HTTPClient.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 NIOHTTPCompression
import NIOPosix
import NIOSSL
import NIOTLS
import NIOTransportServices

extension Logger {
    private func requestInfo(_ request: HTTPClient.Request) -> Logger.Metadata.Value {
        return "\(request.method) \(request.url)"
    }

    func attachingRequestInformation(_ request: HTTPClient.Request, requestID: Int) -> Logger {
        var modified = self
        modified[metadataKey: "ahc-prev-request"] = nil
        modified[metadataKey: "ahc-request-id"] = "\(requestID)"
        return modified
    }
}

let globalRequestID = NIOAtomic<Int>.makeAtomic(value: 0)

/// HTTPClient class provides API for request execution.
///
/// Example:
///
/// ```swift
///     let client = HTTPClient(eventLoopGroupProvider: .createNew)
///     client.get(url: "https://swift.org", deadline: .now() + .seconds(1)).whenComplete { result in
///         switch result {
///         case .failure(let error):
///             // process error
///         case .success(let response):
///             if let response.status == .ok {
///                 // handle response
///             } else {
///                 // handle remote error
///             }
///         }
///     }
/// ```
///
/// It is important to close the client instance, for example in a defer statement, after use to cleanly shutdown the underlying NIO `EventLoopGroup`:
///
/// ```swift
///     try client.syncShutdown()
/// ```
public class HTTPClient {
    public let eventLoopGroup: EventLoopGroup
    let eventLoopGroupProvider: EventLoopGroupProvider
    let configuration: Configuration
    let poolManager: HTTPConnectionPool.Manager
    var state: State
    private let stateLock = Lock()

    internal static let loggingDisabled = Logger(label: "AHC-do-not-log", factory: { _ in SwiftLogNoOpLogHandler() })

    /// Create an `HTTPClient` with specified `EventLoopGroup` provider and configuration.
    ///
    /// - parameters:
    ///     - eventLoopGroupProvider: Specify how `EventLoopGroup` will be created.
    ///     - configuration: Client configuration.
    public convenience init(eventLoopGroupProvider: EventLoopGroupProvider,
                            configuration: Configuration = Configuration()) {
        self.init(eventLoopGroupProvider: eventLoopGroupProvider,
                  configuration: configuration,
                  backgroundActivityLogger: HTTPClient.loggingDisabled)
    }

    /// Create an `HTTPClient` with specified `EventLoopGroup` provider and configuration.
    ///
    /// - parameters:
    ///     - eventLoopGroupProvider: Specify how `EventLoopGroup` will be created.
    ///     - configuration: Client configuration.
    public required init(eventLoopGroupProvider: EventLoopGroupProvider,
                         configuration: Configuration = Configuration(),
                         backgroundActivityLogger: Logger) {
        self.eventLoopGroupProvider = eventLoopGroupProvider
        switch self.eventLoopGroupProvider {
        case .shared(let group):
            self.eventLoopGroup = group
        case .createNew:
            #if canImport(Network)
                if #available(OSX 10.14, iOS 12.0, tvOS 12.0, watchOS 6.0, *) {
                    self.eventLoopGroup = NIOTSEventLoopGroup()
                } else {
                    self.eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: 1)
                }
            #else
                self.eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: 1)
            #endif
        }
        self.configuration = configuration
        self.poolManager = HTTPConnectionPool.Manager(
            eventLoopGroup: self.eventLoopGroup,
            configuration: self.configuration,
            backgroundActivityLogger: backgroundActivityLogger
        )
        self.state = .upAndRunning
    }

    deinit {
        guard case .shutDown = self.state else {
            preconditionFailure("Client not shut down before the deinit. Please call client.syncShutdown() when no longer needed.")
        }
    }

    /// Shuts down the client and `EventLoopGroup` if it was created by the client.
    public func syncShutdown() throws {
        try self.syncShutdown(requiresCleanClose: false)
    }

    /// Shuts down the client and `EventLoopGroup` if it was created by the client.
    ///
    /// - parameters:
    ///     - requiresCleanClose: Determine if the client should throw when it is shutdown in a non-clean state
    ///
    /// - Note:
    /// The `requiresCleanClose` will let the client do additional checks about its internal consistency on shutdown and
    /// throw the appropriate error if needed. For instance, if its internal connection pool has any non-released connections,
    /// this indicate shutdown was called too early before tasks were completed or explicitly canceled.
    /// In general, setting this parameter to `true` should make it easier and faster to catch related programming errors.
    internal func syncShutdown(requiresCleanClose: Bool) throws {
        if let eventLoop = MultiThreadedEventLoopGroup.currentEventLoop {
            preconditionFailure("""
            BUG DETECTED: syncShutdown() must not be called when on an EventLoop.
            Calling syncShutdown() on any EventLoop can lead to deadlocks.
            Current eventLoop: \(eventLoop)
            """)
        }
        let errorStorageLock = Lock()
        var errorStorage: Error?
        let continuation = DispatchWorkItem {}
        self.shutdown(requiresCleanClose: requiresCleanClose, queue: DispatchQueue(label: "async-http-client.shutdown")) { error in
            if let error = error {
                errorStorageLock.withLock {
                    errorStorage = error
                }
            }
            continuation.perform()
        }
        continuation.wait()
        try errorStorageLock.withLock {
            if let error = errorStorage {
                throw error
            }
        }
    }

    /// Shuts down the client and event loop gracefully. This function is clearly an outlier in that it uses a completion
    /// callback instead of an EventLoopFuture. The reason for that is that NIO's EventLoopFutures will call back on an event loop.
    /// The virtue of this function is to shut the event loop down. To work around that we call back on a DispatchQueue
    /// instead.
    public func shutdown(queue: DispatchQueue = .global(), _ callback: @escaping (Error?) -> Void) {
        self.shutdown(requiresCleanClose: false, queue: queue, callback)
    }

    private func shutdownEventLoop(queue: DispatchQueue, _ callback: @escaping (Error?) -> Void) {
        self.stateLock.withLock {
            switch self.eventLoopGroupProvider {
            case .shared:
                self.state = .shutDown
                queue.async {
                    callback(nil)
                }
            case .createNew:
                switch self.state {
                case .shuttingDown:
                    self.state = .shutDown
                    self.eventLoopGroup.shutdownGracefully(queue: queue, callback)
                case .shutDown, .upAndRunning:
                    assertionFailure("The only valid state at this point is \(String(describing: State.shuttingDown))")
                }
            }
        }
    }

    private func shutdown(requiresCleanClose: Bool, queue: DispatchQueue, _ callback: @escaping (Error?) -> Void) {
        do {
            try self.stateLock.withLock {
                guard case .upAndRunning = self.state else {
                    throw HTTPClientError.alreadyShutdown
                }
                self.state = .shuttingDown(requiresCleanClose: requiresCleanClose, callback: callback)
            }
        } catch {
            callback(error)
            return
        }

        let promise = self.eventLoopGroup.next().makePromise(of: Bool.self)
        self.poolManager.shutdown(promise: promise)
        promise.futureResult.whenComplete { result in
            switch result {
            case .failure:
                preconditionFailure("Shutting down the connection pool must not fail, ever.")
            case .success(let unclean):
                let (callback, uncleanError) = self.stateLock.withLock { () -> ((Error?) -> Void, Error?) in
                    guard case .shuttingDown(let requiresClean, callback: let callback) = self.state else {
                        preconditionFailure("Why did the pool manager shut down, if it was not instructed to")
                    }

                    let error: Error? = (requiresClean && unclean) ? HTTPClientError.uncleanShutdown : nil
                    return (callback, error)
                }

                self.shutdownEventLoop(queue: queue) { error in
                    let reportedError = error ?? uncleanError
                    callback(reportedError)
                }
            }
        }
    }

    /// Execute `GET` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - deadline: Point in time by which the request must complete.
    public func get(url: String, deadline: NIODeadline? = nil) -> EventLoopFuture<Response> {
        return self.get(url: url, deadline: deadline, logger: HTTPClient.loggingDisabled)
    }

    /// Execute `GET` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func get(url: String, deadline: NIODeadline? = nil, logger: Logger) -> EventLoopFuture<Response> {
        return self.execute(.GET, url: url, deadline: deadline, logger: logger)
    }

    /// Execute `POST` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    public func post(url: String, body: Body? = nil, deadline: NIODeadline? = nil) -> EventLoopFuture<Response> {
        return self.post(url: url, body: body, deadline: deadline, logger: HTTPClient.loggingDisabled)
    }

    /// Execute `POST` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func post(url: String, body: Body? = nil, deadline: NIODeadline? = nil, logger: Logger) -> EventLoopFuture<Response> {
        return self.execute(.POST, url: url, body: body, deadline: deadline, logger: logger)
    }

    /// Execute `PATCH` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    public func patch(url: String, body: Body? = nil, deadline: NIODeadline? = nil) -> EventLoopFuture<Response> {
        return self.patch(url: url, body: body, deadline: deadline, logger: HTTPClient.loggingDisabled)
    }

    /// Execute `PATCH` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func patch(url: String, body: Body? = nil, deadline: NIODeadline? = nil, logger: Logger) -> EventLoopFuture<Response> {
        return self.execute(.PATCH, url: url, body: body, deadline: deadline, logger: logger)
    }

    /// Execute `PUT` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    public func put(url: String, body: Body? = nil, deadline: NIODeadline? = nil) -> EventLoopFuture<Response> {
        return self.put(url: url, body: body, deadline: deadline, logger: HTTPClient.loggingDisabled)
    }

    /// Execute `PUT` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func put(url: String, body: Body? = nil, deadline: NIODeadline? = nil, logger: Logger) -> EventLoopFuture<Response> {
        return self.execute(.PUT, url: url, body: body, deadline: deadline, logger: logger)
    }

    /// Execute `DELETE` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - deadline: The time when the request must have been completed by.
    public func delete(url: String, deadline: NIODeadline? = nil) -> EventLoopFuture<Response> {
        return self.delete(url: url, deadline: deadline, logger: HTTPClient.loggingDisabled)
    }

    /// Execute `DELETE` request using specified URL.
    ///
    /// - parameters:
    ///     - url: Remote URL.
    ///     - deadline: The time when the request must have been completed by.
    ///     - logger: The logger to use for this request.
    public func delete(url: String, deadline: NIODeadline? = nil, logger: Logger) -> EventLoopFuture<Response> {
        return self.execute(.DELETE, url: url, deadline: deadline, logger: logger)
    }

    /// Execute arbitrary HTTP request using specified URL.
    ///
    /// - parameters:
    ///     - method: Request method.
    ///     - url: Request url.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func execute(_ method: HTTPMethod = .GET, url: String, body: Body? = nil, deadline: NIODeadline? = nil, logger: Logger? = nil) -> EventLoopFuture<Response> {
        do {
            let request = try Request(url: url, method: method, body: body)
            return self.execute(request: request, deadline: deadline, logger: logger ?? HTTPClient.loggingDisabled)
        } catch {
            return self.eventLoopGroup.next().makeFailedFuture(error)
        }
    }

    /// Execute arbitrary HTTP+UNIX request to a unix domain socket path, using the specified URL as the request to send to the server.
    ///
    /// - parameters:
    ///     - method: Request method.
    ///     - socketPath: The path to the unix domain socket to connect to.
    ///     - urlPath: The URL path and query that will be sent to the server.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func execute(_ method: HTTPMethod = .GET, socketPath: String, urlPath: String, body: Body? = nil, deadline: NIODeadline? = nil, logger: Logger? = nil) -> EventLoopFuture<Response> {
        do {
            guard let url = URL(httpURLWithSocketPath: socketPath, uri: urlPath) else {
                throw HTTPClientError.invalidURL
            }
            let request = try Request(url: url, method: method, body: body)
            return self.execute(request: request, deadline: deadline, logger: logger ?? HTTPClient.loggingDisabled)
        } catch {
            return self.eventLoopGroup.next().makeFailedFuture(error)
        }
    }

    /// Execute arbitrary HTTPS+UNIX request to a unix domain socket path over TLS, using the specified URL as the request to send to the server.
    ///
    /// - parameters:
    ///     - method: Request method.
    ///     - secureSocketPath: The path to the unix domain socket to connect to.
    ///     - urlPath: The URL path and query that will be sent to the server.
    ///     - body: Request body.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func execute(_ method: HTTPMethod = .GET, secureSocketPath: String, urlPath: String, body: Body? = nil, deadline: NIODeadline? = nil, logger: Logger? = nil) -> EventLoopFuture<Response> {
        do {
            guard let url = URL(httpsURLWithSocketPath: secureSocketPath, uri: urlPath) else {
                throw HTTPClientError.invalidURL
            }
            let request = try Request(url: url, method: method, body: body)
            return self.execute(request: request, deadline: deadline, logger: logger ?? HTTPClient.loggingDisabled)
        } catch {
            return self.eventLoopGroup.next().makeFailedFuture(error)
        }
    }

    /// Execute arbitrary HTTP request using specified URL.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - deadline: Point in time by which the request must complete.
    public func execute(request: Request, deadline: NIODeadline? = nil) -> EventLoopFuture<Response> {
        return self.execute(request: request, deadline: deadline, logger: HTTPClient.loggingDisabled)
    }

    /// Execute arbitrary HTTP request using specified URL.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func execute(request: Request, deadline: NIODeadline? = nil, logger: Logger) -> EventLoopFuture<Response> {
        let accumulator = ResponseAccumulator(request: request)
        return self.execute(request: request, delegate: accumulator, deadline: deadline, logger: logger).futureResult
    }

    /// Execute arbitrary HTTP request using specified URL.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - eventLoop: NIO Event Loop preference.
    ///     - deadline: Point in time by which the request must complete.
    public func execute(request: Request, eventLoop: EventLoopPreference, deadline: NIODeadline? = nil) -> EventLoopFuture<Response> {
        return self.execute(request: request,
                            eventLoop: eventLoop,
                            deadline: deadline,
                            logger: HTTPClient.loggingDisabled)
    }

    /// Execute arbitrary HTTP request and handle response processing using provided delegate.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - eventLoop: NIO Event Loop preference.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func execute(request: Request,
                        eventLoop eventLoopPreference: EventLoopPreference,
                        deadline: NIODeadline? = nil,
                        logger: Logger?) -> EventLoopFuture<Response> {
        let accumulator = ResponseAccumulator(request: request)
        return self.execute(request: request, delegate: accumulator, eventLoop: eventLoopPreference, deadline: deadline, logger: logger).futureResult
    }

    /// Execute arbitrary HTTP request and handle response processing using provided delegate.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - delegate: Delegate to process response parts.
    ///     - deadline: Point in time by which the request must complete.
    public func execute<Delegate: HTTPClientResponseDelegate>(request: Request,
                                                              delegate: Delegate,
                                                              deadline: NIODeadline? = nil) -> Task<Delegate.Response> {
        return self.execute(request: request, delegate: delegate, deadline: deadline, logger: HTTPClient.loggingDisabled)
    }

    /// Execute arbitrary HTTP request and handle response processing using provided delegate.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - delegate: Delegate to process response parts.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func execute<Delegate: HTTPClientResponseDelegate>(request: Request,
                                                              delegate: Delegate,
                                                              deadline: NIODeadline? = nil,
                                                              logger: Logger) -> Task<Delegate.Response> {
        return self.execute(request: request, delegate: delegate, eventLoop: .indifferent, deadline: deadline, logger: logger)
    }

    /// Execute arbitrary HTTP request and handle response processing using provided delegate.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - delegate: Delegate to process response parts.
    ///     - eventLoop: NIO Event Loop preference.
    ///     - deadline: Point in time by which the request must complete.
    ///     - logger: The logger to use for this request.
    public func execute<Delegate: HTTPClientResponseDelegate>(request: Request,
                                                              delegate: Delegate,
                                                              eventLoop eventLoopPreference: EventLoopPreference,
                                                              deadline: NIODeadline? = nil) -> Task<Delegate.Response> {
        return self.execute(request: request,
                            delegate: delegate,
                            eventLoop: eventLoopPreference,
                            deadline: deadline,
                            logger: HTTPClient.loggingDisabled)
    }

    /// Execute arbitrary HTTP request and handle response processing using provided delegate.
    ///
    /// - parameters:
    ///     - request: HTTP request to execute.
    ///     - delegate: Delegate to process response parts.
    ///     - eventLoop: NIO Event Loop preference.
    ///     - deadline: Point in time by which the request must complete.
    public func execute<Delegate: HTTPClientResponseDelegate>(request: Request,
                                                              delegate: Delegate,
                                                              eventLoop eventLoopPreference: EventLoopPreference,
                                                              deadline: NIODeadline? = nil,
                                                              logger originalLogger: Logger?) -> Task<Delegate.Response> {
        let logger = (originalLogger ?? HTTPClient.loggingDisabled).attachingRequestInformation(request, requestID: globalRequestID.add(1))
        let taskEL: EventLoop
        switch eventLoopPreference.preference {
        case .indifferent:
            taskEL = self.eventLoopGroup.next()
        case .delegate(on: let eventLoop):
            precondition(self.eventLoopGroup.makeIterator().contains { $0 === eventLoop }, "Provided EventLoop must be part of clients EventLoopGroup.")
            taskEL = eventLoop
        case .delegateAndChannel(on: let eventLoop):
            precondition(self.eventLoopGroup.makeIterator().contains { $0 === eventLoop }, "Provided EventLoop must be part of clients EventLoopGroup.")
            taskEL = eventLoop
        case .testOnly_exact(_, delegateOn: let delegateEL):
            taskEL = delegateEL
        }
        logger.trace("selected EventLoop for task given the preference",
                     metadata: ["ahc-eventloop": "\(taskEL)",
                                "ahc-el-preference": "\(eventLoopPreference)"])

        let failedTask: Task<Delegate.Response>? = self.stateLock.withLock {
            switch state {
            case .upAndRunning:
                return nil
            case .shuttingDown, .shutDown:
                logger.debug("client is shutting down, failing request")
                return Task<Delegate.Response>.failedTask(eventLoop: taskEL,
                                                          error: HTTPClientError.alreadyShutdown,
                                                          logger: logger)
            }
        }

        if let failedTask = failedTask {
            return failedTask
        }

        let redirectHandler: RedirectHandler<Delegate.Response>?
        switch self.configuration.redirectConfiguration.configuration {
        case .follow(let max, let allowCycles):
            var request = request
            if request.redirectState == nil {
                request.redirectState = .init(count: max, visited: allowCycles ? nil : Set())
            }
            redirectHandler = RedirectHandler<Delegate.Response>(request: request) { newRequest in
                self.execute(request: newRequest,
                             delegate: delegate,
                             eventLoop: eventLoopPreference,
                             deadline: deadline)
            }
        case .disallow:
            redirectHandler = nil
        }

        let task = Task<Delegate.Response>(eventLoop: taskEL, logger: logger)
        do {
            let requestBag = try RequestBag(
                request: request,
                eventLoopPreference: eventLoopPreference,
                task: task,
                redirectHandler: redirectHandler,
                connectionDeadline: .now() + (self.configuration.timeout.connect ?? .seconds(10)),
                requestOptions: .fromClientConfiguration(self.configuration),
                delegate: delegate
            )

            var deadlineSchedule: Scheduled<Void>?
            if let deadline = deadline {
                deadlineSchedule = taskEL.scheduleTask(deadline: deadline) {
                    requestBag.fail(HTTPClientError.deadlineExceeded)
                }

                task.promise.futureResult.whenComplete { _ in
                    deadlineSchedule?.cancel()
                }
            }

            self.poolManager.executeRequest(requestBag)
        } catch {
            task.fail(with: error, delegateType: Delegate.self)
        }

        return task
    }

    /// `HTTPClient` configuration.
    public struct Configuration {
        /// TLS configuration, defaults to `TLSConfiguration.makeClientConfiguration()`.
        public var tlsConfiguration: Optional<TLSConfiguration>
        /// Enables following 3xx redirects automatically, defaults to `RedirectConfiguration()`.
        ///
        /// Following redirects are supported:
        ///  - `301: Moved Permanently`
        ///  - `302: Found`
        ///  - `303: See Other`
        ///  - `304: Not Modified`
        ///  - `305: Use Proxy`
        ///  - `307: Temporary Redirect`
        ///  - `308: Permanent Redirect`
        public var redirectConfiguration: RedirectConfiguration
        /// Default client timeout, defaults to no `read` timeout and 10 seconds `connect` timeout.
        public var timeout: Timeout
        /// Connection pool configuration.
        public var connectionPool: ConnectionPool
        /// Upstream proxy, defaults to no proxy.
        public var proxy: Proxy?
        /// Enables automatic body decompression. Supported algorithms are gzip and deflate.
        public var decompression: Decompression
        /// Ignore TLS unclean shutdown error, defaults to `false`.
        public var ignoreUncleanSSLShutdown: Bool

        // TODO: make public
        // TODO: set to automatic by default
        /// HTTP/2 is by default disabled
        internal var httpVersion: HTTPVersion

        public init(
            tlsConfiguration: TLSConfiguration? = nil,
            redirectConfiguration: RedirectConfiguration? = nil,
            timeout: Timeout = Timeout(),
            connectionPool: ConnectionPool = ConnectionPool(),
            proxy: Proxy? = nil,
            ignoreUncleanSSLShutdown: Bool = false,
            decompression: Decompression = .disabled
        ) {
            self.init(
                tlsConfiguration: tlsConfiguration,
                redirectConfiguration: redirectConfiguration,
                timeout: timeout, connectionPool: connectionPool,
                proxy: proxy,
                ignoreUncleanSSLShutdown: ignoreUncleanSSLShutdown,
                decompression: decompression,
                // TODO: set to automatic by default
                httpVersion: .http1Only
            )
        }

        // TODO: make public
        internal init(
            tlsConfiguration: TLSConfiguration? = nil,
            redirectConfiguration: RedirectConfiguration? = nil,
            timeout: Timeout = Timeout(),
            connectionPool: ConnectionPool = ConnectionPool(),
            proxy: Proxy? = nil,
            ignoreUncleanSSLShutdown: Bool = false,
            decompression: Decompression = .disabled,
            httpVersion: HTTPVersion
        ) {
            self.tlsConfiguration = tlsConfiguration
            self.redirectConfiguration = redirectConfiguration ?? RedirectConfiguration()
            self.timeout = timeout
            self.connectionPool = connectionPool
            self.proxy = proxy
            self.ignoreUncleanSSLShutdown = ignoreUncleanSSLShutdown
            self.decompression = decompression
            self.httpVersion = httpVersion
        }

        public init(tlsConfiguration: TLSConfiguration? = nil,
                    redirectConfiguration: RedirectConfiguration? = nil,
                    timeout: Timeout = Timeout(),
                    proxy: Proxy? = nil,
                    ignoreUncleanSSLShutdown: Bool = false,
                    decompression: Decompression = .disabled) {
            self.init(
                tlsConfiguration: tlsConfiguration,
                redirectConfiguration: redirectConfiguration,
                timeout: timeout,
                connectionPool: ConnectionPool(),
                proxy: proxy,
                ignoreUncleanSSLShutdown: ignoreUncleanSSLShutdown,
                decompression: decompression
            )
        }

        public init(certificateVerification: CertificateVerification,
                    redirectConfiguration: RedirectConfiguration? = nil,
                    timeout: Timeout = Timeout(),
                    maximumAllowedIdleTimeInConnectionPool: TimeAmount = .seconds(60),
                    proxy: Proxy? = nil,
                    ignoreUncleanSSLShutdown: Bool = false,
                    decompression: Decompression = .disabled) {
            var tlsConfig = TLSConfiguration.makeClientConfiguration()
            tlsConfig.certificateVerification = certificateVerification
            self.init(tlsConfiguration: tlsConfig,
                      redirectConfiguration: redirectConfiguration,
                      timeout: timeout,
                      connectionPool: ConnectionPool(),
                      proxy: proxy,
                      ignoreUncleanSSLShutdown: ignoreUncleanSSLShutdown,
                      decompression: decompression)
        }

        public init(certificateVerification: CertificateVerification,
                    redirectConfiguration: RedirectConfiguration? = nil,
                    timeout: Timeout = Timeout(),
                    connectionPool: TimeAmount = .seconds(60),
                    proxy: Proxy? = nil,
                    ignoreUncleanSSLShutdown: Bool = false,
                    decompression: Decompression = .disabled,
                    backgroundActivityLogger: Logger?) {
            var tlsConfig = TLSConfiguration.makeClientConfiguration()
            tlsConfig.certificateVerification = certificateVerification
            self.init(tlsConfiguration: tlsConfig,
                      redirectConfiguration: redirectConfiguration,
                      timeout: timeout,
                      connectionPool: ConnectionPool(),
                      proxy: proxy,
                      ignoreUncleanSSLShutdown: ignoreUncleanSSLShutdown,
                      decompression: decompression)
        }

        public init(certificateVerification: CertificateVerification,
                    redirectConfiguration: RedirectConfiguration? = nil,
                    timeout: Timeout = Timeout(),
                    proxy: Proxy? = nil,
                    ignoreUncleanSSLShutdown: Bool = false,
                    decompression: Decompression = .disabled) {
            self.init(
                certificateVerification: certificateVerification,
                redirectConfiguration: redirectConfiguration,
                timeout: timeout,
                maximumAllowedIdleTimeInConnectionPool: .seconds(60),
                proxy: proxy,
                ignoreUncleanSSLShutdown: ignoreUncleanSSLShutdown,
                decompression: decompression
            )
        }
    }

    /// Specifies how `EventLoopGroup` will be created and establishes lifecycle ownership.
    public enum EventLoopGroupProvider {
        /// `EventLoopGroup` will be provided by the user. Owner of this group is responsible for its lifecycle.
        case shared(EventLoopGroup)
        /// `EventLoopGroup` will be created by the client. When `syncShutdown` is called, created `EventLoopGroup` will be shut down as well.
        case createNew
    }

    /// Specifies how the library will treat event loop passed by the user.
    public struct EventLoopPreference {
        enum Preference {
            /// Event Loop will be selected by the library.
            case indifferent
            /// The delegate will be run on the specified EventLoop (and the Channel if possible).
            case delegate(on: EventLoop)
            /// The delegate and the `Channel` will be run on the specified EventLoop.
            case delegateAndChannel(on: EventLoop)

            case testOnly_exact(channelOn: EventLoop, delegateOn: EventLoop)
        }

        var preference: Preference

        init(_ preference: Preference) {
            self.preference = preference
        }

        /// Event Loop will be selected by the library.
        public static let indifferent = EventLoopPreference(.indifferent)

        /// The delegate will be run on the specified EventLoop (and the Channel if possible).
        ///
        /// This will call the configured delegate on `eventLoop` and will try to use a `Channel` on the same
        /// `EventLoop` but will not establish a new network connection just to satisfy the `EventLoop` preference if
        /// another existing connection on a different `EventLoop` is readily available from a connection pool.
        public static func delegate(on eventLoop: EventLoop) -> EventLoopPreference {
            return EventLoopPreference(.delegate(on: eventLoop))
        }

        /// The delegate and the `Channel` will be run on the specified EventLoop.
        ///
        /// Use this for use-cases where you prefer a new connection to be established over re-using an existing
        /// connection that might be on a different `EventLoop`.
        public static func delegateAndChannel(on eventLoop: EventLoop) -> EventLoopPreference {
            return EventLoopPreference(.delegateAndChannel(on: eventLoop))
        }
    }

    /// Specifies decompression settings.
    public enum Decompression {
        /// Decompression is disabled.
        case disabled
        /// Decompression is enabled.
        case enabled(limit: NIOHTTPDecompression.DecompressionLimit)
    }

    enum State {
        case upAndRunning
        case shuttingDown(requiresCleanClose: Bool, callback: (Error?) -> Void)
        case shutDown
    }
}

extension HTTPClient.Configuration {
    /// Timeout configuration.
    public struct Timeout {
        /// Specifies connect timeout. If no connect timeout is given, a default 30 seconds timeout will applied.
        public var connect: TimeAmount?
        /// Specifies read timeout.
        public var read: TimeAmount?

        /// internal connection creation timeout. Defaults the connect timeout to always contain a value.
        var connectionCreationTimeout: TimeAmount {
            self.connect ?? .seconds(10)
        }

        /// Create timeout.
        ///
        /// - parameters:
        ///     - connect: `connect` timeout. Will default to 10 seconds, if no value is
        ///       provided. See `var connectionCreationTimeout`
        ///     - read: `read` timeout.
        public init(connect: TimeAmount? = nil, read: TimeAmount? = nil) {
            self.connect = connect
            self.read = read
        }
    }

    /// Specifies redirect processing settings.
    public struct RedirectConfiguration {
        enum Configuration {
            /// Redirects are not followed.
            case disallow
            /// Redirects are followed with a specified limit.
            case follow(max: Int, allowCycles: Bool)
        }

        var configuration: Configuration

        init() {
            self.configuration = .follow(max: 5, allowCycles: false)
        }

        init(configuration: Configuration) {
            self.configuration = configuration
        }

        /// Redirects are not followed.
        public static let disallow = RedirectConfiguration(configuration: .disallow)

        /// Redirects are followed with a specified limit.
        ///
        /// - parameters:
        ///     - max: The maximum number of allowed redirects.
        ///     - allowCycles: Whether cycles are allowed.
        ///
        /// - warning: Cycle detection will keep all visited URLs in memory which means a malicious server could use this as a denial-of-service vector.
        public static func follow(max: Int, allowCycles: Bool) -> RedirectConfiguration { return .init(configuration: .follow(max: max, allowCycles: allowCycles)) }
    }

    /// Connection pool configuration.
    public struct ConnectionPool: Hashable {
        /// Specifies amount of time connections are kept idle in the pool. After this time has passed without a new
        /// request the connections are closed.
        public var idleTimeout: TimeAmount

        /// The maximum number of connections that are kept alive in the connection pool per host. If requests with
        /// an explicit eventLoopRequirement are sent, this number might be exceeded due to overflow connections.
        public var concurrentHTTP1ConnectionsPerHostSoftLimit: Int

        public init(idleTimeout: TimeAmount = .seconds(60)) {
            self.init(idleTimeout: idleTimeout, concurrentHTTP1ConnectionsPerHostSoftLimit: 8)
        }

        public init(idleTimeout: TimeAmount, concurrentHTTP1ConnectionsPerHostSoftLimit: Int) {
            self.idleTimeout = idleTimeout
            self.concurrentHTTP1ConnectionsPerHostSoftLimit = concurrentHTTP1ConnectionsPerHostSoftLimit
        }
    }

    // TODO: make this struct and its static properties public
    internal struct HTTPVersion {
        internal enum Configuration {
            case http1Only
            case automatic
        }

        /// we only use HTTP/1, even if the server would supports HTTP/2
        internal static let http1Only: Self = .init(configuration: .http1Only)

        /// HTTP/2 is used if we connect to a server with HTTPS and the server supports HTTP/2, otherwise we use HTTP/1
        internal static let automatic: Self = .init(configuration: .automatic)

        internal var configuration: Configuration
    }
}

/// Possible client errors.
public struct HTTPClientError: Error, Equatable, CustomStringConvertible {
    private enum Code: Equatable {
        case invalidURL
        case emptyHost
        case missingSocketPath
        case alreadyShutdown
        case emptyScheme
        case unsupportedScheme(String)
        case readTimeout
        case remoteConnectionClosed
        case cancelled
        case identityCodingIncorrectlyPresent
        case chunkedSpecifiedMultipleTimes
        case invalidProxyResponse
        case contentLengthMissing
        case proxyAuthenticationRequired
        case redirectLimitReached
        case redirectCycleDetected
        case uncleanShutdown
        case traceRequestWithBody
        case invalidHeaderFieldNames([String])
        case bodyLengthMismatch
        case writeAfterRequestSent
        case incompatibleHeaders
        case connectTimeout
        case socksHandshakeTimeout
        case httpProxyHandshakeTimeout
        case tlsHandshakeTimeout
        case serverOfferedUnsupportedApplicationProtocol(String)
        case requestStreamCancelled
        case getConnectionFromPoolTimeout
        case deadlineExceeded
        case httpEndReceivedAfterHeadWith1xx
    }

    private var code: Code

    private init(code: Code) {
        self.code = code
    }

    public var description: String {
        return "HTTPClientError.\(String(describing: self.code))"
    }

    /// URL provided is invalid.
    public static let invalidURL = HTTPClientError(code: .invalidURL)
    /// URL does not contain host.
    public static let emptyHost = HTTPClientError(code: .emptyHost)
    /// URL does not contain a socketPath as a host for http(s)+unix shemes.
    public static let missingSocketPath = HTTPClientError(code: .missingSocketPath)
    /// Client is shutdown and cannot be used for new requests.
    public static let alreadyShutdown = HTTPClientError(code: .alreadyShutdown)
    /// URL does not contain scheme.
    public static let emptyScheme = HTTPClientError(code: .emptyScheme)
    /// Provided URL scheme is not supported, supported schemes are: `http` and `https`
    public static func unsupportedScheme(_ scheme: String) -> HTTPClientError { return HTTPClientError(code: .unsupportedScheme(scheme)) }
    /// Request timed out.
    public static let readTimeout = HTTPClientError(code: .readTimeout)
    /// Remote connection was closed unexpectedly.
    public static let remoteConnectionClosed = HTTPClientError(code: .remoteConnectionClosed)
    /// Request was cancelled.
    public static let cancelled = HTTPClientError(code: .cancelled)
    /// Request contains invalid identity encoding.
    public static let identityCodingIncorrectlyPresent = HTTPClientError(code: .identityCodingIncorrectlyPresent)
    /// Request contains multiple chunks definitions.
    public static let chunkedSpecifiedMultipleTimes = HTTPClientError(code: .chunkedSpecifiedMultipleTimes)
    /// Proxy response was invalid.
    public static let invalidProxyResponse = HTTPClientError(code: .invalidProxyResponse)
    /// Request does not contain `Content-Length` header.
    public static let contentLengthMissing = HTTPClientError(code: .contentLengthMissing)
    /// Proxy Authentication Required.
    public static let proxyAuthenticationRequired = HTTPClientError(code: .proxyAuthenticationRequired)
    /// Redirect Limit reached.
    public static let redirectLimitReached = HTTPClientError(code: .redirectLimitReached)
    /// Redirect Cycle detected.
    public static let redirectCycleDetected = HTTPClientError(code: .redirectCycleDetected)
    /// Unclean shutdown.
    public static let uncleanShutdown = HTTPClientError(code: .uncleanShutdown)
    /// A body was sent in a request with method TRACE.
    public static let traceRequestWithBody = HTTPClientError(code: .traceRequestWithBody)
    /// Header field names contain invalid characters.
    public static func invalidHeaderFieldNames(_ names: [String]) -> HTTPClientError { return HTTPClientError(code: .invalidHeaderFieldNames(names)) }
    /// Body length is not equal to `Content-Length`.
    public static let bodyLengthMismatch = HTTPClientError(code: .bodyLengthMismatch)
    /// Body part was written after request was fully sent.
    public static let writeAfterRequestSent = HTTPClientError(code: .writeAfterRequestSent)
    /// Incompatible headers specified, for example `Transfer-Encoding` and `Content-Length`.
    public static let incompatibleHeaders = HTTPClientError(code: .incompatibleHeaders)
    /// Creating a new tcp connection timed out
    public static let connectTimeout = HTTPClientError(code: .connectTimeout)
    /// The socks handshake timed out.
    public static let socksHandshakeTimeout = HTTPClientError(code: .socksHandshakeTimeout)
    /// The http proxy connection creation timed out.
    public static let httpProxyHandshakeTimeout = HTTPClientError(code: .httpProxyHandshakeTimeout)
    /// The tls handshake timed out.
    public static let tlsHandshakeTimeout = HTTPClientError(code: .tlsHandshakeTimeout)
    /// The remote server only offered an unsupported application protocol
    public static func serverOfferedUnsupportedApplicationProtocol(_ proto: String) -> HTTPClientError {
        return HTTPClientError(code: .serverOfferedUnsupportedApplicationProtocol(proto))
    }

    /// The request deadline was exceeded. The request was cancelled because of this.
    public static let deadlineExceeded = HTTPClientError(code: .deadlineExceeded)

    /// The remote server responded with a status code >= 300, before the full request was sent. The request stream
    /// was therefore cancelled
    public static let requestStreamCancelled = HTTPClientError(code: .requestStreamCancelled)

    /// Aquiring a HTTP connection from the connection pool timed out.
    ///
    /// This can have multiple reasons:
    ///  - A connection could not be created within the timout period.
    ///  - Tasks are not processed fast enough on the existing connections, to process all waiters in time
    public static let getConnectionFromPoolTimeout = HTTPClientError(code: .getConnectionFromPoolTimeout)

    public static let httpEndReceivedAfterHeadWith1xx = HTTPClientError(code: .httpEndReceivedAfterHeadWith1xx)
}