http_client_asio.cpp

/***
 * Copyright (C) Microsoft. All rights reserved.
 * Licensed under the MIT license. See LICENSE.txt file in the project root for full license information.
 *
 * =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 *
 * HTTP Library: Client-side APIs.
 *
 * This file contains a cross platform implementation based on Boost.ASIO.
 *
 * For the latest on this and related APIs, please see: https://github.com/Microsoft/cpprestsdk
 *
 * =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 ****/

#include "stdafx.h"

#include "../common/connection_pool_helpers.h"
#include "../common/internal_http_helpers.h"
#include "cpprest/asyncrt_utils.h"
#include <sstream>

#if defined(__clang__)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-local-typedef"
#pragma clang diagnostic ignored "-Winfinite-recursion"
#endif
#include <boost/algorithm/string.hpp>
#include <boost/asio.hpp>
#include <boost/asio/ssl.hpp>
#include <boost/asio/ssl/error.hpp>
#include <boost/asio/steady_timer.hpp>
#include <boost/bind.hpp>
#if defined(__clang__)
#pragma clang diagnostic pop
#endif

#if defined(BOOST_NO_CXX11_SMART_PTR)
#error "Cpp rest SDK requires c++11 smart pointer support from boost"
#endif

#include "../common/x509_cert_utilities.h"
#include "cpprest/base_uri.h"
#include "cpprest/details/http_helpers.h"
#include "http_client_impl.h"
#include "pplx/threadpool.h"
#include <memory>
#include <unordered_set>

#if defined(__GNUC__) && !defined(__clang__)

#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8)
#define AND_CAPTURE_MEMBER_FUNCTION_POINTERS
#else
// GCC Bug 56222 - Pointer to member in lambda should not require this to be captured
// See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56222
// GCC Bug 51494 - Legal program rejection - capturing "this" when using static method inside lambda
// See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51494
#define AND_CAPTURE_MEMBER_FUNCTION_POINTERS , this
#endif

#elif defined(_MSC_VER)

#if _MSC_VER >= 1900
#define AND_CAPTURE_MEMBER_FUNCTION_POINTERS
#else
// This bug also afflicts VS2013 which incorrectly reports "warning C4573: the usage of 'symbol' requires the compiler
// to capture 'this' but the current default capture mode does not allow it"
#define AND_CAPTURE_MEMBER_FUNCTION_POINTERS , this
#endif

#else

#define AND_CAPTURE_MEMBER_FUNCTION_POINTERS

#endif

using boost::asio::ip::tcp;

#ifdef __ANDROID__
using utility::conversions::details::to_string;
#else
using std::to_string;
#endif

namespace
{
const std::string CRLF("\r\n");

std::string calc_cn_host(const web::http::uri& baseUri, const web::http::http_headers& requestHeaders)
{
    std::string result;
    if (baseUri.scheme() == U("https"))
    {
        const utility::string_t* encResult;
        const auto hostHeader = requestHeaders.find(_XPLATSTR("Host"));
        if (hostHeader == requestHeaders.end())
        {
            encResult = &baseUri.host();
        }
        else
        {
            encResult = &hostHeader->second;
        }

        result = utility::conversions::to_utf8string(*encResult);
        utility::details::inplace_tolower(result);
    }

    return result;
}
} // namespace

namespace web
{
namespace http
{
namespace client
{
namespace details
{
enum class httpclient_errorcode_context
{
    none = 0,
    connect,
    handshake,
    writeheader,
    writebody,
    readheader,
    readbody,
    close
};

static std::string generate_base64_userpass(const ::web::credentials& creds)
{
    auto userpass = creds.username() + U(":") + *creds._internal_decrypt();
    auto&& u8_userpass = utility::conversions::to_utf8string(userpass);
    std::vector<unsigned char> credentials_buffer(u8_userpass.begin(), u8_userpass.end());
    return utility::conversions::to_utf8string(utility::conversions::to_base64(credentials_buffer));
}

class asio_connection_pool;

class asio_connection
{
    friend class asio_client;

public:
    asio_connection(boost::asio::io_service& io_service)
        : m_socket_lock()
        , m_socket(io_service)
        , m_ssl_stream()
        , m_cn_hostname()
        , m_is_reused(false)
        , m_keep_alive(true)
        , m_closed(false)
    {
    }

    ~asio_connection() { close(); }

    // This simply instantiates the internal state to support ssl. It does not perform the handshake.
    void upgrade_to_ssl(std::string&& cn_hostname,
                        const std::function<void(boost::asio::ssl::context&)>& ssl_context_callback)
    {
        std::lock_guard<std::mutex> lock(m_socket_lock);
        assert(!is_ssl());
        boost::asio::ssl::context ssl_context(boost::asio::ssl::context::sslv23);
        ssl_context.set_default_verify_paths();
        ssl_context.set_options(boost::asio::ssl::context::default_workarounds);
        if (ssl_context_callback)
        {
            ssl_context_callback(ssl_context);
        }
        m_ssl_stream = utility::details::make_unique<boost::asio::ssl::stream<boost::asio::ip::tcp::socket&>>(
            m_socket, ssl_context);
        m_cn_hostname = std::move(cn_hostname);
    }

    void close()
    {
        std::lock_guard<std::mutex> lock(m_socket_lock);

        // Ensures closed connections owned by request_context will not be put to pool when they are released.
        m_keep_alive = false;
        m_closed = true;

        boost::system::error_code error;
        m_socket.shutdown(tcp::socket::shutdown_both, error);
        m_socket.close(error);
    }

    boost::system::error_code cancel()
    {
        std::lock_guard<std::mutex> lock(m_socket_lock);
        boost::system::error_code error;
        m_socket.cancel(error);
        return error;
    }

    bool is_reused() const { return m_is_reused; }
    void set_keep_alive(bool keep_alive) { m_keep_alive = keep_alive; }
    bool keep_alive() const { return m_keep_alive; }
    bool is_ssl() const { return m_ssl_stream ? true : false; }
    const std::string& cn_hostname() const { return m_cn_hostname; }

    // Check if the error code indicates that the connection was closed by the
    // server: this is used to detect if a connection in the pool was closed during
    // its period of inactivity and we should reopen it.
    bool was_reused_and_closed_by_server(const boost::system::error_code& ec) const
    {
        if (!is_reused())
        {
            // Don't bother reopening the connection if it's a new one: in this
            // case, even if the connection was really lost, it's still a real
            // error and we shouldn't try to reopen it.
            return false;
        }

        // These errors tell if connection was closed.
        if ((boost::asio::error::eof == ec) || (boost::asio::error::connection_reset == ec) ||
            (boost::asio::error::connection_aborted == ec))
        {
            return true;
        }

        if (is_ssl())
        {
            // For SSL connections, we can also get a different error due to
            // incorrect secure connection shutdown if it was closed by the
            // server due to inactivity. Unfortunately, the exact error we get
            // in this case depends on the Boost.Asio version used.
#if BOOST_ASIO_VERSION >= 101008
            if (boost::asio::ssl::error::stream_truncated == ec) return true;
#else // Asio < 1.10.8 didn't have ssl::error::stream_truncated
            if (boost::system::error_code(ERR_PACK(ERR_LIB_SSL, 0, SSL_R_SHORT_READ),
                                          boost::asio::error::get_ssl_category()) == ec)
                return true;
#endif
        }

        return false;
    }

    template<typename Iterator, typename Handler>
    void async_connect(const Iterator& begin, const Handler& handler)
    {
        {
            std::lock_guard<std::mutex> lock(m_socket_lock);
            if (!m_closed)
            {
                m_socket.async_connect(begin, handler);
                return;
            }
        } // unlock

        handler(boost::asio::error::operation_aborted);
    }

    template<typename HandshakeHandler, typename CertificateHandler>
    void async_handshake(boost::asio::ssl::stream_base::handshake_type type,
                         const http_client_config& config,
                         const HandshakeHandler& handshake_handler,
                         const CertificateHandler& cert_handler)
    {
        std::lock_guard<std::mutex> lock(m_socket_lock);
        assert(is_ssl());

        // Check to turn on/off server certificate verification.
        if (config.validate_certificates())
        {
            m_ssl_stream->set_verify_mode(boost::asio::ssl::context::verify_peer);
            m_ssl_stream->set_verify_callback(cert_handler);
        }
        else
        {
            m_ssl_stream->set_verify_mode(boost::asio::ssl::context::verify_none);
        }

        // Check to set host name for Server Name Indication (SNI)
        if (config.is_tlsext_sni_enabled())
        {
            SSL_set_tlsext_host_name(m_ssl_stream->native_handle(), &m_cn_hostname[0]);
        }

        m_ssl_stream->async_handshake(type, handshake_handler);
    }

    template<typename ConstBufferSequence, typename Handler>
    void async_write(ConstBufferSequence& buffer, const Handler& writeHandler)
    {
        std::lock_guard<std::mutex> lock(m_socket_lock);
        if (m_ssl_stream)
        {
            boost::asio::async_write(*m_ssl_stream, buffer, writeHandler);
        }
        else
        {
            boost::asio::async_write(m_socket, buffer, writeHandler);
        }
    }

    template<typename MutableBufferSequence, typename CompletionCondition, typename Handler>
    void async_read(MutableBufferSequence& buffer, const CompletionCondition& condition, const Handler& readHandler)
    {
        std::lock_guard<std::mutex> lock(m_socket_lock);
        if (m_ssl_stream)
        {
            boost::asio::async_read(*m_ssl_stream, buffer, condition, readHandler);
        }
        else
        {
            boost::asio::async_read(m_socket, buffer, condition, readHandler);
        }
    }

    template<typename Handler>
    void async_read_until(boost::asio::streambuf& buffer, const std::string& delim, const Handler& readHandler)
    {
        std::lock_guard<std::mutex> lock(m_socket_lock);
        if (m_ssl_stream)
        {
            boost::asio::async_read_until(*m_ssl_stream, buffer, delim, readHandler);
        }
        else
        {
            boost::asio::async_read_until(m_socket, buffer, delim, readHandler);
        }
    }

    void start_reuse() { m_is_reused = true; }

    void enable_no_delay()
    {
        boost::asio::ip::tcp::no_delay option(true);
        boost::system::error_code error_ignored;
        m_socket.set_option(option, error_ignored);
    }

private:
    // Guards concurrent access to socket/ssl::stream. This is necessary
    // because timeouts and cancellation can touch the socket at the same time
    // as normal message processing.
    std::mutex m_socket_lock;
    tcp::socket m_socket;
    std::unique_ptr<boost::asio::ssl::stream<tcp::socket&>> m_ssl_stream;
    std::string m_cn_hostname;

    bool m_is_reused;
    bool m_keep_alive;
    bool m_closed;
};

/// <summary>Implements a connection pool with adaptive connection removal</summary>
/// <remarks>
/// Every 30 seconds, the lambda in `start_epoch_interval` fires, triggering the
/// cleanup of any connections that have resided in the pool since the last
/// cleanup phase.
///
/// During the cleanup phase, connections are removed starting with the oldest. This
/// ensures that if a high intensity workload is followed by a low intensity workload,
/// the connection pool will correctly adapt to the low intensity workload.
///
/// Specifically, the following code will eventually result in a maximum of one pooled
/// connection regardless of the initial number of pooled connections:
/// <code>
///   while(1)
///   {
///     auto conn = pool.try_acquire();
///     if (!conn) conn = new_conn();
///     pool.release(std::move(conn));
///   }
/// </code>
/// </remarks>
class asio_connection_pool final : public std::enable_shared_from_this<asio_connection_pool>
{
public:
    asio_connection_pool()
        : m_lock()
        , m_connections()
        , m_is_timer_running(false)
        , m_pool_epoch_timer(crossplat::threadpool::shared_instance().service())
    {
    }

    asio_connection_pool(const asio_connection_pool&) = delete;
    asio_connection_pool& operator=(const asio_connection_pool&) = delete;

    std::shared_ptr<asio_connection> try_acquire(const std::string& cn_hostname)
    {
        std::lock_guard<std::mutex> lock(m_lock);
        if (m_connections.empty())
        {
            return nullptr;
        }

        auto conn = m_connections[cn_hostname].try_acquire();
        if (conn)
        {
            conn->start_reuse();
        }

        return conn;
    }

    void release(std::shared_ptr<asio_connection>&& connection)
    {
        connection->cancel();
        if (!connection->keep_alive())
        {
            connection.reset();
            return;
        }

        std::lock_guard<std::mutex> lock(m_lock);
        if (!m_is_timer_running)
        {
            start_epoch_interval(shared_from_this());
            m_is_timer_running = true;
        }

        m_connections[connection->cn_hostname()].release(std::move(connection));
    }

private:
    // Note: must be called under m_lock
    static void start_epoch_interval(const std::shared_ptr<asio_connection_pool>& pool)
    {
        auto& self = *pool;
        std::weak_ptr<asio_connection_pool> weak_pool = pool;

        self.m_pool_epoch_timer.expires_from_now(boost::posix_time::seconds(30));
        self.m_pool_epoch_timer.async_wait([weak_pool](const boost::system::error_code& ec) {
            if (ec)
            {
                return;
            }

            auto pool = weak_pool.lock();
            if (!pool)
            {
                return;
            }

            auto& self = *pool;
            std::lock_guard<std::mutex> lock(self.m_lock);
            bool restartTimer = false;
            for (auto& entry : self.m_connections)
            {
                if (entry.second.free_stale_connections())
                {
                    restartTimer = true;
                }
            }

            if (restartTimer)
            {
                start_epoch_interval(pool);
            }
            else
            {
                self.m_is_timer_running = false;
            }
        });
    }

    std::mutex m_lock;
    std::map<std::string, connection_pool_stack<asio_connection>> m_connections;
    bool m_is_timer_running;
    boost::asio::deadline_timer m_pool_epoch_timer;
};

class asio_client final : public _http_client_communicator
{
public:
    asio_client(http::uri&& address, http_client_config&& client_config)
        : _http_client_communicator(std::move(address), std::move(client_config))
        , m_pool(std::make_shared<asio_connection_pool>())
    {
    }

    virtual void send_request(const std::shared_ptr<request_context>& request_ctx) override;

    void release_connection(std::shared_ptr<asio_connection>&& conn) { m_pool->release(std::move(conn)); }

    std::shared_ptr<asio_connection> obtain_connection(const http_request& req)
    {
        std::string cn_host = calc_cn_host(base_uri(), req.headers());
        std::shared_ptr<asio_connection> conn = m_pool->try_acquire(cn_host);
        if (conn == nullptr)
        {
            // Pool was empty. Create a new connection
            conn = std::make_shared<asio_connection>(crossplat::threadpool::shared_instance().service());
            if (base_uri().scheme() == U("https") && !this->client_config().proxy().is_specified())
            {
                conn->upgrade_to_ssl(std::move(cn_host), this->client_config().get_ssl_context_callback());
            }
        }

        return conn;
    }

    virtual pplx::task<http_response> propagate(http_request request) override;

private:
    const std::shared_ptr<asio_connection_pool> m_pool;
};

class asio_context final : public request_context, public std::enable_shared_from_this<asio_context>
{
    friend class asio_client;

public:
    asio_context(const std::shared_ptr<_http_client_communicator>& client,
                 http_request& request,
                 const std::shared_ptr<asio_connection>& connection)
        : request_context(client, request)
        , m_content_length(0)
        , m_needChunked(false)
        , m_timer(client->client_config().timeout<std::chrono::microseconds>())
        , m_resolver(crossplat::threadpool::shared_instance().service())
        , m_connection(connection)
#ifdef CPPREST_PLATFORM_ASIO_CERT_VERIFICATION_AVAILABLE
        , m_openssl_failed(false)
#endif // CPPREST_PLATFORM_ASIO_CERT_VERIFICATION_AVAILABLE
    {
    }

    virtual ~asio_context()
    {
        m_timer.stop();
        // Release connection back to the pool. If connection was not closed, it will be put to the pool for reuse.
        std::static_pointer_cast<asio_client>(m_http_client)->release_connection(std::move(m_connection));
    }

    static std::shared_ptr<request_context> create_request_context(std::shared_ptr<_http_client_communicator>& client,
                                                                   http_request& request)
    {
        auto client_cast(std::static_pointer_cast<asio_client>(client));
        auto connection(client_cast->obtain_connection(request));
        auto ctx = std::make_shared<asio_context>(client, request, connection);
        ctx->m_timer.set_ctx(std::weak_ptr<asio_context>(ctx));
        return ctx;
    }

    class ssl_proxy_tunnel final : public std::enable_shared_from_this<ssl_proxy_tunnel>
    {
    public:
        ssl_proxy_tunnel(std::shared_ptr<asio_context> context,
                         std::function<void(std::shared_ptr<asio_context>)> ssl_tunnel_established)
            : m_ssl_tunnel_established(ssl_tunnel_established), m_context(context)
        {
        }

        void start_proxy_connect()
        {
            auto proxy = m_context->m_http_client->client_config().proxy();
            auto proxy_uri = proxy.address();

            utility::string_t proxy_host = proxy_uri.host();
            int proxy_port = proxy_uri.port() == -1 ? 8080 : proxy_uri.port();

            const auto& base_uri = m_context->m_http_client->base_uri();
            const auto& host = utility::conversions::to_utf8string(base_uri.host());
            const int portRaw = base_uri.port();
            const int port = (portRaw != 0) ? portRaw : 443;

            std::ostream request_stream(&m_request);
            request_stream.imbue(std::locale::classic());

            request_stream << "CONNECT " << host << ":" << port << " HTTP/1.1\r\n";
            request_stream << "Host: " << host << ":" << port << CRLF;
            request_stream << "Proxy-Connection: Keep-Alive\r\n";

            if (m_context->m_http_client->client_config().proxy().credentials().is_set())
            {
                request_stream << m_context->generate_basic_proxy_auth_header();
            }

            request_stream << CRLF;

            m_context->m_timer.start();

            tcp::resolver::query query(utility::conversions::to_utf8string(proxy_host), to_string(proxy_port));

            auto client = std::static_pointer_cast<asio_client>(m_context->m_http_client);
            m_context->m_resolver.async_resolve(query,
                                                boost::bind(&ssl_proxy_tunnel::handle_resolve,
                                                            shared_from_this(),
                                                            boost::asio::placeholders::error,
                                                            boost::asio::placeholders::iterator));
        }

    private:
        void handle_resolve(const boost::system::error_code& ec, tcp::resolver::iterator endpoints)
        {
            if (ec)
            {
                m_context->report_error("Error resolving proxy address", ec, httpclient_errorcode_context::connect);
            }
            else
            {
                m_context->m_timer.reset();
                auto endpoint = *endpoints;
                m_context->m_connection->async_connect(endpoint,
                                                       boost::bind(&ssl_proxy_tunnel::handle_tcp_connect,
                                                                   shared_from_this(),
                                                                   boost::asio::placeholders::error,
                                                                   ++endpoints));
            }
        }

        void handle_tcp_connect(const boost::system::error_code& ec, tcp::resolver::iterator endpoints)
        {
            if (!ec)
            {
                m_context->m_timer.reset();
                m_context->m_connection->enable_no_delay();
                m_context->m_connection->async_write(m_request,
                                                     boost::bind(&ssl_proxy_tunnel::handle_write_request,
                                                                 shared_from_this(),
                                                                 boost::asio::placeholders::error));
            }
            else if (endpoints == tcp::resolver::iterator())
            {
                m_context->report_error(
                    "Failed to connect to any resolved proxy endpoint", ec, httpclient_errorcode_context::connect);
            }
            else
            {
                m_context->m_timer.reset();
                //// Replace the connection. This causes old connection object to go out of scope.
                auto client = std::static_pointer_cast<asio_client>(m_context->m_http_client);
                try
                {
                    m_context->m_connection = client->obtain_connection(m_context->m_request);
                }
                catch (...)
                {
                    m_context->report_exception(std::current_exception());
                    return;
                }

                auto endpoint = *endpoints;
                m_context->m_connection->async_connect(endpoint,
                                                       boost::bind(&ssl_proxy_tunnel::handle_tcp_connect,
                                                                   shared_from_this(),
                                                                   boost::asio::placeholders::error,
                                                                   ++endpoints));
            }
        }

        void handle_write_request(const boost::system::error_code& err)
        {
            if (!err)
            {
                m_context->m_timer.reset();
                m_context->m_connection->async_read_until(m_response,
                                                          CRLF + CRLF,
                                                          boost::bind(&ssl_proxy_tunnel::handle_status_line,
                                                                      shared_from_this(),
                                                                      boost::asio::placeholders::error));
            }
            else
            {
                m_context->report_error(
                    "Failed to send connect request to proxy.", err, httpclient_errorcode_context::writebody);
            }
        }

        void handle_status_line(const boost::system::error_code& ec)
        {
            if (!ec)
            {
                m_context->m_timer.reset();
                std::istream response_stream(&m_response);
                response_stream.imbue(std::locale::classic());
                std::string http_version;
                response_stream >> http_version;
                status_code status_code;
                response_stream >> status_code;

                if (!response_stream || http_version.substr(0, 5) != "HTTP/")
                {
                    m_context->report_error("Invalid HTTP status line during proxy connection",
                                            ec,
                                            httpclient_errorcode_context::readheader);
                    return;
                }

                if (status_code != 200)
                {
                    m_context->report_error("Expected a 200 response from proxy, received: " + to_string(status_code),
                                            ec,
                                            httpclient_errorcode_context::readheader);
                    return;
                }

                try
                {
                    m_context->upgrade_to_ssl();
                }
                catch (...)
                {
                    m_context->report_exception(std::current_exception());
                    return;
                }

                m_ssl_tunnel_established(m_context);
            }
            else
            {
                m_context->handle_failed_read_status_line(ec, "Failed to read HTTP status line from proxy");
            }
        }

        std::function<void(std::shared_ptr<asio_context>)> m_ssl_tunnel_established;
        std::shared_ptr<asio_context> m_context;

        boost::asio::streambuf m_request;
        boost::asio::streambuf m_response;
    };

    enum class http_proxy_type
    {
        none,
        http,
        ssl_tunnel
    };

    void start_request()
    {
        if (m_request._cancellation_token().is_canceled())
        {
            request_context::report_error(make_error_code(std::errc::operation_canceled).value(),
                                          "Request canceled by user.");
            return;
        }

        http_proxy_type proxy_type = http_proxy_type::none;
        std::string proxy_host;
        int proxy_port = -1;

        // There is no support for auto-detection of proxies on non-windows platforms, it must be specified explicitly
        // from the client code.
        if (m_http_client->client_config().proxy().is_specified())
        {
            proxy_type =
                m_http_client->base_uri().scheme() == U("https") ? http_proxy_type::ssl_tunnel : http_proxy_type::http;
            auto proxy = m_http_client->client_config().proxy();
            auto proxy_uri = proxy.address();
            proxy_port = proxy_uri.port() == -1 ? 8080 : proxy_uri.port();
            proxy_host = utility::conversions::to_utf8string(proxy_uri.host());
        }

        auto start_http_request_flow = [proxy_type, proxy_host, proxy_port AND_CAPTURE_MEMBER_FUNCTION_POINTERS](
                                           std::shared_ptr<asio_context> ctx) {
            if (ctx->m_request._cancellation_token().is_canceled())
            {
                ctx->request_context::report_error(make_error_code(std::errc::operation_canceled).value(),
                                                   "Request canceled by user.");
                return;
            }

            const auto& base_uri = ctx->m_http_client->base_uri();
            const auto full_uri = uri_builder(base_uri).append(ctx->m_request.relative_uri()).to_uri();

            // For a normal http proxy, we need to specify the full request uri, otherwise just specify the resource
            auto encoded_resource =
                proxy_type == http_proxy_type::http ? full_uri.to_string() : full_uri.resource().to_string();

            if (encoded_resource.empty())
            {
                encoded_resource = U("/");
            }

            const auto& method = ctx->m_request.method();

            // stop injection of headers via method
            // resource should be ok, since it's been encoded
            // and host won't resolve
            if (!::web::http::details::validate_method(method))
            {
                ctx->report_exception(http_exception("The method string is invalid."));
                return;
            }

            std::ostream request_stream(&ctx->m_body_buf);
            request_stream.imbue(std::locale::classic());
            const auto& host = utility::conversions::to_utf8string(base_uri.host());

            request_stream << utility::conversions::to_utf8string(method) << " "
                           << utility::conversions::to_utf8string(encoded_resource) << " "
                           << "HTTP/1.1\r\n";

            int port = base_uri.port();

            if (base_uri.is_port_default())
            {
                port = (ctx->m_connection->is_ssl() ? 443 : 80);
            }

            // Add the Host header if user has not specified it explicitly
            if (!ctx->m_request.headers().has(header_names::host))
            {
                request_stream << "Host: " << host;
                if (!base_uri.is_port_default())
                {
                    request_stream << ":" << port;
                }
                request_stream << CRLF;
            }

            // Extra request headers are constructed here.
            std::string extra_headers;

            // Add header for basic proxy authentication
            if (proxy_type == http_proxy_type::http &&
                ctx->m_http_client->client_config().proxy().credentials().is_set())
            {
                extra_headers.append(ctx->generate_basic_proxy_auth_header());
            }

            if (ctx->m_http_client->client_config().credentials().is_set())
            {
                extra_headers.append(ctx->generate_basic_auth_header());
            }

            extra_headers += utility::conversions::to_utf8string(ctx->get_compression_header());

            // Check user specified transfer-encoding.
            std::string transferencoding;
            if (ctx->m_request.headers().match(header_names::transfer_encoding, transferencoding) &&
                boost::icontains(transferencoding, U("chunked")))
            {
                ctx->m_needChunked = true;
            }
            else if (!ctx->m_request.headers().match(header_names::content_length, ctx->m_content_length))
            {
                // Stream without content length is the signal of requiring transfer encoding chunked.
                if (ctx->m_request.body())
                {
                    ctx->m_needChunked = true;
                    extra_headers.append("Transfer-Encoding:chunked\r\n");
                }
                else if (ctx->m_request.method() == methods::POST || ctx->m_request.method() == methods::PUT)
                {
                    // Some servers do not accept POST/PUT requests with a content length of 0, such as
                    // lighttpd - http://serverfault.com/questions/315849/curl-post-411-length-required
                    // old apache versions - https://issues.apache.org/jira/browse/TS-2902
                    extra_headers.append("Content-Length: 0\r\n");
                }
            }

            if (proxy_type == http_proxy_type::http)
            {
                extra_headers.append("Cache-Control: no-store, no-cache\r\n"
                                     "Pragma: no-cache\r\n");
            }

            request_stream << utility::conversions::to_utf8string(
                ::web::http::details::flatten_http_headers(ctx->m_request.headers()));
            request_stream << extra_headers;
            // Enforce HTTP connection keep alive (even for the old HTTP/1.0 protocol).
            request_stream << "Connection: Keep-Alive\r\n\r\n";

            // Start connection timeout timer.
            if (!ctx->m_timer.has_started())
            {
                ctx->m_timer.start();
            }

            if (ctx->m_connection->is_reused() || proxy_type == http_proxy_type::ssl_tunnel)
            {
                // If socket is a reused connection or we're connected via an ssl-tunneling proxy, try to write the
                // request directly. In both cases we have already established a tcp connection.
                ctx->write_request();
            }
            else
            {
                // If the connection is new (unresolved and unconnected socket), then start async
                // call to resolve first, leading eventually to request write.

                // For normal http proxies, we want to connect directly to the proxy server. It will relay our request.
                auto tcp_host = proxy_type == http_proxy_type::http ? proxy_host : host;
                auto tcp_port = proxy_type == http_proxy_type::http ? proxy_port : port;

                tcp::resolver::query query(tcp_host, to_string(tcp_port));
                ctx->m_resolver.async_resolve(query,
                                              boost::bind(&asio_context::handle_resolve,
                                                          ctx,
                                                          boost::asio::placeholders::error,
                                                          boost::asio::placeholders::iterator));
            }

            // Register for notification on cancellation to abort this request.
            if (ctx->m_request._cancellation_token() != pplx::cancellation_token::none())
            {
                // weak_ptr prevents lambda from taking shared ownership of the context.
                // Otherwise context replacement in the handle_status_line() would leak the objects.
                std::weak_ptr<asio_context> ctx_weak(ctx);
                ctx->m_cancellationRegistration = ctx->m_request._cancellation_token().register_callback([ctx_weak]() {
                    if (auto ctx_lock = ctx_weak.lock())
                    {
                        // Shut down transmissions, close the socket and prevent connection from being pooled.
                        ctx_lock->m_connection->close();
                    }
                });
            }
        };

        // Note that we must not try to CONNECT using an already established connection via proxy -- this would send
        // CONNECT to the end server which is definitely not what we want.
        if (proxy_type == http_proxy_type::ssl_tunnel && !m_connection->is_reused())
        {
            // The ssl_tunnel_proxy keeps the context alive and then calls back once the ssl tunnel is established via
            // 'start_http_request_flow'
            std::shared_ptr<ssl_proxy_tunnel> ssl_tunnel =
                std::make_shared<ssl_proxy_tunnel>(shared_from_this(), start_http_request_flow);
            ssl_tunnel->start_proxy_connect();
        }
        else
        {
            start_http_request_flow(shared_from_this());
        }
    }

    template<typename _ExceptionType>
    void report_exception(const _ExceptionType& e)
    {
        report_exception(std::make_exception_ptr(e));
    }

    void report_exception(std::exception_ptr exceptionPtr) override
    {
        // Don't recycle connections that had an error into the connection pool.
        m_connection->close();
        request_context::report_exception(exceptionPtr);
    }

private:
    void upgrade_to_ssl()
    {
        auto& client = static_cast<asio_client&>(*m_http_client);
        m_connection->upgrade_to_ssl(calc_cn_host(client.base_uri(), m_request.headers()),
                                     client.client_config().get_ssl_context_callback());
    }

    std::string generate_basic_auth_header()
    {
        std::string header;
        header.append("Authorization: Basic ");
        header.append(generate_base64_userpass(m_http_client->client_config().credentials()));
        header.append(CRLF);
        return header;
    }

    std::string generate_basic_proxy_auth_header()
    {
        std::string header;
        header.append("Proxy-Authorization: Basic ");
        header.append(generate_base64_userpass(m_http_client->client_config().proxy().credentials()));
        header.append(CRLF);
        return header;
    }

    void report_error(const std::string& message,
                      const boost::system::error_code& ec,
                      httpclient_errorcode_context context = httpclient_errorcode_context::none)
    {
        // By default, errorcodeValue don't need to converted
        long errorcodeValue = ec.value();

        // map timer cancellation to time_out
        if (m_timer.has_timedout())
        {
            errorcodeValue = make_error_code(std::errc::timed_out).value();
        }
        else
        {
            // We need to correct inaccurate ASIO error code base on context information
            switch (context)
            {
                case httpclient_errorcode_context::writeheader:
                    if (ec == boost::system::errc::broken_pipe)
                    {
                        errorcodeValue = make_error_code(std::errc::host_unreachable).value();
                    }
                    break;
                case httpclient_errorcode_context::connect:
                    if (ec == boost::system::errc::connection_refused)
                    {
                        errorcodeValue = make_error_code(std::errc::host_unreachable).value();
                    }
                    break;
                case httpclient_errorcode_context::readheader:
                    if (ec.default_error_condition().value() ==
                        boost::system::errc::no_such_file_or_directory) // bug in boost error_code mapping
                    {
                        errorcodeValue = make_error_code(std::errc::connection_aborted).value();
                    }
                    break;
                default: break;
            }
        }
        request_context::report_error(errorcodeValue, message);
    }

    void handle_connect(const boost::system::error_code& ec, tcp::resolver::iterator endpoints)
    {
        m_timer.reset();
        if (!ec)
        {
            m_connection->enable_no_delay();
            write_request();
        }
        else if (ec.value() == boost::system::errc::operation_canceled ||
                 ec.value() == boost::asio::error::operation_aborted)
        {
            report_error("Request canceled by user.", ec, httpclient_errorcode_context::connect);
        }
        else if (endpoints == tcp::resolver::iterator())
        {
            report_error("Failed to connect to any resolved endpoint", ec, httpclient_errorcode_context::connect);
        }
        else
        {
            // Replace the connection. This causes old connection object to go out of scope.
            auto client = std::static_pointer_cast<asio_client>(m_http_client);
            try
            {
                m_connection = client->obtain_connection(m_request);
            }
            catch (...)
            {
                request_context::report_exception(std::current_exception());
                return;
            }

            auto endpoint = *endpoints;
            m_connection->async_connect(
                endpoint,
                boost::bind(
                    &asio_context::handle_connect, shared_from_this(), boost::asio::placeholders::error, ++endpoints));
        }
    }

    void handle_resolve(const boost::system::error_code& ec, tcp::resolver::iterator endpoints)
    {
        if (ec)
        {
            report_error("Error resolving address", ec, httpclient_errorcode_context::connect);
        }
        else if (endpoints == tcp::resolver::iterator())
        {
            report_error("Failed to resolve address", ec, httpclient_errorcode_context::connect);
        }
        else
        {
            m_timer.reset();
            auto endpoint = *endpoints;
            m_connection->async_connect(
                endpoint,
                boost::bind(
                    &asio_context::handle_connect, shared_from_this(), boost::asio::placeholders::error, ++endpoints));
        }
    }

    void write_request()
    {
        // Only perform handshake if a TLS connection and not being reused.
        if (m_connection->is_ssl() && !m_connection->is_reused())
        {
            const auto weakCtx = std::weak_ptr<asio_context>(shared_from_this());
            m_connection->async_handshake(
                boost::asio::ssl::stream_base::client,
                m_http_client->client_config(),
                boost::bind(&asio_context::handle_handshake, shared_from_this(), boost::asio::placeholders::error),

                // Use a weak_ptr since the verify_callback is stored until the connection is
                // destroyed. This avoids creating a circular reference since we pool connection
                // objects.
                [weakCtx](bool preverified, boost::asio::ssl::verify_context& verify_context) {
                    auto this_request = weakCtx.lock();
                    if (this_request)
                    {
                        return this_request->handle_cert_verification(preverified, verify_context);
                    }
                    return false;
                });
        }
        else
        {
            m_connection->async_write(
                m_body_buf,
                boost::bind(&asio_context::handle_write_headers, shared_from_this(), boost::asio::placeholders::error));
        }
    }

    void handle_handshake(const boost::system::error_code& ec)
    {
        if (!ec)
        {
            m_connection->async_write(
                m_body_buf,
                boost::bind(&asio_context::handle_write_headers, shared_from_this(), boost::asio::placeholders::error));
        }
        else
        {
            report_error("Error in SSL handshake", ec, httpclient_errorcode_context::handshake);
        }
    }

    bool handle_cert_verification(bool preverified, boost::asio::ssl::verify_context& verifyCtx)
    {
        // OpenSSL calls the verification callback once per certificate in the chain,
        // starting with the root CA certificate. The 'leaf', non-Certificate Authority (CA)
        // certificate, i.e. actual server certificate is at the '0' position in the
        // certificate chain, the rest are optional intermediate certificates, followed
        // finally by the root CA self signed certificate.

#ifdef CPPREST_PLATFORM_ASIO_CERT_VERIFICATION_AVAILABLE
        // If OpenSSL fails we will doing verification at the end using the whole certificate
        // chain so wait until the 'leaf' cert. For now return true so OpenSSL continues down
        // the certificate chain.
        if (!preverified)
        {
            m_openssl_failed = true;
        }

        if (m_openssl_failed)
        {
            return verify_cert_chain_platform_specific(verifyCtx, m_connection->cn_hostname());
        }
#endif // CPPREST_PLATFORM_ASIO_CERT_VERIFICATION_AVAILABLE

        boost::asio::ssl::rfc2818_verification rfc2818(m_connection->cn_hostname());
        return rfc2818(preverified, verifyCtx);
    }

    void handle_write_headers(const boost::system::error_code& ec)
    {
        if (ec)
        {
            report_error("Failed to write request headers", ec, httpclient_errorcode_context::writeheader);
        }
        else
        {
            if (m_needChunked)
            {
                handle_write_chunked_body(ec);
            }
            else
            {
                handle_write_large_body(ec);
            }
        }
    }

    void handle_write_chunked_body(const boost::system::error_code& ec)
    {
        if (ec)
        {
            // Reuse error handling.
            return handle_write_body(ec);
        }

        m_timer.reset();
        const auto& progress = m_request._get_impl()->_progress_handler();
        if (progress)
        {
            try
            {
                (*progress)(message_direction::upload, m_uploaded);
            }
            catch (...)
            {
                report_exception(std::current_exception());
                return;
            }
        }

        const auto& chunkSize = m_http_client->client_config().chunksize();
        auto readbuf = _get_readbuffer();
        uint8_t* buf = boost::asio::buffer_cast<uint8_t*>(
            m_body_buf.prepare(chunkSize + http::details::chunked_encoding::additional_encoding_space));
        const auto this_request = shared_from_this();
        readbuf.getn(buf + http::details::chunked_encoding::data_offset, chunkSize)
            .then([this_request, buf, chunkSize AND_CAPTURE_MEMBER_FUNCTION_POINTERS](pplx::task<size_t> op) {
                size_t readSize = 0;
                try
                {
                    readSize = op.get();
                }
                catch (...)
                {
                    this_request->report_exception(std::current_exception());
                    return;
                }

                const size_t offset = http::details::chunked_encoding::add_chunked_delimiters(
                    buf, chunkSize + http::details::chunked_encoding::additional_encoding_space, readSize);
                this_request->m_body_buf.commit(readSize + http::details::chunked_encoding::additional_encoding_space);
                this_request->m_body_buf.consume(offset);
                this_request->m_uploaded += static_cast<uint64_t>(readSize);

                if (readSize != 0)
                {
                    this_request->m_connection->async_write(this_request->m_body_buf,
                                                            boost::bind(&asio_context::handle_write_chunked_body,
                                                                        this_request,
                                                                        boost::asio::placeholders::error));
                }
                else
                {
                    this_request->m_connection->async_write(
                        this_request->m_body_buf,
                        boost::bind(&asio_context::handle_write_body, this_request, boost::asio::placeholders::error));
                }
            });
    }

    void handle_write_large_body(const boost::system::error_code& ec)
    {
        if (ec || m_uploaded >= m_content_length)
        {
            // Reuse error handling.
            return handle_write_body(ec);
        }

        m_timer.reset();
        const auto& progress = m_request._get_impl()->_progress_handler();
        if (progress)
        {
            try
            {
                (*progress)(message_direction::upload, m_uploaded);
            }
            catch (...)
            {
                report_exception(std::current_exception());
                return;
            }
        }

        const auto this_request = shared_from_this();
        const auto readSize = static_cast<size_t>((std::min)(
            static_cast<uint64_t>(m_http_client->client_config().chunksize()), m_content_length - m_uploaded));
        auto readbuf = _get_readbuffer();
        readbuf.getn(boost::asio::buffer_cast<uint8_t*>(m_body_buf.prepare(readSize)), readSize)
            .then([this_request AND_CAPTURE_MEMBER_FUNCTION_POINTERS](pplx::task<size_t> op) {
                try
                {
                    const auto actualReadSize = op.get();
                    if (actualReadSize == 0)
                    {
                        this_request->report_exception(http_exception(
                            "Unexpected end of request body stream encountered before Content-Length satisfied."));
                        return;
                    }
                    this_request->m_uploaded += static_cast<uint64_t>(actualReadSize);
                    this_request->m_body_buf.commit(actualReadSize);
                    this_request->m_connection->async_write(this_request->m_body_buf,
                                                            boost::bind(&asio_context::handle_write_large_body,
                                                                        this_request,
                                                                        boost::asio::placeholders::error));
                }
                catch (...)
                {
                    this_request->report_exception(std::current_exception());
                    return;
                }
            });
    }

    void handle_write_body(const boost::system::error_code& ec)
    {
        if (!ec)
        {
            m_timer.reset();
            const auto& progress = m_request._get_impl()->_progress_handler();
            if (progress)
            {
                try
                {
                    (*progress)(message_direction::upload, m_uploaded);
                }
                catch (...)
                {
                    report_exception(std::current_exception());
                    return;
                }
            }

            // Read until the end of entire headers
            m_connection->async_read_until(
                m_body_buf,
                CRLF + CRLF,
                boost::bind(&asio_context::handle_status_line, shared_from_this(), boost::asio::placeholders::error));
        }
        else
        {
            report_error("Failed to write request body", ec, httpclient_errorcode_context::writebody);
        }
    }

    void handle_status_line(const boost::system::error_code& ec)
    {
        if (!ec)
        {
            m_timer.reset();

            std::istream response_stream(&m_body_buf);
            response_stream.imbue(std::locale::classic());
            std::string http_version;
            response_stream >> http_version;
            status_code status_code;
            response_stream >> status_code;

            std::string status_message;
            std::getline(response_stream, status_message);

            m_response.set_status_code(status_code);

            ::web::http::details::trim_whitespace(status_message);
            m_response.set_reason_phrase(utility::conversions::to_string_t(std::move(status_message)));

            if (!response_stream || http_version.substr(0, 5) != "HTTP/")
            {
                report_error("Invalid HTTP status line", ec, httpclient_errorcode_context::readheader);
                return;
            }

            web::http::http_version parsed_version = web::http::http_version::from_string(http_version);
            m_response._get_impl()->_set_http_version(parsed_version);

            // if HTTP version is 1.0 then disable 'Keep-Alive' by default
            if (parsed_version == web::http::http_versions::HTTP_1_0)
            {
                m_connection->set_keep_alive(false);
            }

            read_headers();
        }
        else
        {
            handle_failed_read_status_line(ec, "Failed to read HTTP status line");
        }
    }

    void handle_failed_read_status_line(const boost::system::error_code& ec, const char* generic_error_message)
    {
        if (m_connection->was_reused_and_closed_by_server(ec))
        {
            // Failed to write to socket because connection was already closed while it was in the pool.
            // close() here ensures socket is closed in a robust way and prevents the connection from being put to the
            // pool again.
            m_connection->close();

            // Create a new context and copy the request object, completion event and
            // cancellation registration to maintain the old state.
            // This also obtains a new connection from pool.
            std::shared_ptr<request_context> new_ctx;
            try
            {
                new_ctx = create_request_context(m_http_client, m_request);
            }
            catch (...)
            {
                report_exception(std::current_exception());
                return;
            }

            // If the request contains a valid instream, we try to rewind it to
            // replay the just-failed request. Otherwise we assume that no data
            // was sent in the first place.
            const auto& instream = new_ctx->m_request._get_impl()->instream();
            if (instream)
            {
                // As stated in the commit message of f4f2348, we might encounter
                // streams that are not capable of rewinding and hence resending the
                // request is not possible. We cannot recover from this condition and
                // need to escalate it to the using code.
                if (!instream.can_seek())
                {
                    report_error("cannot rewind input stream for connection re-establishment",
                                 ec,
                                 httpclient_errorcode_context::readheader);
                    return;
                }

                try
                {
                    // Rewinding the stream might throw, in which case we cannot do the
                    // connection re-establishment transparently. I.e. report the exception
                    // to the calling code.
                    instream.seek(0);
                }
                catch (...)
                {
                    report_exception(std::current_exception());
                    return;
                }
            }

            new_ctx->m_request_completion = m_request_completion;
            new_ctx->m_cancellationRegistration = m_cancellationRegistration;

            auto client = std::static_pointer_cast<asio_client>(m_http_client);
            // Resend the request using the new context.
            client->send_request(new_ctx);
        }
        else
        {
            report_error(generic_error_message, ec, httpclient_errorcode_context::readheader);
        }
    }

    void read_headers()
    {
        auto needChunked = false;
        std::istream response_stream(&m_body_buf);
        response_stream.imbue(std::locale::classic());
        std::string header;
        while (std::getline(response_stream, header) && header != "\r")
        {
            const auto colon = header.find(':');
            if (colon != std::string::npos)
            {
                auto name = header.substr(0, colon);
                auto value = header.substr(colon + 1, header.size() - colon - 2);
                boost::algorithm::trim(name);
                boost::algorithm::trim(value);

                if (boost::iequals(name, header_names::transfer_encoding))
                {
                    needChunked = boost::icontains(value, U("chunked"));
                }

                if (boost::iequals(name, header_names::connection))
                {
                    // If the server uses HTTP/1.1, then 'Keep-Alive' is the default,
                    // so connection is explicitly closed only if we get "Connection: close".
                    // If the server uses HTTP/1.0, it would need to respond using
                    // 'Connection: Keep-Alive' every time.
                    if (m_response._get_impl()->http_version() != web::http::http_versions::HTTP_1_0)
                        m_connection->set_keep_alive(!boost::iequals(value, U("close")));
                    else
                        m_connection->set_keep_alive(boost::iequals(value, U("Keep-Alive")));
                }

                m_response.headers().add(utility::conversions::to_string_t(std::move(name)),
                                         utility::conversions::to_string_t(std::move(value)));
            }
        }

        m_content_length = (std::numeric_limits<size_t>::max)(); // Without Content-Length header, size should be same
                                                                 // as TCP stream - set it size_t max.
        m_response.headers().match(header_names::content_length, m_content_length);

        if (!this->handle_compression())
        {
            // false indicates report_exception was called
            return;
        }

        complete_headers();

        // Check for HEAD requests and status codes which cannot contain a
        // message body in HTTP/1.1 (see 3.3.3/1 of the RFC 7230).
        //
        // note: need to check for 'chunked' here as well, azure storage sends both
        // transfer-encoding:chunked and content-length:0 (although HTTP says not to)
        const auto status = m_response.status_code();
        if (m_request.method() == U("HEAD") || (status >= 100 && status < 200) || status == status_codes::NoContent ||
            status == status_codes::NotModified || (!needChunked && m_content_length == 0))
        {
            // we can stop early - no body
            const auto& progress = m_request._get_impl()->_progress_handler();
            if (progress)
            {
                try
                {
                    (*progress)(message_direction::download, 0);
                }
                catch (...)
                {
                    report_exception(std::current_exception());
                    return;
                }
            }

            complete_request(0);
        }
        else
        {
            if (!needChunked)
            {
                async_read_until_buffersize(
                    static_cast<size_t>((std::min)(m_content_length,
                                                   static_cast<uint64_t>(m_http_client->client_config().chunksize()))),
                    boost::bind(
                        &asio_context::handle_read_content, shared_from_this(), boost::asio::placeholders::error));
            }
            else
            {
                m_connection->async_read_until(m_body_buf,
                                               CRLF,
                                               boost::bind(&asio_context::handle_chunk_header,
                                                           shared_from_this(),
                                                           boost::asio::placeholders::error));
            }
        }
    }

    template<typename ReadHandler>
    void async_read_until_buffersize(size_t size, const ReadHandler& handler)
    {
        size_t size_to_read = 0;
        if (m_body_buf.size() < size)
        {
            size_to_read = size - m_body_buf.size();
        }

        m_connection->async_read(m_body_buf, boost::asio::transfer_exactly(size_to_read), handler);
    }

    void handle_chunk_header(const boost::system::error_code& ec)
    {
        if (!ec)
        {
            m_timer.reset();

            std::istream response_stream(&m_body_buf);
            response_stream.imbue(std::locale::classic());
            std::string line;
            std::getline(response_stream, line);

            std::istringstream octetLine(std::move(line));
            octetLine.imbue(std::locale::classic());
            int octets = 0;
            octetLine >> std::hex >> octets;

            if (octetLine.fail())
            {
                report_error("Invalid chunked response header",
                             boost::system::error_code(),
                             httpclient_errorcode_context::readbody);
            }
            else
            {
                async_read_until_buffersize(
                    octets + CRLF.size(),
                    boost::bind(
                        &asio_context::handle_chunk, shared_from_this(), boost::asio::placeholders::error, octets));
            }
        }
        else
        {
            report_error("Retrieving message chunk header", ec, httpclient_errorcode_context::readbody);
        }
    }

    bool decompress(const uint8_t* input, size_t input_size, std::vector<uint8_t>& output)
    {
        // Need to guard against attempting to decompress when we're already finished or encountered an error!
        if (input == nullptr || input_size == 0)
        {
            return false;
        }

        size_t processed;
        size_t got;
        size_t inbytes = 0;
        size_t outbytes = 0;
        bool done;

        try
        {
            output.resize(input_size * 3);
            do
            {
                if (inbytes)
                {
                    output.resize(output.size() + (std::max)(input_size, static_cast<size_t>(1024)));
                }
                got = m_decompressor->decompress(input + inbytes,
                                                 input_size - inbytes,
                                                 output.data() + outbytes,
                                                 output.size() - outbytes,
                                                 web::http::compression::operation_hint::has_more,
                                                 processed,
                                                 done);
                inbytes += processed;
                outbytes += got;
            } while (got && !done);
            output.resize(outbytes);
        }
        catch (...)
        {
            return false;
        }

        return true;
    }

    void handle_chunk(const boost::system::error_code& ec, int to_read)
    {
        if (!ec)
        {
            m_timer.reset();

            m_downloaded += static_cast<uint64_t>(to_read);
            const auto& progress = m_request._get_impl()->_progress_handler();
            if (progress)
            {
                try
                {
                    (*progress)(message_direction::download, m_downloaded);
                }
                catch (...)
                {
                    report_exception(std::current_exception());
                    return;
                }
            }

            if (to_read == 0)
            {
                m_body_buf.consume(CRLF.size());
                complete_request(m_downloaded);
            }
            else
            {
                auto writeBuffer = _get_writebuffer();
                const auto this_request = shared_from_this();
                if (m_decompressor)
                {
                    std::vector<uint8_t> decompressed;

                    bool boo =
                        decompress(boost::asio::buffer_cast<const uint8_t*>(m_body_buf.data()), to_read, decompressed);
                    if (!boo)
                    {
                        report_exception(std::runtime_error("Failed to decompress the response body"));
                        return;
                    }

                    // It is valid for the decompressor to sometimes return an empty output for a given chunk, the data
                    // will be flushed when the next chunk is received
                    if (decompressed.empty())
                    {
                        m_body_buf.consume(to_read + CRLF.size()); // consume crlf
                        m_connection->async_read_until(m_body_buf,
                                                       CRLF,
                                                       boost::bind(&asio_context::handle_chunk_header,
                                                                   this_request,
                                                                   boost::asio::placeholders::error));
                    }
                    else
                    {
                        // Move the decompressed buffer into a shared_ptr to keep it alive until putn_nocopy completes.
                        // When VS 2013 support is dropped, this should be changed to a unique_ptr plus a move capture.
                        auto shared_decompressed = std::make_shared<std::vector<uint8_t>>(std::move(decompressed));

                        writeBuffer.putn_nocopy(shared_decompressed->data(), shared_decompressed->size())
                            .then([this_request, to_read, shared_decompressed AND_CAPTURE_MEMBER_FUNCTION_POINTERS](
                                      pplx::task<size_t> op) {
                                try
                                {
                                    op.get();
                                    this_request->m_body_buf.consume(to_read + CRLF.size()); // consume crlf
                                    this_request->m_connection->async_read_until(
                                        this_request->m_body_buf,
                                        CRLF,
                                        boost::bind(&asio_context::handle_chunk_header,
                                                    this_request,
                                                    boost::asio::placeholders::error));
                                }
                                catch (...)
                                {
                                    this_request->report_exception(std::current_exception());
                                    return;
                                }
                            });
                    }
                }
                else
                {
                    writeBuffer.putn_nocopy(boost::asio::buffer_cast<const uint8_t*>(m_body_buf.data()), to_read)
                        .then([this_request, to_read AND_CAPTURE_MEMBER_FUNCTION_POINTERS](pplx::task<size_t> op) {
                            try
                            {
                                op.wait();
                            }
                            catch (...)
                            {
                                this_request->report_exception(std::current_exception());
                                return;
                            }
                            this_request->m_body_buf.consume(to_read + CRLF.size()); // consume crlf
                            this_request->m_connection->async_read_until(this_request->m_body_buf,
                                                                         CRLF,
                                                                         boost::bind(&asio_context::handle_chunk_header,
                                                                                     this_request,
                                                                                     boost::asio::placeholders::error));
                        });
                }
            }
        }
        else
        {
            report_error("Failed to read chunked response part", ec, httpclient_errorcode_context::readbody);
        }
    }

    void handle_read_content(const boost::system::error_code& ec)
    {
        auto writeBuffer = _get_writebuffer();

        if (ec)
        {
            if (ec == boost::asio::error::eof && m_content_length == (std::numeric_limits<size_t>::max)())
            {
                m_content_length = m_downloaded + m_body_buf.size();
            }
            else
            {
                report_error("Failed to read response body", ec, httpclient_errorcode_context::readbody);
                return;
            }
        }

        m_timer.reset();
        const auto& progress = m_request._get_impl()->_progress_handler();
        if (progress)
        {
            try
            {
                (*progress)(message_direction::download, m_downloaded);
            }
            catch (...)
            {
                report_exception(std::current_exception());
                return;
            }
        }

        if (m_downloaded < m_content_length)
        {
            // more data need to be read
            const auto this_request = shared_from_this();

            auto read_size = static_cast<size_t>(
                (std::min)(static_cast<uint64_t>(m_body_buf.size()), m_content_length - m_downloaded));

            if (m_decompressor)
            {
                std::vector<uint8_t> decompressed;

                bool boo =
                    decompress(boost::asio::buffer_cast<const uint8_t*>(m_body_buf.data()), read_size, decompressed);
                if (!boo)
                {
                    this_request->report_exception(std::runtime_error("Failed to decompress the response body"));
                    return;
                }

                // It is valid for the decompressor to sometimes return an empty output for a given chunk, the data will
                // be flushed when the next chunk is received
                if (decompressed.empty())
                {
                    try
                    {
                        this_request->m_downloaded += static_cast<uint64_t>(read_size);

                        this_request->async_read_until_buffersize(
                            static_cast<size_t>((std::min)(
                                static_cast<uint64_t>(this_request->m_http_client->client_config().chunksize()),
                                this_request->m_content_length - this_request->m_downloaded)),
                            boost::bind(
                                &asio_context::handle_read_content, this_request, boost::asio::placeholders::error));
                    }
                    catch (...)
                    {
                        this_request->report_exception(std::current_exception());
                        return;
                    }
                }
                else
                {
                    // Move the decompressed buffer into a shared_ptr to keep it alive until putn_nocopy completes.
                    // When VS 2013 support is dropped, this should be changed to a unique_ptr plus a move capture.
                    auto shared_decompressed = std::make_shared<std::vector<uint8_t>>(std::move(decompressed));

                    writeBuffer.putn_nocopy(shared_decompressed->data(), shared_decompressed->size())
                        .then([this_request, read_size, shared_decompressed AND_CAPTURE_MEMBER_FUNCTION_POINTERS](
                                  pplx::task<size_t> op) {
                            size_t writtenSize = 0;
                            (void)writtenSize;
                            try
                            {
                                writtenSize = op.get();
                                this_request->m_downloaded += static_cast<uint64_t>(read_size);
                                this_request->m_body_buf.consume(read_size);
                                this_request->async_read_until_buffersize(
                                    static_cast<size_t>((std::min)(
                                        static_cast<uint64_t>(this_request->m_http_client->client_config().chunksize()),
                                        this_request->m_content_length - this_request->m_downloaded)),
                                    boost::bind(&asio_context::handle_read_content,
                                                this_request,
                                                boost::asio::placeholders::error));
                            }
                            catch (...)
                            {
                                this_request->report_exception(std::current_exception());
                                return;
                            }
                        });
                }
            }
            else
            {
                writeBuffer.putn_nocopy(boost::asio::buffer_cast<const uint8_t*>(m_body_buf.data()), read_size)
                    .then([this_request AND_CAPTURE_MEMBER_FUNCTION_POINTERS](pplx::task<size_t> op) {
                        size_t writtenSize = 0;
                        try
                        {
                            writtenSize = op.get();
                            this_request->m_downloaded += static_cast<uint64_t>(writtenSize);
                            this_request->m_body_buf.consume(writtenSize);
                            this_request->async_read_until_buffersize(
                                static_cast<size_t>((std::min)(
                                    static_cast<uint64_t>(this_request->m_http_client->client_config().chunksize()),
                                    this_request->m_content_length - this_request->m_downloaded)),
                                boost::bind(&asio_context::handle_read_content,
                                            this_request,
                                            boost::asio::placeholders::error));
                        }
                        catch (...)
                        {
                            this_request->report_exception(std::current_exception());
                            return;
                        }
                    });
            }
        }
        else
        {
            // Request is complete no more data to read.
            complete_request(m_downloaded);
        }
    }

    // Simple timer class wrapping Boost deadline timer.
    // Closes the connection when timer fires.
    class timeout_timer
    {
    public:
        timeout_timer(const std::chrono::microseconds& timeout)
            : m_duration(timeout.count()), m_state(created), m_timer(crossplat::threadpool::shared_instance().service())
        {
        }

        void set_ctx(const std::weak_ptr<asio_context>& ctx) { m_ctx = ctx; }

        void start()
        {
            assert(m_state == created);
            assert(!m_ctx.expired());
            m_state = started;

            m_timer.expires_from_now(m_duration);
            auto ctx = m_ctx;
            m_timer.async_wait([ctx AND_CAPTURE_MEMBER_FUNCTION_POINTERS](const boost::system::error_code& ec) {
                handle_timeout(ec, ctx);
            });
        }

        void reset()
        {
            assert(m_state == started || m_state == timedout);
            assert(!m_ctx.expired());
            if (m_timer.expires_from_now(m_duration) > 0)
            {
                // The existing handler was canceled so schedule a new one.
                assert(m_state == started);
                auto ctx = m_ctx;
                m_timer.async_wait([ctx AND_CAPTURE_MEMBER_FUNCTION_POINTERS](const boost::system::error_code& ec) {
                    handle_timeout(ec, ctx);
                });
            }
        }

        bool has_timedout() const { return m_state == timedout; }

        bool has_started() const { return m_state == started; }

        void stop()
        {
            m_state = stopped;
            m_timer.cancel();
        }

        static void handle_timeout(const boost::system::error_code& ec, const std::weak_ptr<asio_context>& ctx)
        {
            if (!ec)
            {
                auto shared_ctx = ctx.lock();
                if (shared_ctx)
                {
                    assert(shared_ctx->m_timer.m_state != timedout);
                    shared_ctx->m_timer.m_state = timedout;
                    shared_ctx->m_connection->close();
                }
            }
        }

    private:
        enum timer_state
        {
            created,
            started,
            stopped,
            timedout
        };

#if (defined(ANDROID) || defined(__ANDROID__)) && !defined(_LIBCPP_VERSION)
        boost::chrono::microseconds m_duration;
#else
        std::chrono::microseconds m_duration;
#endif
        std::atomic<timer_state> m_state;
        std::weak_ptr<asio_context> m_ctx;
        boost::asio::steady_timer m_timer;
    };

    uint64_t m_content_length;
    bool m_needChunked;
    timeout_timer m_timer;
    tcp::resolver m_resolver;
    boost::asio::streambuf m_body_buf;
    std::shared_ptr<asio_connection> m_connection;

#ifdef CPPREST_PLATFORM_ASIO_CERT_VERIFICATION_AVAILABLE
    bool m_openssl_failed;
#endif // CPPREST_PLATFORM_ASIO_CERT_VERIFICATION_AVAILABLE
};

std::shared_ptr<_http_client_communicator> create_platform_final_pipeline_stage(uri&& base_uri,
                                                                                http_client_config&& client_config)
{
    return std::make_shared<asio_client>(std::move(base_uri), std::move(client_config));
}

void asio_client::send_request(const std::shared_ptr<request_context>& request_ctx)
{
    auto ctx = std::static_pointer_cast<asio_context>(request_ctx);

    try
    {
        if (ctx->m_connection->is_ssl())
        {
            client_config().invoke_nativehandle_options(ctx->m_connection->m_ssl_stream.get());
        }
        else
        {
            client_config().invoke_nativehandle_options(&(ctx->m_connection->m_socket));
        }
    }
    catch (...)
    {
        request_ctx->report_exception(std::current_exception());
        return;
    }

    ctx->start_request();
}

static bool is_retrieval_redirection(status_code code)
{
    // See https://tools.ietf.org/html/rfc7231#section-6.4

    switch (code)
    {
    case status_codes::MovedPermanently:
        // "For historical reasons, a user agent MAY change the request method
        // from POST to GET for the subsequent request."
        return true;
    case status_codes::Found:
        // "For historical reasons, a user agent MAY change the request method
        // from POST to GET for the subsequent request."
        return true;
    case status_codes::SeeOther:
        // "A user agent can perform a [GET or HEAD] request. It is primarily
        // used to allow the output of a POST action to redirect the user agent
        // to a selected resource."
        return true;
    default:
        return false;
    }
}

static bool is_unchanged_redirection(status_code code)
{
    // See https://tools.ietf.org/html/rfc7231#section-6.4
    // and https://tools.ietf.org/html/rfc7538#section-3

    switch (code)
    {
    case status_codes::TemporaryRedirect:
        // "The user agent MUST NOT change the request method if it performs an
        // automatic redirection to that URI."
        return true;
    case status_codes::PermanentRedirect:
        // This status code "does not allow changing the request method from POST
        // to GET."
        return true;
    default:
        return false;
    }
}

static bool is_recognized_redirection(status_code code)
{
    // other 3xx status codes, e.g. 300 Multiple Choices, are not handled
    // and should be handled externally
    return is_retrieval_redirection(code) || is_unchanged_redirection(code);
}

static bool is_retrieval_request(method method)
{
    return methods::GET == method || methods::HEAD == method;
}

static const std::vector<utility::string_t> request_body_header_names =
{
    header_names::content_encoding,
    header_names::content_language,
    header_names::content_length,
    header_names::content_location,
    header_names::content_type
};

// A request continuation that follows redirects according to the specified configuration.
// This implementation only supports retrieval redirects, as it cannot redirect e.g. a POST request
// using the same method since the request body may have been consumed.
struct http_redirect_follower
{
    http_client_config config;
    std::vector<uri> followed_urls;
    http_request redirect;

    http_redirect_follower(http_client_config config, const http_request& request);

    uri url_to_follow(const http_response& response) const;

    pplx::task<http_response> operator()(http_response response);
};

http_redirect_follower::http_redirect_follower(http_client_config config, const http_request& request)
    : config(std::move(config))
    , followed_urls(1, request.absolute_uri())
    , redirect(request.method())
{
    // Stash the original request URL, etc. to be prepared for an automatic redirect

    // Basically, it makes sense to send the redirects with the same headers as the original request
    redirect.headers() = request.headers();
    // However, this implementation only supports retrieval redirects, with no body, so Content-* headers
    // should be removed
    for (const auto& content_header : request_body_header_names)
    {
        redirect.headers().remove(content_header);
    }

    redirect._set_cancellation_token(request._cancellation_token());
}

uri http_redirect_follower::url_to_follow(const http_response& response) const
{
    // Return immediately if the response is not a supported redirection
    if (!is_recognized_redirection(response.status_code()))
        return{};

    // Although not required by RFC 7231, config may limit the number of automatic redirects
    // (followed_urls includes the initial request URL, hence '<' here)
    if (config.max_redirects() < followed_urls.size())
        return{};

    // Can't very well automatically redirect if the server hasn't provided a Location
    const auto location = response.headers().find(header_names::location);
    if (response.headers().end() == location)
        return{};

    uri to_follow(followed_urls.back().resolve_uri(location->second));

    // Config may prohibit automatic redirects from HTTPS to HTTP
    if (!config.https_to_http_redirects() && followed_urls.back().scheme() == _XPLATSTR("https")
        && to_follow.scheme() != _XPLATSTR("https"))
        return{};

    // "A client SHOULD detect and intervene in cyclical redirections."
    if (followed_urls.end() != std::find(followed_urls.begin(), followed_urls.end(), to_follow))
        return{};

    return to_follow;
}

pplx::task<http_response> http_redirect_follower::operator()(http_response response)
{
    // Return immediately if the response doesn't indicate a valid automatic redirect
    uri to_follow = url_to_follow(response);
    if (to_follow.is_empty())
        return pplx::task_from_result(response);

    // This implementation only supports retrieval redirects, as it cannot redirect e.g. a POST request
    // using the same method since the request body may have been consumed.
    if (!is_retrieval_request(redirect.method()) && !is_retrieval_redirection(response.status_code()))
        return pplx::task_from_result(response);

    if (!is_retrieval_request(redirect.method()))
        redirect.set_method(methods::GET);

    // If the reply to this request is also a redirect, we want visibility of that
    auto config_no_redirects = config;
    config_no_redirects.set_max_redirects(0);
    http_client client(to_follow, config_no_redirects);

    // Stash the redirect request URL and make the request with the same continuation
    auto request_task = client.request(redirect, redirect._cancellation_token());
    followed_urls.push_back(std::move(to_follow));
    return request_task.then(std::move(*this));
}

pplx::task<http_response> asio_client::propagate(http_request request)
{
    auto self = std::static_pointer_cast<_http_client_communicator>(shared_from_this());
    std::shared_ptr<request_context> context;
    try
    {
        context = details::asio_context::create_request_context(self, request);
    }
    catch (...)
    {
        return pplx::task_from_exception<http_response>(std::current_exception());
    }

    // Use a task to externally signal the final result and completion of the task.
    auto result_task = pplx::create_task(context->m_request_completion);

    // Asynchronously send the response with the HTTP client implementation.
    this->async_send_request(context);

    return client_config().max_redirects() > 0
        ? result_task.then(http_redirect_follower(client_config(), request))
        : result_task;
}
} // namespace details
} // namespace client
} // namespace http
} // namespace web