ssl_) : nullptr;
}

/* static */
const char* AsyncSSLSocket::getSSLServerNameFromSSL(SSL* ssl) {
  if (ssl == nullptr) {
    return nullptr;
  }
#ifdef SSL_CTRL_SET_TLSEXT_SERVERNAME_CB
  return SSL_get_servername(ssl, TLSEXT_NAMETYPE_host_name);
#else
  return nullptr;
#endif
}

const char *AsyncSSLSocket::getSSLServerName() const {
#ifdef SSL_CTRL_SET_TLSEXT_SERVERNAME_CB
  return getSSLServerNameFromSSL(ssl_);
#else
  throw AsyncSocketException(AsyncSocketException::NOT_SUPPORTED,
                             "SNI not supported");
#endif
}

const char *AsyncSSLSocket::getSSLServerNameNoThrow() const {
  return getSSLServerNameFromSSL(ssl_);
}

int AsyncSSLSocket::getSSLVersion() const {
  return (ssl_ != nullptr) ? SSL_version(ssl_) : 0;
}

const char *AsyncSSLSocket::getSSLCertSigAlgName() const {
  X509 *cert = (ssl_ != nullptr) ? SSL_get_certificate(ssl_) : nullptr;
  if (cert) {
    int nid = OBJ_obj2nid(cert->sig_alg->algorithm);
    return OBJ_nid2ln(nid);
  }
  return nullptr;
}

int AsyncSSLSocket::getSSLCertSize() const {
  int certSize = 0;
  X509 *cert = (ssl_ != nullptr) ? SSL_get_certificate(ssl_) : nullptr;
  if (cert) {
    EVP_PKEY *key = X509_get_pubkey(cert);
    certSize = EVP_PKEY_bits(key);
    EVP_PKEY_free(key);
  }
  return certSize;
}

bool AsyncSSLSocket::willBlock(int ret,
                               int* sslErrorOut,
                               unsigned long* errErrorOut) noexcept {
  *errErrorOut = 0;
  int error = *sslErrorOut = SSL_get_error(ssl_, ret);
  if (error == SSL_ERROR_WANT_READ) {
    // Register for read event if not already.
    updateEventRegistration(EventHandler::READ, EventHandler::WRITE);
    return true;
  } else if (error == SSL_ERROR_WANT_WRITE) {
    VLOG(3) << "AsyncSSLSocket(fd=" << fd_
            << ", state=" << int(state_) << ", sslState="
            << sslState_ << ", events=" << eventFlags_ << "): "
            << "SSL_ERROR_WANT_WRITE";
    // Register for write event if not already.
    updateEventRegistration(EventHandler::WRITE, EventHandler::READ);
    return true;
#ifdef SSL_ERROR_WANT_SESS_CACHE_LOOKUP
  } else if (error == SSL_ERROR_WANT_SESS_CACHE_LOOKUP) {
    // We will block but we can't register our own socket.  The callback that
    // triggered this code will re-call handleAccept at the appropriate time.

    // We can only get here if the linked libssl.so has support for this feature
    // as well, otherwise SSL_get_error cannot return our error code.
    sslState_ = STATE_CACHE_LOOKUP;

    // Unregister for all events while blocked here
    updateEventRegistration(EventHandler::NONE,
                            EventHandler::READ | EventHandler::WRITE);

    // The timeout (if set) keeps running here
    return true;
#endif
  } else if (0
#ifdef SSL_ERROR_WANT_RSA_ASYNC_PENDING
      || error == SSL_ERROR_WANT_RSA_ASYNC_PENDING
#endif
#ifdef SSL_ERROR_WANT_ECDSA_ASYNC_PENDING
      || error == SSL_ERROR_WANT_ECDSA_ASYNC_PENDING
#endif
      ) {
    // Our custom openssl function has kicked off an async request to do
    // rsa/ecdsa private key operation.  When that call returns, a callback will
    // be invoked that will re-call handleAccept.
    sslState_ = STATE_ASYNC_PENDING;

    // Unregister for all events while blocked here
    updateEventRegistration(
      EventHandler::NONE,
      EventHandler::READ | EventHandler::WRITE
    );

    // The timeout (if set) keeps running here
    return true;
  } else {
    unsigned long lastError = *errErrorOut = ERR_get_error();
    VLOG(6) << "AsyncSSLSocket(fd=" << fd_ << ", "
            << "state=" << state_ << ", "
            << "sslState=" << sslState_ << ", "
            << "events=" << std::hex << eventFlags_ << "): "
            << "SSL error: " << error << ", "
            << "errno: " << errno << ", "
            << "ret: " << ret << ", "
            << "read: " << BIO_number_read(SSL_get_rbio(ssl_)) << ", "
            << "written: " << BIO_number_written(SSL_get_wbio(ssl_)) << ", "
            << "func: " << ERR_func_error_string(lastError) << ", "
            << "reason: " << ERR_reason_error_string(lastError);
    return false;
  }
}

void AsyncSSLSocket::checkForImmediateRead() noexcept {
  // openssl may have buffered data that it read from the socket already.
  // In this case we have to process it immediately, rather than waiting for
  // the socket to become readable again.
  if (ssl_ != nullptr && SSL_pending(ssl_) > 0) {
    AsyncSocket::handleRead();
  }
}

void
AsyncSSLSocket::restartSSLAccept()
{
  VLOG(3) << "AsyncSSLSocket::restartSSLAccept() this=" << this
          << ", fd=" << fd_ << ", state=" << int(state_) << ", "
          << "sslState=" << sslState_ << ", events=" << eventFlags_;
  DestructorGuard dg(this);
  assert(
    sslState_ == STATE_CACHE_LOOKUP ||
    sslState_ == STATE_ASYNC_PENDING ||
    sslState_ == STATE_ERROR ||
    sslState_ == STATE_CLOSED);
  if (sslState_ == STATE_CLOSED) {
    // I sure hope whoever closed this socket didn't delete it already,
    // but this is not strictly speaking an error
    return;
  }
  if (sslState_ == STATE_ERROR) {
    // go straight to fail if timeout expired during lookup
    AsyncSocketException ex(AsyncSocketException::TIMED_OUT,
                           "SSL accept timed out");
    failHandshake(__func__, ex);
    return;
  }
  sslState_ = STATE_ACCEPTING;
  this->handleAccept();
}

void
AsyncSSLSocket::handleAccept() noexcept {
  VLOG(3) << "AsyncSSLSocket::handleAccept() this=" << this
          << ", fd=" << fd_ << ", state=" << int(state_) << ", "
          << "sslState=" << sslState_ << ", events=" << eventFlags_;
  assert(server_);
  assert(state_ == StateEnum::ESTABLISHED &&
         sslState_ == STATE_ACCEPTING);
  if (!ssl_) {
    /* lazily create the SSL structure */
    try {
      ssl_ = ctx_->createSSL();
    } catch (std::exception &e) {
      sslState_ = STATE_ERROR;
      AsyncSocketException ex(AsyncSocketException::INTERNAL_ERROR,
                             "error calling SSLContext::createSSL()");
      LOG(ERROR) << "AsyncSSLSocket::handleAccept(this=" << this
                 << ", fd=" << fd_ << "): " << e.what();
      return failHandshake(__func__, ex);
    }

    if (!setupSSLBio()) {
      sslState_ = STATE_ERROR;
      AsyncSocketException ex(
          AsyncSocketException::INTERNAL_ERROR, "error creating write bio");
      return failHandshake(__func__, ex);
    }

    SSL_set_ex_data(ssl_, getSSLExDataIndex(), this);

    applyVerificationOptions(ssl_);
  }

  if (server_ && parseClientHello_) {
    SSL_set_msg_callback(ssl_, &AsyncSSLSocket::clientHelloParsingCallback);
    SSL_set_msg_callback_arg(ssl_, this);
  }

  int ret = SSL_accept(ssl_);
  if (ret <= 0) {
    int sslError;
    unsigned long errError;
    int errnoCopy = errno;
    if (willBlock(ret, &sslError, &errError)) {
      return;
    } else {
      sslState_ = STATE_ERROR;
      SSLException ex(sslError, errError, ret, errnoCopy);
      return failHandshake(__func__, ex);
    }
  }

  handshakeComplete_ = true;
  updateEventRegistration(0, EventHandler::READ | EventHandler::WRITE);

  // Move into STATE_ESTABLISHED in the normal case that we are in
  // STATE_ACCEPTING.
  sslState_ = STATE_ESTABLISHED;

  VLOG(3) << "AsyncSSLSocket " << this << ": fd " << fd_
          << " successfully accepted; state=" << int(state_)
          << ", sslState=" << sslState_ << ", events=" << eventFlags_;

  // Remember the EventBase we are attached to, before we start invoking any
  // callbacks (since the callbacks may call detachEventBase()).
  EventBase* originalEventBase = eventBase_;

  // Call the accept callback.
  invokeHandshakeCB();

  // Note that the accept callback may have changed our state.
  // (set or unset the read callback, called write(), closed the socket, etc.)
  // The following code needs to handle these situations correctly.
  //
  // If the socket has been closed, readCallback_ and writeReqHead_ will
  // always be nullptr, so that will prevent us from trying to read or write.
  //
  // The main thing to check for is if eventBase_ is still originalEventBase.
  // If not, we have been detached from this event base, so we shouldn't
  // perform any more operations.
  if (eventBase_ != originalEventBase) {
    return;
  }

  AsyncSocket::handleInitialReadWrite();
}

void
AsyncSSLSocket::handleConnect() noexcept {
  VLOG(3) <<  "AsyncSSLSocket::handleConnect() this=" << this
          << ", fd=" << fd_ << ", state=" << int(state_) << ", "
          << "sslState=" << sslState_ << ", events=" << eventFlags_;
  assert(!server_);
  if (state_ < StateEnum::ESTABLISHED) {
    return AsyncSocket::handleConnect();
  }

  assert(
      (state_ == StateEnum::FAST_OPEN || state_ == StateEnum::ESTABLISHED) &&
      sslState_ == STATE_CONNECTING);
  assert(ssl_);

  int ret = SSL_connect(ssl_);
  if (ret <= 0) {
    int sslError;
    unsigned long errError;
    int errnoCopy = errno;
    if (willBlock(ret, &sslError, &errError)) {
      return;
    } else {
      sslState_ = STATE_ERROR;
      SSLException ex(sslError, errError, ret, errnoCopy);
      return failHandshake(__func__, ex);
    }
  }

  handshakeComplete_ = true;
  updateEventRegistration(0, EventHandler::READ | EventHandler::WRITE);

  // Move into STATE_ESTABLISHED in the normal case that we are in
  // STATE_CONNECTING.
  sslState_ = STATE_ESTABLISHED;

  VLOG(3) << "AsyncSSLSocket " << this << ": "
          << "fd " << fd_ << " successfully connected; "
          << "state=" << int(state_) << ", sslState=" << sslState_
          << ", events=" << eventFlags_;

  // Remember the EventBase we are attached to, before we start invoking any
  // callbacks (since the callbacks may call detachEventBase()).
  EventBase* originalEventBase = eventBase_;

  // Call the handshake callback.
  invokeHandshakeCB();

  // Note that the connect callback may have changed our state.
  // (set or unset the read callback, called write(), closed the socket, etc.)
  // The following code needs to handle these situations correctly.
  //
  // If the socket has been closed, readCallback_ and writeReqHead_ will
  // always be nullptr, so that will prevent us from trying to read or write.
  //
  // The main thing to check for is if eventBase_ is still originalEventBase.
  // If not, we have been detached from this event base, so we shouldn't
  // perform any more operations.
  if (eventBase_ != originalEventBase) {
    return;
  }

  AsyncSocket::handleInitialReadWrite();
}

void AsyncSSLSocket::invokeConnectSuccess() {
  if (sslState_ == SSLStateEnum::STATE_CONNECTING) {
    // If we failed TFO, we'd fall back to trying to connect the socket,
    // when we succeed we should handle the writes that caused us to start
    // TFO.
    handleWrite();
  }
  AsyncSocket::invokeConnectSuccess();
}

void AsyncSSLSocket::setReadCB(ReadCallback *callback) {
#ifdef SSL_MODE_MOVE_BUFFER_OWNERSHIP
  // turn on the buffer movable in openssl
  if (bufferMovableEnabled_ && ssl_ != nullptr && !isBufferMovable_ &&
      callback != nullptr && callback->isBufferMovable()) {
    SSL_set_mode(ssl_, SSL_get_mode(ssl_) | SSL_MODE_MOVE_BUFFER_OWNERSHIP);
    isBufferMovable_ = true;
  }
#endif

  AsyncSocket::setReadCB(callback);
}

void AsyncSSLSocket::setBufferMovableEnabled(bool enabled) {
  bufferMovableEnabled_ = enabled;
}

void AsyncSSLSocket::prepareReadBuffer(void** buf, size_t* buflen) noexcept {
  CHECK(readCallback_);
  if (isBufferMovable_) {
    *buf = nullptr;
    *buflen = 0;
  } else {
    // buf is necessary for SSLSocket without SSL_MODE_MOVE_BUFFER_OWNERSHIP
    readCallback_->getReadBuffer(buf, buflen);
  }
}

void
AsyncSSLSocket::handleRead() noexcept {
  VLOG(5) << "AsyncSSLSocket::handleRead() this=" << this << ", fd=" << fd_
          << ", state=" << int(state_) << ", "
          << "sslState=" << sslState_ << ", events=" << eventFlags_;
  if (state_ < StateEnum::ESTABLISHED) {
    return AsyncSocket::handleRead();
  }


  if (sslState_ == STATE_ACCEPTING) {
    assert(server_);
    handleAccept();
    return;
  }
  else if (sslState_ == STATE_CONNECTING) {
    assert(!server_);
    handleConnect();
    return;
  }

  // Normal read
  AsyncSocket::handleRead();
}

AsyncSocket::ReadResult
AsyncSSLSocket::performRead(void** buf, size_t* buflen, size_t* offset) {
  VLOG(4) << "AsyncSSLSocket::performRead() this=" << this << ", buf=" << *buf
          << ", buflen=" << *buflen;

  if (sslState_ == STATE_UNENCRYPTED) {
    return AsyncSocket::performRead(buf, buflen, offset);
  }

  ssize_t bytes = 0;
  if (!isBufferMovable_) {
    bytes = SSL_read(ssl_, *buf, *buflen);
  }
#ifdef SSL_MODE_MOVE_BUFFER_OWNERSHIP
  else {
    bytes = SSL_read_buf(ssl_, buf, (int *) offset, (int *) buflen);
  }
#endif

  if (server_ && renegotiateAttempted_) {
    LOG(ERROR) << "AsyncSSLSocket(fd=" << fd_ << ", state=" << int(state_)
               << ", sslstate=" << sslState_ << ", events=" << eventFlags_
               << "): client intitiated SSL renegotiation not permitted";
    return ReadResult(
        READ_ERROR,
        folly::make_unique<SSLException>(SSLError::CLIENT_RENEGOTIATION));
  }
  if (bytes <= 0) {
    int error = SSL_get_error(ssl_, bytes);
    if (error == SSL_ERROR_WANT_READ) {
      // The caller will register for read event if not already.
      if (errno == EWOULDBLOCK || errno == EAGAIN) {
        return ReadResult(READ_BLOCKING);
      } else {
        return ReadResult(READ_ERROR);
      }
    } else if (error == SSL_ERROR_WANT_WRITE) {
      // TODO: Even though we are attempting to read data, SSL_read() may
      // need to write data if renegotiation is being performed.  We currently
      // don't support this and just fail the read.
      LOG(ERROR) << "AsyncSSLSocket(fd=" << fd_ << ", state=" << int(state_)
                 << ", sslState=" << sslState_ << ", events=" << eventFlags_
                 << "): unsupported SSL renegotiation during read";
      return ReadResult(
          READ_ERROR,
          folly::make_unique<SSLException>(SSLError::INVALID_RENEGOTIATION));
    } else {
      if (zero_return(error, bytes)) {
        return ReadResult(bytes);
      }
      long errError = ERR_get_error();
      VLOG(6) << "AsyncSSLSocket(fd=" << fd_ << ", "
              << "state=" << state_ << ", "
              << "sslState=" << sslState_ << ", "
              << "events=" << std::hex << eventFlags_ << "): "
              << "bytes: " << bytes << ", "
              << "error: " << error << ", "
              << "errno: " << errno << ", "
              << "func: " << ERR_func_error_string(errError) << ", "
              << "reason: " << ERR_reason_error_string(errError);
      return ReadResult(
          READ_ERROR,
          folly::make_unique<SSLException>(error, errError, bytes, errno));
    }
  } else {
    appBytesReceived_ += bytes;
    return ReadResult(bytes);
  }
}

void AsyncSSLSocket::handleWrite() noexcept {
  VLOG(5) << "AsyncSSLSocket::handleWrite() this=" << this << ", fd=" << fd_
          << ", state=" << int(state_) << ", "
          << "sslState=" << sslState_ << ", events=" << eventFlags_;
  if (state_ < StateEnum::ESTABLISHED) {
    return AsyncSocket::handleWrite();
  }

  if (sslState_ == STATE_ACCEPTING) {
    assert(server_);
    handleAccept();
    return;
  }

  if (sslState_ == STATE_CONNECTING) {
    assert(!server_);
    handleConnect();
    return;
  }

  // Normal write
  AsyncSocket::handleWrite();
}

AsyncSocket::WriteResult AsyncSSLSocket::interpretSSLError(int rc, int error) {
  if (error == SSL_ERROR_WANT_READ) {
    // Even though we are attempting to write data, SSL_write() may
    // need to read data if renegotiation is being performed.  We currently
    // don't support this and just fail the write.
    LOG(ERROR) << "AsyncSSLSocket(fd=" << fd_ << ", state=" << int(state_)
               << ", sslState=" << sslState_ << ", events=" << eventFlags_
               << "): "
               << "unsupported SSL renegotiation during write";
    return WriteResult(
        WRITE_ERROR,
        folly::make_unique<SSLException>(SSLError::INVALID_RENEGOTIATION));
  } else {
    if (zero_return(error, rc)) {
      return WriteResult(0);
    }
    auto errError = ERR_get_error();
    VLOG(3) << "ERROR: AsyncSSLSocket(fd=" << fd_ << ", state=" << int(state_)
            << ", sslState=" << sslState_ << ", events=" << eventFlags_ << "): "
            << "SSL error: " << error << ", errno: " << errno
            << ", func: " << ERR_func_error_string(errError)
            << ", reason: " << ERR_reason_error_string(errError);
    return WriteResult(
        WRITE_ERROR,
        folly::make_unique<SSLException>(error, errError, rc, errno));
  }
}

AsyncSocket::WriteResult AsyncSSLSocket::performWrite(
    const iovec* vec,
    uint32_t count,
    WriteFlags flags,
    uint32_t* countWritten,
    uint32_t* partialWritten) {
  if (sslState_ == STATE_UNENCRYPTED) {
    return AsyncSocket::performWrite(
      vec, count, flags, countWritten, partialWritten);
  }
  if (sslState_ != STATE_ESTABLISHED) {
    LOG(ERROR) << "AsyncSSLSocket(fd=" << fd_ << ", state=" << int(state_)
               << ", sslState=" << sslState_
               << ", events=" << eventFlags_ << "): "
               << "TODO: AsyncSSLSocket currently does not support calling "
               << "write() before the handshake has fully completed";
    return WriteResult(
        WRITE_ERROR, folly::make_unique<SSLException>(SSLError::EARLY_WRITE));
  }

  bool cork = isSet(flags, WriteFlags::CORK);
  CorkGuard guard(fd_, count > 1, cork, &corked_);

  // Declare a buffer used to hold small write requests.  It could point to a
  // memory block either on stack or on heap. If it is on heap, we release it
  // manually when scope exits
  char* combinedBuf{nullptr};
  SCOPE_EXIT {
    // Note, always keep this check consistent with what we do below
    if (combinedBuf != nullptr && minWriteSize_ > MAX_STACK_BUF_SIZE) {
      delete[] combinedBuf;
    }
  };

  *countWritten = 0;
  *partialWritten = 0;
  ssize_t totalWritten = 0;
  size_t bytesStolenFromNextBuffer = 0;
  for (uint32_t i = 0; i < count; i++) {
    const iovec* v = vec + i;
    size_t offset = bytesStolenFromNextBuffer;
    bytesStolenFromNextBuffer = 0;
    size_t len = v->iov_len - offset;
    const void* buf;
    if (len == 0) {
      (*countWritten)++;
      continue;
    }
    buf = ((const char*)v->iov_base) + offset;

    ssize_t bytes;
    uint32_t buffersStolen = 0;
    if ((len < minWriteSize_) && ((i + 1) < count)) {
      // Combine this buffer with part or all of the next buffers in
      // order to avoid really small-grained calls to SSL_write().
      // Each call to SSL_write() produces a separate record in
      // the egress SSL stream, and we've found that some low-end
      // mobile clients can't handle receiving an HTTP response
      // header and the first part of the response body in two
      // separate SSL records (even if those two records are in
      // the same TCP packet).

      if (combinedBuf == nullptr) {
        if (minWriteSize_ > MAX_STACK_BUF_SIZE) {
          // Allocate the buffer on heap
          combinedBuf = new char[minWriteSize_];
        } else {
          // Allocate the buffer on stack
          combinedBuf = (char*)alloca(minWriteSize_);
        }
      }
      assert(combinedBuf != nullptr);

      memcpy(combinedBuf, buf, len);
      do {
        // INVARIANT: i + buffersStolen == complete chunks serialized
        uint32_t nextIndex = i + buffersStolen + 1;
        bytesStolenFromNextBuffer = std::min(vec[nextIndex].iov_len,
                                             minWriteSize_ - len);
        memcpy(combinedBuf + len, vec[nextIndex].iov_base,
               bytesStolenFromNextBuffer);
        len += bytesStolenFromNextBuffer;
        if (bytesStolenFromNextBuffer < vec[nextIndex].iov_len) {
          // couldn't steal the whole buffer
          break;
        } else {
          bytesStolenFromNextBuffer = 0;
          buffersStolen++;
        }
      } while ((i + buffersStolen + 1) < count && (len < minWriteSize_));
      bytes = eorAwareSSLWrite(
        ssl_, combinedBuf, len,
        (isSet(flags, WriteFlags::EOR) && i + buffersStolen + 1 == count));

    } else {
      bytes = eorAwareSSLWrite(ssl_, buf, len,
                           (isSet(flags, WriteFlags::EOR) && i + 1 == count));
    }

    if (bytes <= 0) {
      int error = SSL_get_error(ssl_, bytes);
      if (error == SSL_ERROR_WANT_WRITE) {
        // The caller will register for write event if not already.
        *partialWritten = offset;
        return WriteResult(totalWritten);
      }
      auto writeResult = interpretSSLError(bytes, error);
      if (writeResult.writeReturn < 0) {
        return writeResult;
      } // else fall through to below to correctly record totalWritten
    }

    totalWritten += bytes;

    if (bytes == (ssize_t)len) {
      // The full iovec is written.
      (*countWritten) += 1 + buffersStolen;
      i += buffersStolen;
      // continue
    } else {
      bytes += offset; // adjust bytes to account for all of v
      while (bytes >= (ssize_t)v->iov_len) {
        // We combined this buf with part or all of the next one, and
        // we managed to write all of this buf but not all of the bytes
        // from the next one that we'd hoped to write.
        bytes -= v->iov_len;
        (*countWritten)++;
        v = &(vec[++i]);
      }
      *partialWritten = bytes;
      return WriteResult(totalWritten);
    }
  }

  return WriteResult(totalWritten);
}

int AsyncSSLSocket::eorAwareSSLWrite(SSL *ssl, const void *buf, int n,
                                      bool eor) {
  if (eor && trackEor_) {
    if (appEorByteNo_) {
      // cannot track for more than one app byte EOR
      CHECK(appEorByteNo_ == appBytesWritten_ + n);
    } else {
      appEorByteNo_ = appBytesWritten_ + n;
    }

    // 1. It is fine to keep updating minEorRawByteNo_.
    // 2. It is _min_ in the sense that SSL record will add some overhead.
    minEorRawByteNo_ = getRawBytesWritten() + n;
  }

  n = sslWriteImpl(ssl, buf, n);
  if (n > 0) {
    appBytesWritten_ += n;
    if (appEorByteNo_) {
      if (getRawBytesWritten() >= minEorRawByteNo_) {
        minEorRawByteNo_ = 0;
      }
      if(appBytesWritten_ == appEorByteNo_) {
        appEorByteNo_ = 0;
      } else {
        CHECK(appBytesWritten_ < appEorByteNo_);
      }
    }
  }
  return n;
}

void AsyncSSLSocket::sslInfoCallback(const SSL* ssl, int where, int ret) {
  AsyncSSLSocket *sslSocket = AsyncSSLSocket::getFromSSL(ssl);
  if (sslSocket->handshakeComplete_ && (where & SSL_CB_HANDSHAKE_START)) {
    sslSocket->renegotiateAttempted_ = true;
  }
  if (where & SSL_CB_READ_ALERT) {
    const char* type = SSL_alert_type_string(ret);
    if (type) {
      const char* desc = SSL_alert_desc_string(ret);
      sslSocket->alertsReceived_.emplace_back(
          *type, StringPiece(desc, std::strlen(desc)));
    }
  }
}

int AsyncSSLSocket::bioWrite(BIO* b, const char* in, int inl) {
  struct msghdr msg;
  struct iovec iov;
  int flags = 0;
  AsyncSSLSocket* tsslSock;

  iov.iov_base = const_cast<char*>(in);
  iov.iov_len = inl;
  memset(&msg, 0, sizeof(msg));
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;

  auto appData = OpenSSLUtils::getBioAppData(b);
  CHECK(appData);

  tsslSock = reinterpret_cast<AsyncSSLSocket*>(appData);
  CHECK(tsslSock);

  if (tsslSock->trackEor_ && tsslSock->minEorRawByteNo_ &&
      tsslSock->minEorRawByteNo_ <= BIO_number_written(b) + inl) {
    flags = MSG_EOR;
  }

  auto result =
      tsslSock->sendSocketMessage(BIO_get_fd(b, nullptr), &msg, flags);
  BIO_clear_retry_flags(b);
  if (!result.exception && result.writeReturn <= 0) {
    if (OpenSSLUtils::getBioShouldRetryWrite(result.writeReturn)) {
      BIO_set_retry_write(b);
    }
  }
  return result.writeReturn;
}

int AsyncSSLSocket::sslVerifyCallback(
    int preverifyOk,
    X509_STORE_CTX* x509Ctx) {
  SSL* ssl = (SSL*) X509_STORE_CTX_get_ex_data(
    x509Ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
  AsyncSSLSocket* self = AsyncSSLSocket::getFromSSL(ssl);

  VLOG(3) <<  "AsyncSSLSocket::sslVerifyCallback() this=" << self << ", "
          << "fd=" << self->fd_ << ", preverifyOk=" << preverifyOk;
  return (self->handshakeCallback_) ?
    self->handshakeCallback_->handshakeVer(self, preverifyOk, x509Ctx) :
    preverifyOk;
}

void AsyncSSLSocket::enableClientHelloParsing()  {
    parseClientHello_ = true;
    clientHelloInfo_.reset(new ssl::ClientHelloInfo());
}

void AsyncSSLSocket::resetClientHelloParsing(SSL *ssl)  {
  SSL_set_msg_callback(ssl, nullptr);
  SSL_set_msg_callback_arg(ssl, nullptr);
  clientHelloInfo_->clientHelloBuf_.clear();
}

void AsyncSSLSocket::clientHelloParsingCallback(int written,
                                                int /* version */,
                                                int contentType,
                                                const void* buf,
                                                size_t len,
                                                SSL* ssl,
                                                void* arg) {
  AsyncSSLSocket *sock = static_cast<AsyncSSLSocket*>(arg);
  if (written != 0) {
    sock->resetClientHelloParsing(ssl);
    return;
  }
  if (contentType != SSL3_RT_HANDSHAKE) {
    return;
  }
  if (len == 0) {
    return;
  }

  auto& clientHelloBuf = sock->clientHelloInfo_->clientHelloBuf_;
  clientHelloBuf.append(IOBuf::wrapBuffer(buf, len));
  try {
    Cursor cursor(clientHelloBuf.front());
    if (cursor.read<uint8_t>() != SSL3_MT_CLIENT_HELLO) {
      sock->resetClientHelloParsing(ssl);
      return;
    }

    if (cursor.totalLength() < 3) {
      clientHelloBuf.trimEnd(len);
      clientHelloBuf.append(IOBuf::copyBuffer(buf, len));
      return;
    }

    uint32_t messageLength = cursor.read<uint8_t>();
    messageLength <<= 8;
    messageLength |= cursor.read<uint8_t>();
    messageLength <<= 8;
    messageLength |= cursor.read<uint8_t>();
    if (cursor.totalLength() < messageLength) {
      clientHelloBuf.trimEnd(len);
      clientHelloBuf.append(IOBuf::copyBuffer(buf, len));
      return;
    }

    sock->clientHelloInfo_->clientHelloMajorVersion_ = cursor.read<uint8_t>();
    sock->clientHelloInfo_->clientHelloMinorVersion_ = cursor.read<uint8_t>();

    cursor.skip(4); // gmt_unix_time
    cursor.skip(28); // random_bytes

    cursor.skip(cursor.read<uint8_t>()); // session_id

    uint16_t cipherSuitesLength = cursor.readBE<uint16_t>();
    for (int i = 0; i < cipherSuitesLength; i += 2) {
      sock->clientHelloInfo_->
        clientHelloCipherSuites_.push_back(cursor.readBE<uint16_t>());
    }

    uint8_t compressionMethodsLength = cursor.read<uint8_t>();
    for (int i = 0; i < compressionMethodsLength; ++i) {
      sock->clientHelloInfo_->
        clientHelloCompressionMethods_.push_back(cursor.readBE<uint8_t>());
    }

    if (cursor.totalLength() > 0) {
      uint16_t extensionsLength = cursor.readBE<uint16_t>();
      while (extensionsLength) {
        ssl::TLSExtension extensionType =
            static_cast<ssl::TLSExtension>(cursor.readBE<uint16_t>());
        sock->clientHelloInfo_->
          clientHelloExtensions_.push_back(extensionType);
        extensionsLength -= 2;
        uint16_t extensionDataLength = cursor.readBE<uint16_t>();
        extensionsLength -= 2;

        if (extensionType == ssl::TLSExtension::SIGNATURE_ALGORITHMS) {
          cursor.skip(2);
          extensionDataLength -= 2;
          while (extensionDataLength) {
            ssl::HashAlgorithm hashAlg =
                static_cast<ssl::HashAlgorithm>(cursor.readBE<uint8_t>());
            ssl::SignatureAlgorithm sigAlg =
                static_cast<ssl::SignatureAlgorithm>(cursor.readBE<uint8_t>());
            extensionDataLength -= 2;
            sock->clientHelloInfo_->
              clientHelloSigAlgs_.emplace_back(hashAlg, sigAlg);
          }
        } else {
          cursor.skip(extensionDataLength);
          extensionsLength -= extensionDataLength;
        }
      }
    }
  } catch (std::out_of_range& e) {
    // we'll use what we found and cleanup below.
    VLOG(4) << "AsyncSSLSocket::clientHelloParsingCallback(): "
      << "buffer finished unexpectedly." << " AsyncSSLSocket socket=" << sock;
  }

  sock->resetClientHelloParsing(ssl);
}

} // namespace
