[Proposal] New HTTP loader for URLSession (#1135)

Author: guoye-zhang

Proposals/0018-urlsession-new-loader.md

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+# New HTTP loader for URLSession
+
+* Proposal: [SF-0018](0018-urlsession-new-loader.md)
+* Authors: [Guoye Zhang](https://github.com/guoye-zhang)
+* Review Manager: [Tina L](https://github.com/itingliu)
+* Status: **Accepted**
+* Implementation: [swiftlang/swift-corelibs-foundation#5162](https://github.com/swiftlang/swift-corelibs-foundation/pull/5162)
+* Review: ([pitch](https://forums.swift.org/t/pitch-new-http-loader-for-urlsession-on-darwin/77440))
+
+## Introduction
+
+The new HTTP stack built in Network framework was enabled in Safari on iOS 18 and macOS Sequoia. It supports new features like 0-RTT early data, oblivious HTTP, WebSocket over HTTP/2, and unix domain socket. Support for FTP and HTTP/1 pipelining was dropped.
+
+In this proposal, we are introducing an API to enable the new HTTP loader in URLSession on Darwin, as well as deprecating several legacy features not supported in the new stack.
+
+## Proposed solution
+
+Introducing a new flag `usesClassicLoadingMode` that defaults to true on URLSessionConfiguration to allow developers to opt-in to the new stack.
+
+The flag is not available on non-Darwin platforms at this moment.
+
+## Detailed design
+
+### Opt-in flag
+
+The new loader is enabled when `usesClassicLoadingMode` is set to false. `usesClassicLoadingMode` currently defaults to true, but might default to false in a future OS.
+
+```swift
+open class URLSessionConfiguration {
+
+    /* Uses the classic network loader */
+    @available(*, unavailable, message: "Not available on non-Darwin platforms")
+    open var usesClassicLoadingMode: Bool
+}
+```
+
+### Deprecations
+
+* FTP was deprecated in the old stack and unsupported in the new stack.
+* HTTP/1 pipelining is not supported in the new HTTP stack. It is disabled by default in the old stack and has known compatibility issues with some servers.
+* Server push was disabled in both stacks on iOS 17 and aligned releases due to low adoption and web compatibility issues.
+* `NSURLErrorFailingURLStringErrorKey` is redundant with `NSURLErrorFailingURLErrorKey` and it is incompatible with WHATWG URLs.
+* `shouldUseExtendedBackgroundIdleMode` was not supported in either the old stack or the new stack.
+
+```swift
+public struct URLRequest {
+
+    @available(swift, deprecated: 6.1, message: "HTTP/1 pipelining has known compatibility issues, please adopt HTTP/2 and HTTP/3 instead")
+    public var httpShouldUsePipelining: Bool
+}
+
+open class NSURLRequest {
+
+    @available(swift, deprecated: 6.1, message: "HTTP/1 pipelining has known compatibility issues, please adopt HTTP/2 and HTTP/3 instead")
+    open var httpShouldUsePipelining: Bool { get }
+}
+
+open class NSMutableURLRequest {
+
+    @available(swift, deprecated: 6.1, message: "HTTP/1 pipelining has known compatibility issues, please adopt HTTP/2 and HTTP/3 instead")
+    open override var httpShouldUsePipelining: Bool
+}
+
+open class URLSessionConfiguration {
+
+    @available(swift, deprecated: 6.1, message: "HTTP/1 pipelining has known compatibility issues, please adopt HTTP/2 and HTTP/3 instead")
+    open var httpShouldUsePipelining: Bool
+
+    @available(swift, deprecated: 6.1, message: "Not supported")
+    open var shouldUseExtendedBackgroundIdleMode: Bool
+}
+
+public enum URLSessionTaskMetrics.ResourceFetchType {
+
+    @available(swift, deprecated: 6.1, message: "Server push is not supported")
+    case serverPush
+}
+
+public struct URLError {
+
+    @available(swift, deprecated: 6.1, message: "Use failingURL instead")
+    public var failureURLString: String?
+}
+
+@available(swift, deprecated: 6.1, message: "Use NSURLErrorFailingURLErrorKey instead")
+public let NSURLErrorFailingURLStringErrorKey: String
+
+@available(swift, deprecated: 6.1, message: "FTP is deprecated")
+public let NSURLProtectionSpaceFTP: String
+
+@available(swift, deprecated: 6.1, message: "FTP is deprecated")
+public let NSURLProtectionSpaceFTPProxy: String
+```
+
+## Source compatibility
+
+No impact.
+
+## Implications on adoption
+
+This feature can be freely adopted and un-adopted in source code with no deployment constraints and without affecting source compatibility.
+
+## Future directions
+
+The new HTTP stack might become the default on a future Darwin release.
+
+## Alternatives considered
+
+### Alternative spelling `usesNewLoader`
+
+The initial idea was an opt-in flag where YES means the new loader, and we considered multiple variants, including `usesModernLoader`, `usesNWLoader` (NW stands for Network framework). However, this makes it hard to come up with names for newer loaders in the future, and an opt-out flag gives the impression that the new loader is the default, not the exception.