async-http-client/Sources/AsyncHTTPClient/HTTPHandler.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the AsyncHTTPClient open source project
//
// Copyright (c) 2018-2019 Apple Inc. and the AsyncHTTPClient project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of AsyncHTTPClient project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//

import Foundation
import Logging
import NIOConcurrencyHelpers
import NIOCore
import NIOHTTP1
import NIOSSL

extension HTTPClient {
    /// A request body.
    public struct Body {
        /// A streaming uploader.
        ///
        /// ``StreamWriter`` abstracts
        public struct StreamWriter {
            let closure: (IOData) -> EventLoopFuture<Void>

            /// Create new ``HTTPClient/Body/StreamWriter``
            ///
            /// - parameters:
            ///     - closure: function that will be called to write actual bytes to the channel.
            public init(closure: @escaping (IOData) -> EventLoopFuture<Void>) {
                self.closure = closure
            }

            /// Write data to server.
            ///
            /// - parameters:
            ///     - data: `IOData` to write.
            public func write(_ data: IOData) -> EventLoopFuture<Void> {
                return self.closure(data)
            }
        }

        /// Body size. If nil,`Transfer-Encoding` will automatically be set to `chunked`. Otherwise a `Content-Length`
        /// header is set with the given `length`.
        public var length: Int?
        /// Body chunk provider.
        public var stream: (StreamWriter) -> EventLoopFuture<Void>

        @inlinable
        init(length: Int?, stream: @escaping (StreamWriter) -> EventLoopFuture<Void>) {
            self.length = length
            self.stream = stream
        }

        /// Create and stream body using `ByteBuffer`.
        ///
        /// - parameters:
        ///     - buffer: Body `ByteBuffer` representation.
        public static func byteBuffer(_ buffer: ByteBuffer) -> Body {
            return Body(length: buffer.readableBytes) { writer in
                writer.write(.byteBuffer(buffer))
            }
        }

        /// Create and stream body using ``StreamWriter``.
        ///
        /// - parameters:
        ///     - length: Body size. If nil, `Transfer-Encoding` will automatically be set to `chunked`. Otherwise a `Content-Length`
        /// header is set with the given `length`.
        ///     - stream: Body chunk provider.
        public static func stream(length: Int? = nil, _ stream: @escaping (StreamWriter) -> EventLoopFuture<Void>) -> Body {
            return Body(length: length, stream: stream)
        }

        /// Create and stream body using a collection of bytes.
        ///
        /// - parameters:
        ///     - data: Body binary representation.
        @inlinable
        public static func bytes<Bytes>(_ bytes: Bytes) -> Body where Bytes: RandomAccessCollection, Bytes.Element == UInt8 {
            return Body(length: bytes.count) { writer in
                writer.write(.byteBuffer(ByteBuffer(bytes: bytes)))
            }
        }

        /// Create and stream body using `String`.
        ///
        /// - parameters:
        ///     - string: Body `String` representation.
        public static func string(_ string: String) -> Body {
            return Body(length: string.utf8.count) { writer in
                writer.write(.byteBuffer(ByteBuffer(string: string)))
            }
        }
    }

    /// Represents an HTTP request.
    public struct Request {
        /// Request HTTP method, defaults to `GET`.
        public let method: HTTPMethod
        /// Remote URL.
        public let url: URL

        /// Remote HTTP scheme, resolved from `URL`.
        public var scheme: String {
            self.deconstructedURL.scheme.rawValue
        }

        /// Request custom HTTP Headers, defaults to no headers.
        public var headers: HTTPHeaders
        /// Request body, defaults to no body.
        public var body: Body?
        /// Request-specific TLS configuration, defaults to no request-specific TLS configuration.
        public var tlsConfiguration: TLSConfiguration?

        /// Parsed, validated and deconstructed URL.
        let deconstructedURL: DeconstructedURL

        /// Create HTTP request.
        ///
        /// - parameters:
        ///     - url: Remote `URL`.
        ///     - version: HTTP version.
        ///     - method: HTTP method.
        ///     - headers: Custom HTTP headers.
        ///     - body: Request body.
        /// - throws:
        ///     - `invalidURL` if URL cannot be parsed.
        ///     - `emptyScheme` if URL does not contain HTTP scheme.
        ///     - `unsupportedScheme` if URL does contains unsupported HTTP scheme.
        ///     - `emptyHost` if URL does not contains a host.
        public init(url: String, method: HTTPMethod = .GET, headers: HTTPHeaders = HTTPHeaders(), body: Body? = nil) throws {
            try self.init(url: url, method: method, headers: headers, body: body, tlsConfiguration: nil)
        }

        /// Create HTTP request.
        ///
        /// - parameters:
        ///     - url: Remote `URL`.
        ///     - version: HTTP version.
        ///     - method: HTTP method.
        ///     - headers: Custom HTTP headers.
        ///     - body: Request body.
        ///     - tlsConfiguration: Request TLS configuration
        /// - throws:
        ///     - `invalidURL` if URL cannot be parsed.
        ///     - `emptyScheme` if URL does not contain HTTP scheme.
        ///     - `unsupportedScheme` if URL does contains unsupported HTTP scheme.
        ///     - `emptyHost` if URL does not contains a host.
        public init(url: String, method: HTTPMethod = .GET, headers: HTTPHeaders = HTTPHeaders(), body: Body? = nil, tlsConfiguration: TLSConfiguration?) throws {
            guard let url = URL(string: url) else {
                throw HTTPClientError.invalidURL
            }

            try self.init(url: url, method: method, headers: headers, body: body, tlsConfiguration: tlsConfiguration)
        }

        /// Create an HTTP `Request`.
        ///
        /// - parameters:
        ///     - url: Remote `URL`.
        ///     - method: HTTP method.
        ///     - headers: Custom HTTP headers.
        ///     - body: Request body.
        /// - throws:
        ///     - `emptyScheme` if URL does not contain HTTP scheme.
        ///     - `unsupportedScheme` if URL does contains unsupported HTTP scheme.
        ///     - `emptyHost` if URL does not contains a host.
        ///     - `missingSocketPath` if URL does not contains a socketPath as an encoded host.
        public init(url: URL, method: HTTPMethod = .GET, headers: HTTPHeaders = HTTPHeaders(), body: Body? = nil) throws {
            try self.init(url: url, method: method, headers: headers, body: body, tlsConfiguration: nil)
        }

        /// Create an HTTP `Request`.
        ///
        /// - parameters:
        ///     - url: Remote `URL`.
        ///     - method: HTTP method.
        ///     - headers: Custom HTTP headers.
        ///     - body: Request body.
        ///     - tlsConfiguration: Request TLS configuration
        /// - throws:
        ///     - `emptyScheme` if URL does not contain HTTP scheme.
        ///     - `unsupportedScheme` if URL does contains unsupported HTTP scheme.
        ///     - `emptyHost` if URL does not contains a host.
        ///     - `missingSocketPath` if URL does not contains a socketPath as an encoded host.
        public init(url: URL, method: HTTPMethod = .GET, headers: HTTPHeaders = HTTPHeaders(), body: Body? = nil, tlsConfiguration: TLSConfiguration?) throws {
            self.deconstructedURL = try DeconstructedURL(url: url)

            self.url = url
            self.method = method
            self.headers = headers
            self.body = body
            self.tlsConfiguration = tlsConfiguration
        }

        /// Remote host, resolved from `URL`.
        public var host: String {
            self.deconstructedURL.connectionTarget.host ?? ""
        }

        /// Resolved port.
        public var port: Int {
            self.deconstructedURL.connectionTarget.port ?? self.deconstructedURL.scheme.defaultPort
        }

        /// Whether request will be executed using secure socket.
        public var useTLS: Bool { self.deconstructedURL.scheme.usesTLS }

        func createRequestHead() throws -> (HTTPRequestHead, RequestFramingMetadata) {
            var head = HTTPRequestHead(
                version: .http1_1,
                method: self.method,
                uri: self.deconstructedURL.uri,
                headers: self.headers
            )

            head.headers.addHostIfNeeded(for: self.deconstructedURL)

            let metadata = try head.headers.validateAndSetTransportFraming(method: self.method, bodyLength: .init(self.body))

            return (head, metadata)
        }
    }

    /// Represents an HTTP response.
    public struct Response {
        /// Remote host of the request.
        public var host: String
        /// Response HTTP status.
        public var status: HTTPResponseStatus
        /// Response HTTP version.
        public var version: HTTPVersion
        /// Reponse HTTP headers.
        public var headers: HTTPHeaders
        /// Response body.
        public var body: ByteBuffer?

        /// Create HTTP `Response`.
        ///
        /// - parameters:
        ///     - host: Remote host of the request.
        ///     - status: Response HTTP status.
        ///     - headers: Reponse HTTP headers.
        ///     - body: Response body.
        @available(*, deprecated, renamed: "init(host:status:version:headers:body:)")
        public init(host: String, status: HTTPResponseStatus, headers: HTTPHeaders, body: ByteBuffer?) {
            self.host = host
            self.status = status
            self.version = HTTPVersion(major: 1, minor: 1)
            self.headers = headers
            self.body = body
        }

        /// Create HTTP `Response`.
        ///
        /// - parameters:
        ///     - host: Remote host of the request.
        ///     - status: Response HTTP status.
        ///     - version: Response HTTP version.
        ///     - headers: Reponse HTTP headers.
        ///     - body: Response body.
        public init(host: String, status: HTTPResponseStatus, version: HTTPVersion, headers: HTTPHeaders, body: ByteBuffer?) {
            self.host = host
            self.status = status
            self.version = version
            self.headers = headers
            self.body = body
        }
    }

    /// HTTP authentication.
    public struct Authorization: Hashable {
        private enum Scheme: Hashable {
            case Basic(String)
            case Bearer(String)
        }

        private let scheme: Scheme

        private init(scheme: Scheme) {
            self.scheme = scheme
        }

        /// HTTP basic auth.
        public static func basic(username: String, password: String) -> HTTPClient.Authorization {
            return .basic(credentials: Base64.encode(bytes: "\(username):\(password)".utf8))
        }

        /// HTTP basic auth.
        ///
        /// This version uses the raw string directly.
        public static func basic(credentials: String) -> HTTPClient.Authorization {
            return .init(scheme: .Basic(credentials))
        }

        /// HTTP bearer auth
        public static func bearer(tokens: String) -> HTTPClient.Authorization {
            return .init(scheme: .Bearer(tokens))
        }

        /// The header string for this auth field.
        public var headerValue: String {
            switch self.scheme {
            case .Basic(let credentials):
                return "Basic \(credentials)"
            case .Bearer(let tokens):
                return "Bearer \(tokens)"
            }
        }
    }
}

/// The default ``HTTPClientResponseDelegate``.
///
/// This ``HTTPClientResponseDelegate`` buffers a complete HTTP response in memory. It does not stream the response body in.
/// The resulting ``Response`` type is ``HTTPClient/Response``.
public class ResponseAccumulator: HTTPClientResponseDelegate {
    public typealias Response = HTTPClient.Response

    enum State {
        case idle
        case head(HTTPResponseHead)
        case body(HTTPResponseHead, ByteBuffer)
        case end
        case error(Error)
    }

    var state = State.idle
    let request: HTTPClient.Request

    public init(request: HTTPClient.Request) {
        self.request = request
    }

    public func didReceiveHead(task: HTTPClient.Task<Response>, _ head: HTTPResponseHead) -> EventLoopFuture<Void> {
        switch self.state {
        case .idle:
            self.state = .head(head)
        case .head:
            preconditionFailure("head already set")
        case .body:
            preconditionFailure("no head received before body")
        case .end:
            preconditionFailure("request already processed")
        case .error:
            break
        }
        return task.eventLoop.makeSucceededFuture(())
    }

    public func didReceiveBodyPart(task: HTTPClient.Task<Response>, _ part: ByteBuffer) -> EventLoopFuture<Void> {
        switch self.state {
        case .idle:
            preconditionFailure("no head received before body")
        case .head(let head):
            self.state = .body(head, part)
        case .body(let head, var body):
            // The compiler can't prove that `self.state` is dead here (and it kinda isn't, there's
            // a cross-module call in the way) so we need to drop the original reference to `body` in
            // `self.state` or we'll get a CoW. To fix that we temporarily set the state to `.end` (which
            // has no associated data). We'll fix it at the bottom of this block.
            self.state = .end
            var part = part
            body.writeBuffer(&part)
            self.state = .body(head, body)
        case .end:
            preconditionFailure("request already processed")
        case .error:
            break
        }
        return task.eventLoop.makeSucceededFuture(())
    }

    public func didReceiveError(task: HTTPClient.Task<Response>, _ error: Error) {
        self.state = .error(error)
    }

    public func didFinishRequest(task: HTTPClient.Task<Response>) throws -> Response {
        switch self.state {
        case .idle:
            preconditionFailure("no head received before end")
        case .head(let head):
            return Response(host: self.request.host, status: head.status, version: head.version, headers: head.headers, body: nil)
        case .body(let head, let body):
            return Response(host: self.request.host, status: head.status, version: head.version, headers: head.headers, body: body)
        case .end:
            preconditionFailure("request already processed")
        case .error(let error):
            throw error
        }
    }
}

/// ``HTTPClientResponseDelegate`` allows an implementation to receive notifications about request processing and to control how response parts are processed.
///
/// You can implement this protocol if you need fine-grained control over an HTTP request/response, for example, if you want to inspect the response
/// headers before deciding whether to accept a response body, or if you want to stream your request body. Pass an instance of your conforming
/// class to the ``HTTPClient/execute(request:delegate:eventLoop:deadline:)`` method and this package will call each delegate method appropriately as the request takes place.
///
/// ### Backpressure
///
/// A ``HTTPClientResponseDelegate`` can be used to exert backpressure on the server response. This is achieved by way of the futures returned from
/// ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd`` and ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``.
/// The following functions are part of the "backpressure system" in the delegate:
///
/// - ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd``
/// - ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``
/// - ``HTTPClientResponseDelegate/didFinishRequest(task:)``
/// - ``HTTPClientResponseDelegate/didReceiveError(task:_:)-fhsg``
///
/// The first three methods are strictly _exclusive_, with that exclusivity managed by the futures returned by ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd`` and
/// ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``. What this means is that until the returned future is completed, none of these three methods will be called
/// again. This allows delegates to rate limit the server to a capacity it can manage. ``HTTPClientResponseDelegate/didFinishRequest(task:)`` does not return a future,
/// as we are expecting no more data from the server at this time.
///
/// ``HTTPClientResponseDelegate/didReceiveError(task:_:)-fhsg`` is somewhat special: it signals the end of this regime. ``HTTPClientResponseDelegate/didReceiveError(task:_:)-fhsg``
/// is not exclusive: it may be called at any time, even if a returned future is not yet completed. ``HTTPClientResponseDelegate/didReceiveError(task:_:)-fhsg`` is terminal, meaning
/// that once it has been called none of these four methods will be called again. This can be used as a signal to abandon all outstanding work.
///
///  - note: This delegate is strongly held by the `HTTPTaskHandler`
///          for the duration of the ``HTTPClient/Request`` processing and will be
///          released together with the `HTTPTaskHandler` when channel is closed.
///          Users of the library are not required to keep a reference to the
///          object that implements this protocol, but may do so if needed.
public protocol HTTPClientResponseDelegate: AnyObject {
    associatedtype Response

    /// Called when the request head is sent. Will be called once.
    ///
    /// - parameters:
    ///     - task: Current request context.
    ///     - head: Request head.
    func didSendRequestHead(task: HTTPClient.Task<Response>, _ head: HTTPRequestHead)

    /// Called when a part of the request body is sent. Could be called zero or more times.
    ///
    /// - parameters:
    ///     - task: Current request context.
    ///     - part: Request body part.
    func didSendRequestPart(task: HTTPClient.Task<Response>, _ part: IOData)

    /// Called when the request is fully sent. Will be called once.
    ///
    /// - parameters:
    ///     - task: Current request context.
    func didSendRequest(task: HTTPClient.Task<Response>)

    /// Called when response head is received. Will be called once.
    /// You must return an `EventLoopFuture<Void>` that you complete when you have finished processing the body part.
    /// You can create an already succeeded future by calling `task.eventLoop.makeSucceededFuture(())`.
    ///
    /// - parameters:
    ///     - task: Current request context.
    ///     - head: Received reposonse head.
    /// - returns: `EventLoopFuture` that will be used for backpressure.
    func didReceiveHead(task: HTTPClient.Task<Response>, _ head: HTTPResponseHead) -> EventLoopFuture<Void>

    /// Called when part of a response body is received. Could be called zero or more times.
    /// You must return an `EventLoopFuture<Void>` that you complete when you have finished processing the body part.
    /// You can create an already succeeded future by calling `task.eventLoop.makeSucceededFuture(())`.
    ///
    /// This function will not be called until the future returned by ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd`` has completed.
    ///
    /// This function will not be called for subsequent body parts until the previous future returned by a
    /// call to this function completes.
    ///
    /// - parameters:
    ///     - task: Current request context.
    ///     - buffer: Received body `Part`.
    /// - returns: `EventLoopFuture` that will be used for backpressure.
    func didReceiveBodyPart(task: HTTPClient.Task<Response>, _ buffer: ByteBuffer) -> EventLoopFuture<Void>

    /// Called when error was thrown during request execution. Will be called zero or one time only. Request processing will be stopped after that.
    ///
    /// This function may be called at any time: it does not respect the backpressure exerted by ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd``
    /// and ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``.
    /// All outstanding work may be cancelled when this is received. Once called, no further calls will be made to
    /// ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd``, ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``,
    /// or ``HTTPClientResponseDelegate/didFinishRequest(task:)``.
    ///
    /// - parameters:
    ///     - task: Current request context.
    ///     - error: Error that occured during response processing.
    func didReceiveError(task: HTTPClient.Task<Response>, _ error: Error)

    /// Called when the complete HTTP request is finished. You must return an instance of your ``Response`` associated type. Will be called once, except if an error occurred.
    ///
    /// This function will not be called until all futures returned by ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd`` and ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``
    /// have completed. Once called, no further calls will be made to ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd``, ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``,
    /// or ``HTTPClientResponseDelegate/didReceiveError(task:_:)-fhsg``.
    ///
    /// - parameters:
    ///     - task: Current request context.
    /// - returns: Result of processing.
    func didFinishRequest(task: HTTPClient.Task<Response>) throws -> Response
}

extension HTTPClientResponseDelegate {
    /// Default implementation of ``HTTPClientResponseDelegate/didSendRequestHead(task:_:)-6khai``.
    ///
    /// By default, this does nothing.
    public func didSendRequestHead(task: HTTPClient.Task<Response>, _ head: HTTPRequestHead) {}

    /// Default implementation of ``HTTPClientResponseDelegate/didSendRequestPart(task:_:)-4qxap``.
    ///
    /// By default, this does nothing.
    public func didSendRequestPart(task: HTTPClient.Task<Response>, _ part: IOData) {}

    /// Default implementation of ``HTTPClientResponseDelegate/didSendRequest(task:)-3vqgm``.
    ///
    /// By default, this does nothing.
    public func didSendRequest(task: HTTPClient.Task<Response>) {}

    /// Default implementation of ``HTTPClientResponseDelegate/didReceiveHead(task:_:)-9r4xd``.
    ///
    /// By default, this does nothing.
    public func didReceiveHead(task: HTTPClient.Task<Response>, _: HTTPResponseHead) -> EventLoopFuture<Void> {
        return task.eventLoop.makeSucceededVoidFuture()
    }

    /// Default implementation of ``HTTPClientResponseDelegate/didReceiveBodyPart(task:_:)-4fd4v``.
    ///
    /// By default, this does nothing.
    public func didReceiveBodyPart(task: HTTPClient.Task<Response>, _: ByteBuffer) -> EventLoopFuture<Void> {
        return task.eventLoop.makeSucceededVoidFuture()
    }

    /// Default implementation of ``HTTPClientResponseDelegate/didReceiveError(task:_:)-fhsg``.
    ///
    /// By default, this does nothing.
    public func didReceiveError(task: HTTPClient.Task<Response>, _: Error) {}
}

extension URL {
    var percentEncodedPath: String {
        if self.path.isEmpty {
            return "/"
        }
        return URLComponents(url: self, resolvingAgainstBaseURL: false)?.percentEncodedPath ?? self.path
    }

    var uri: String {
        var uri = self.percentEncodedPath

        if let query = self.query {
            uri += "?" + query
        }

        return uri
    }

    func hasTheSameOrigin(as other: URL) -> Bool {
        return self.host == other.host && self.scheme == other.scheme && self.port == other.port
    }

    /// Initializes a newly created HTTP URL connecting to a unix domain socket path. The socket path is encoded as the URL's host, replacing percent encoding invalid path characters, and will use the "http+unix" scheme.
    /// - Parameters:
    ///   - socketPath: The path to the unix domain socket to connect to.
    ///   - uri: The URI path and query that will be sent to the server.
    public init?(httpURLWithSocketPath socketPath: String, uri: String = "/") {
        guard let host = socketPath.addingPercentEncoding(withAllowedCharacters: .urlHostAllowed) else { return nil }
        var urlString: String
        if uri.hasPrefix("/") {
            urlString = "http+unix://\(host)\(uri)"
        } else {
            urlString = "http+unix://\(host)/\(uri)"
        }
        self.init(string: urlString)
    }

    /// Initializes a newly created HTTPS URL connecting to a unix domain socket path over TLS. The socket path is encoded as the URL's host, replacing percent encoding invalid path characters, and will use the "https+unix" scheme.
    /// - Parameters:
    ///   - socketPath: The path to the unix domain socket to connect to.
    ///   - uri: The URI path and query that will be sent to the server.
    public init?(httpsURLWithSocketPath socketPath: String, uri: String = "/") {
        guard let host = socketPath.addingPercentEncoding(withAllowedCharacters: .urlHostAllowed) else { return nil }
        var urlString: String
        if uri.hasPrefix("/") {
            urlString = "https+unix://\(host)\(uri)"
        } else {
            urlString = "https+unix://\(host)/\(uri)"
        }
        self.init(string: urlString)
    }
}

protocol HTTPClientTaskDelegate {
    func cancel()
}

extension HTTPClient {
    /// Response execution context.
    ///
    /// Will be created by the library and could be used for obtaining
    /// `EventLoopFuture<Response>` of the execution or cancellation of the execution.
    public final class Task<Response> {
        /// The `EventLoop` the delegate will be executed on.
        public let eventLoop: EventLoop

        let promise: EventLoopPromise<Response>
        let logger: Logger // We are okay to store the logger here because a Task is for only one request.

        var isCancelled: Bool {
            self.lock.withLock { self._isCancelled }
        }

        var taskDelegate: HTTPClientTaskDelegate? {
            get {
                self.lock.withLock { self._taskDelegate }
            }
            set {
                self.lock.withLock { self._taskDelegate = newValue }
            }
        }

        private var _isCancelled: Bool = false
        private var _taskDelegate: HTTPClientTaskDelegate?
        private let lock = Lock()

        init(eventLoop: EventLoop, logger: Logger) {
            self.eventLoop = eventLoop
            self.promise = eventLoop.makePromise()
            self.logger = logger
        }

        static func failedTask(eventLoop: EventLoop, error: Error, logger: Logger) -> Task<Response> {
            let task = self.init(eventLoop: eventLoop, logger: logger)
            task.promise.fail(error)
            return task
        }

        /// `EventLoopFuture` for the response returned by this request.
        public var futureResult: EventLoopFuture<Response> {
            return self.promise.futureResult
        }

        /// Waits for execution of this request to complete.
        ///
        /// - returns: The value of the `EventLoopFuture` when it completes.
        /// - throws: The error value of the `EventLoopFuture` if it errors.
        public func wait() throws -> Response {
            return try self.promise.futureResult.wait()
        }

        /// Cancels the request execution.
        public func cancel() {
            let taskDelegate = self.lock.withLock { () -> HTTPClientTaskDelegate? in
                self._isCancelled = true
                return self._taskDelegate
            }

            taskDelegate?.cancel()
        }

        func succeed<Delegate: HTTPClientResponseDelegate>(promise: EventLoopPromise<Response>?,
                                                           with value: Response,
                                                           delegateType: Delegate.Type,
                                                           closing: Bool) {
            promise?.succeed(value)
        }

        func fail<Delegate: HTTPClientResponseDelegate>(with error: Error,
                                                        delegateType: Delegate.Type) {
            self.promise.fail(error)
        }
    }
}

internal struct TaskCancelEvent {}

// MARK: - RedirectHandler

internal struct RedirectHandler<ResponseType> {
    let request: HTTPClient.Request
    let redirectState: RedirectState
    let execute: (HTTPClient.Request, RedirectState) -> HTTPClient.Task<ResponseType>

    func redirectTarget(status: HTTPResponseStatus, responseHeaders: HTTPHeaders) -> URL? {
        responseHeaders.extractRedirectTarget(
            status: status,
            originalURL: self.request.url,
            originalScheme: self.request.deconstructedURL.scheme
        )
    }

    func redirect(
        status: HTTPResponseStatus,
        to redirectURL: URL,
        promise: EventLoopPromise<ResponseType>
    ) {
        do {
            var redirectState = self.redirectState
            try redirectState.redirect(to: redirectURL.absoluteString)

            let (method, headers, body) = transformRequestForRedirect(
                from: request.url,
                method: self.request.method,
                headers: self.request.headers,
                body: self.request.body,
                to: redirectURL,
                status: status
            )

            let newRequest = try HTTPClient.Request(
                url: redirectURL,
                method: method,
                headers: headers,
                body: body
            )
            self.execute(newRequest, redirectState).futureResult.whenComplete { result in
                promise.futureResult.eventLoop.execute {
                    promise.completeWith(result)
                }
            }
        } catch {
            promise.fail(error)
        }
    }
}

extension RequestBodyLength {
    init(_ body: HTTPClient.Body?) {
        guard let body = body else {
            self = .known(0)
            return
        }
        guard let length = body.length else {
            self = .unknown
            return
        }
        self = .known(length)
    }
}