Replace AsyncBackpressuredStream with updated implementation

[simonjbeaumont]

Motivation

As a follow up to #27, we noted it would be good to align on the latest draft implementation of SE-0406 (AsyncStream with backpressure) to both pickup the latest improvements in performance and correctness, and to minimise the churn if/when this lands in the standard library or standalone package.

Modifications

In order to simplify reviewing the following modifications have been made in independent commits:

• Add updated SE-0406 implementation as BufferedStream, incl. tests: Skim over this—it's vendored in wholesale.
• Port the custom watermark support to BufferedStream: Skim over this—it's a 1:1 port of the logic that was added to AsyncBackpressuredStream.
• Switch from AsyncBackpressuredStream to BufferedStream in delegate: Review this—it's a minimal change.
• Remove AsyncBackpressuredStream and its vendored locks: Skim over this—it's removing the old implementation.

Result

No functional change, but the internal async sequence we're using should be more robust, performant, and more likely to match a future standard library type.

Test Plan

• The new vendored BufferedStream actually comes with a much greater number of vendored tests than the previous revision.
• Also ported the tests from this repo for the custom watermark logic.
• All our URLSessionTransport-specific tests continue to pass.

[czechboy0]

lgtm. Please once this is released, double check that NIO isn't brought in during package resolution when depending on the transport as a library.

[simonjbeaumont]

lgtm. Please once this is released, double check that NIO isn't brought in during package resolution when depending on the transport as a library.

So here's an example package that depends on the source branch for this PR, showing that it doesn't clone NIO. The only new dependency this release will bring in is swift-collections.

❯ tree . ├── Package.swift └── Sources  └── OpenAPIURLSessionAdopter  └── Example.swift ❯ cat Package.swift // swift-tools-version: 5.9 import PackageDescription let package = Package(  name: "swift-openapi-urlsession-adopter",  platforms: [.macOS(.v12)],  dependencies: [  .package(  url: "https://github.com/simonjbeaumont/swift-openapi-urlsession",  branch: "sb/update-async-stream-implementation"  ),  ],  targets: [  .target(  name: "OpenAPIURLSessionAdopter",  dependencies: [  .product(name: "OpenAPIURLSession", package: "swift-openapi-urlsession"),  ]  ),  ] ) ❯ cat Sources/OpenAPIURLSessionAdopter/Example.swift import OpenAPIURLSession func f() {  _ = OpenAPIURLSession.URLSessionTransport() } ❯ swift build Fetching https://github.com/simonjbeaumont/swift-openapi-urlsession from cache Fetched https://github.com/simonjbeaumont/swift-openapi-urlsession (0.53s) Fetching https://github.com/apple/swift-collections from cache Fetching https://github.com/apple/swift-openapi-runtime from cache Fetching https://github.com/apple/swift-http-types from cache Fetched https://github.com/apple/swift-http-types (0.52s) Fetched https://github.com/apple/swift-collections (0.54s) Fetched https://github.com/apple/swift-openapi-runtime (0.65s) Computing version for https://github.com/apple/swift-collections Computed https://github.com/apple/swift-collections at 1.0.5 (0.50s) Computing version for https://github.com/apple/swift-openapi-runtime Computed https://github.com/apple/swift-openapi-runtime at 0.3.6 (0.43s) Computing version for https://github.com/apple/swift-http-types Computed https://github.com/apple/swift-http-types at 1.0.0 (0.52s) Creating working copy for https://github.com/simonjbeaumont/swift-openapi-urlsession Working copy of https://github.com/simonjbeaumont/swift-openapi-urlsession resolved at sb/update-async-stream-implementation Creating working copy for https://github.com/apple/swift-openapi-runtime Working copy of https://github.com/apple/swift-openapi-runtime resolved at 0.3.6 Creating working copy for https://github.com/apple/swift-collections Working copy of https://github.com/apple/swift-collections resolved at 1.0.5 Creating working copy for https://github.com/apple/swift-http-types Working copy of https://github.com/apple/swift-http-types resolved at 1.0.0 Building for debugging... [84/84] Compiling OpenAPIURLSessionAdopter Example.swift Build complete! (17.54s) ❯ cat Package.resolved | jq .pins[].location "https://github.com/apple/swift-collections" "https://github.com/apple/swift-http-types" "https://github.com/apple/swift-openapi-runtime" "https://github.com/simonjbeaumont/swift-openapi-urlsession" ❯ swift package show-dependencies . └── swift-openapi-urlsession<https://github.com/simonjbeaumont/swift-openapi-urlsession@unspecified>  ├── swift-openapi-runtime<https://github.com/apple/swift-openapi-runtime@0.3.6>  │ └── swift-http-types<https://github.com/apple/swift-http-types@1.0.0>  └── swift-collections<https://github.com/apple/swift-collections@1.0.5>

[simonjbeaumont]

@swift-server-bot test this please