Revert "Back out new typed HTTP protocol upgrader (#2579)"

# Motivation
We have reverted the typed HTTP protocol upgrader pieces since adopters were running into a compiler bug (swiftlang/swift#69459) that caused the compiler to emit strong references to `swift_getExtendedExistentialTypeMetadata`. The problem is that `swift_getExtendedExistentialTypeMetadata` is not available on older runtimes before constrained existentials have been introduced. This caused adopters to run into runtime crashes when loading any library compiled with this NIO code.

# Modifications
This PR reverts the revert and guard all new code in a compiler guard that checks that we are either on non-Darwin platforms or on a new enough Swift compiler that contains the fix.

# Result
We can offer the typed HTTP upgrade code to our adopters again.

Sources/NIOHTTP1/HTTPTypedPipelineSetup.swift

@@ -0,0 +1,250 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftNIO open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftNIO project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+#if !canImport(Darwin) || (canImport(Darwin) && swift(>=5.10))
+import NIOCore
+
+// MARK: - Server pipeline configuration
+
+/// Configuration for an upgradable HTTP pipeline.
+@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+public struct NIOUpgradableHTTPServerPipelineConfiguration<UpgradeResult: Sendable> {
+    /// Whether to provide assistance handling HTTP clients that pipeline
+    /// their requests. Defaults to `true`. If `false`, users will need to handle clients that pipeline themselves.
+    public var enablePipelining = true
+
+    /// Whether to provide assistance handling protocol errors (e.g. failure to parse the HTTP
+    /// request) by sending 400 errors. Defaults to `true`.
+    public var enableErrorHandling = true
+
+    /// Whether to validate outbound response headers to confirm that they are
+    /// spec compliant. Defaults to `true`.
+    public var enableResponseHeaderValidation = true
+
+    /// The configuration for the ``HTTPResponseEncoder``.
+    public var encoderConfiguration = HTTPResponseEncoder.Configuration()
+
+    /// The configuration for the ``NIOTypedHTTPServerUpgradeHandler``.
+    public var upgradeConfiguration: NIOTypedHTTPServerUpgradeConfiguration<UpgradeResult>
+
+    /// Initializes a new ``NIOUpgradableHTTPServerPipelineConfiguration`` with default values.
+    ///
+    /// The current defaults provide the following features:
+    /// 1. Assistance handling clients that pipeline HTTP requests.
+    /// 2. Assistance handling protocol errors.
+    /// 3. Outbound header fields validation to protect against response splitting attacks.
+    public init(
+        upgradeConfiguration: NIOTypedHTTPServerUpgradeConfiguration<UpgradeResult>
+    ) {
+        self.upgradeConfiguration = upgradeConfiguration
+    }
+}
+
+extension ChannelPipeline {
+    /// Configure a `ChannelPipeline` for use as an HTTP server.
+    ///
+    /// - Parameters:
+    ///   - configuration: The HTTP pipeline's configuration.
+    /// - Returns: An `EventLoopFuture` that will fire when the pipeline is configured. The future contains an `EventLoopFuture`
+    /// that is fired once the pipeline has been upgraded or not and contains the `UpgradeResult`.
+    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    public func configureUpgradableHTTPServerPipeline<UpgradeResult: Sendable>(
+        configuration: NIOUpgradableHTTPServerPipelineConfiguration<UpgradeResult>
+    ) -> EventLoopFuture<EventLoopFuture<UpgradeResult>> {
+        self._configureUpgradableHTTPServerPipeline(
+            configuration: configuration
+        )
+    }
+
+    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    private func _configureUpgradableHTTPServerPipeline<UpgradeResult: Sendable>(
+        configuration: NIOUpgradableHTTPServerPipelineConfiguration<UpgradeResult>
+    ) -> EventLoopFuture<EventLoopFuture<UpgradeResult>> {
+        let future: EventLoopFuture<EventLoopFuture<UpgradeResult>>
+
+        if self.eventLoop.inEventLoop {
+            let result = Result<EventLoopFuture<UpgradeResult>, Error> {
+                try self.syncOperations.configureUpgradableHTTPServerPipeline(
+                    configuration: configuration
+                )
+            }
+            future = self.eventLoop.makeCompletedFuture(result)
+        } else {
+            future = self.eventLoop.submit {
+                try self.syncOperations.configureUpgradableHTTPServerPipeline(
+                    configuration: configuration
+                )
+            }
+        }
+
+        return future
+    }
+}
+
+extension ChannelPipeline.SynchronousOperations {
+    /// Configure a `ChannelPipeline` for use as an HTTP server.
+    ///
+    /// - Parameters:
+    ///   - configuration: The HTTP pipeline's configuration.
+    /// - Returns: An `EventLoopFuture` that is fired once the pipeline has been upgraded or not and contains the `UpgradeResult`.
+    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    public func configureUpgradableHTTPServerPipeline<UpgradeResult: Sendable>(
+        configuration: NIOUpgradableHTTPServerPipelineConfiguration<UpgradeResult>
+    ) throws -> EventLoopFuture<UpgradeResult> {
+        self.eventLoop.assertInEventLoop()
+
+        let responseEncoder = HTTPResponseEncoder(configuration: configuration.encoderConfiguration)
+        let requestDecoder = ByteToMessageHandler(HTTPRequestDecoder(leftOverBytesStrategy: .forwardBytes))
+
+        var extraHTTPHandlers = [RemovableChannelHandler]()
+        extraHTTPHandlers.reserveCapacity(4)
+        extraHTTPHandlers.append(requestDecoder)
+
+        try self.addHandler(responseEncoder)
+        try self.addHandler(requestDecoder)
+
+        if configuration.enablePipelining {
+            let pipeliningHandler = HTTPServerPipelineHandler()
+            try self.addHandler(pipeliningHandler)
+            extraHTTPHandlers.append(pipeliningHandler)
+        }
+
+        if configuration.enableResponseHeaderValidation {
+            let headerValidationHandler = NIOHTTPResponseHeadersValidator()
+            try self.addHandler(headerValidationHandler)
+            extraHTTPHandlers.append(headerValidationHandler)
+        }
+
+        if configuration.enableErrorHandling {
+            let errorHandler = HTTPServerProtocolErrorHandler()
+            try self.addHandler(errorHandler)
+            extraHTTPHandlers.append(errorHandler)
+        }
+
+        let upgrader = NIOTypedHTTPServerUpgradeHandler(
+            httpEncoder: responseEncoder,
+            extraHTTPHandlers: extraHTTPHandlers,
+            upgradeConfiguration: configuration.upgradeConfiguration
+        )
+        try self.addHandler(upgrader)
+
+        return upgrader.upgradeResultFuture
+    }
+}
+
+// MARK: - Client pipeline configuration
+
+/// Configuration for an upgradable HTTP pipeline.
+@available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+public struct NIOUpgradableHTTPClientPipelineConfiguration<UpgradeResult: Sendable> {
+    /// The strategy to use when dealing with leftover bytes after removing the ``HTTPDecoder`` from the pipeline.
+    public var leftOverBytesStrategy = RemoveAfterUpgradeStrategy.dropBytes
+
+    /// Whether to validate outbound response headers to confirm that they are
+    /// spec compliant. Defaults to `true`.
+    public var enableOutboundHeaderValidation = true
+
+    /// The configuration for the ``HTTPRequestEncoder``.
+    public var encoderConfiguration = HTTPRequestEncoder.Configuration()
+
+    /// The configuration for the ``NIOTypedHTTPClientUpgradeHandler``.
+    public var upgradeConfiguration: NIOTypedHTTPClientUpgradeConfiguration<UpgradeResult>
+
+    /// Initializes a new ``NIOUpgradableHTTPClientPipelineConfiguration`` with default values.
+    ///
+    /// The current defaults provide the following features:
+    /// 1. Outbound header fields validation to protect against response splitting attacks.
+    public init(
+        upgradeConfiguration: NIOTypedHTTPClientUpgradeConfiguration<UpgradeResult>
+    ) {
+        self.upgradeConfiguration = upgradeConfiguration
+    }
+}
+
+extension ChannelPipeline {
+    /// Configure a `ChannelPipeline` for use as an HTTP client.
+    ///
+    /// - Parameters:
+    ///   - configuration: The HTTP pipeline's configuration.
+    /// - Returns: An `EventLoopFuture` that will fire when the pipeline is configured. The future contains an `EventLoopFuture`
+    /// that is fired once the pipeline has been upgraded or not and contains the `UpgradeResult`.
+    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    public func configureUpgradableHTTPClientPipeline<UpgradeResult: Sendable>(
+        configuration: NIOUpgradableHTTPClientPipelineConfiguration<UpgradeResult>
+    ) -> EventLoopFuture<EventLoopFuture<UpgradeResult>> {
+        self._configureUpgradableHTTPClientPipeline(configuration: configuration)
+    }
+
+    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    private func _configureUpgradableHTTPClientPipeline<UpgradeResult: Sendable>(
+        configuration: NIOUpgradableHTTPClientPipelineConfiguration<UpgradeResult>
+    ) -> EventLoopFuture<EventLoopFuture<UpgradeResult>> {
+        let future: EventLoopFuture<EventLoopFuture<UpgradeResult>>
+
+        if self.eventLoop.inEventLoop {
+            let result = Result<EventLoopFuture<UpgradeResult>, Error> {
+                try self.syncOperations.configureUpgradableHTTPClientPipeline(
+                    configuration: configuration
+                )
+            }
+            future = self.eventLoop.makeCompletedFuture(result)
+        } else {
+            future = self.eventLoop.submit {
+                try self.syncOperations.configureUpgradableHTTPClientPipeline(
+                    configuration: configuration
+                )
+            }
+        }
+
+        return future
+    }
+}
+
+extension ChannelPipeline.SynchronousOperations {
+    /// Configure a `ChannelPipeline` for use as an HTTP client.
+    ///
+    /// - Parameters:
+    ///   - configuration: The HTTP pipeline's configuration.
+    /// - Returns: An `EventLoopFuture` that is fired once the pipeline has been upgraded or not and contains the `UpgradeResult`.
+    @available(macOS 13, iOS 16, tvOS 16, watchOS 9, *)
+    public func configureUpgradableHTTPClientPipeline<UpgradeResult: Sendable>(
+        configuration: NIOUpgradableHTTPClientPipelineConfiguration<UpgradeResult>
+    ) throws -> EventLoopFuture<UpgradeResult> {
+        self.eventLoop.assertInEventLoop()
+
+        let requestEncoder = HTTPRequestEncoder(configuration: configuration.encoderConfiguration)
+        let responseDecoder = ByteToMessageHandler(HTTPResponseDecoder(leftOverBytesStrategy: configuration.leftOverBytesStrategy))
+        var httpHandlers = [RemovableChannelHandler]()
+        httpHandlers.reserveCapacity(3)
+        httpHandlers.append(requestEncoder)
+        httpHandlers.append(responseDecoder)
+
+        try self.addHandler(requestEncoder)
+        try self.addHandler(responseDecoder)
+
+        if configuration.enableOutboundHeaderValidation {
+            let headerValidationHandler = NIOHTTPRequestHeadersValidator()
+            try self.addHandler(headerValidationHandler)
+            httpHandlers.append(headerValidationHandler)
+        }
+
+        let upgrader = NIOTypedHTTPClientUpgradeHandler(
+            httpHandlers: httpHandlers,
+            upgradeConfiguration: configuration.upgradeConfiguration
+        )
+        try self.addHandler(upgrader)
+
+        return upgrader.upgradeResultFuture
+    }
+}
+#endif