FetchHTTPClient for WASM

[xbhatnag]

Motivation

This PR proposes FetchHTTPClient: a HTTP Client implementation for Swift WASM built on top of Javascript's fetch() API.

Modifications

• Added a barebones FetchHTTPClient implementation of HTTPClient
• Added an example program Examples/WASMClient that uses FetchHTTPClient to make arbitrary network requests (within the confines of what the browser will allow).
• Added Examples/WASMClient/README.md to explain how to build and run the example program.

Note: WASM targets only build when the HTTP_API_ENABLE_WASM environment variable is set on the host system.

Feature set

✅ Request and response headers
✅ Streamed response bodies
⚠️ Request bodies (request body is collected upfront prior to the request because some browsers don't support streaming request bodies)
⚠️ URL parsing (swift-http-types doesn't allow scheme-less URLs, which are valid in Javascript, for example: fetch("/index.html"))
❌ Request options (there are many options that exist in JS RequestInit that still need to be exposed)
❌ Trailers (browsers just don't support them)
❌ Swift Task Cancellation (broken because the cancel doesn't propagate to JS fetch())

[xbhatnag]

large_response_body.mov

post.mov

[xbhatnag]

Online searches suggest I can just do bytes.append(contentsOf: span) but the compiler was not having it, because Span does not implement Sequence?

Still, there's gotta be a better way than adding byte by byte!

[xbhatnag]

This environment variable now conditionally adds the WASM targets in for compilation and is disabled by default.

[xbhatnag]

I hacked up our Client Conformance Tests to work in WASM with the Test Server hosted out of process. Then I ran the test on Safari + Mac.

These tests failed as expected:

• EchoInterleave and SpeakInterleave: Not surprising, because we are collecting the request body upfront. Some browsers cannot stream the request body.
• BasicCookieSetAndUse: Not surprising, because browsers don't reveal the Set-Cookie header with JS/WASM.
• TrailerRead and TrailerWrite: Not surprising, because browsers don't support trailers at all.

These were the unexpected failures that need addressing:

• CancelPreBody and CancelPreHeaders: Cancellation is currently broken with fetch(). We need to setup a JS AbortController and use that when the Swift task is cancelled.
• The browser is modifying the case of the request header names (forcing them to lowercase), which is breaking some assumptions in our conformance tests. To be fair, this is actually an issue with the conformance tests. The tests should be case-insensitive when checking headers.
• ETag: Have yet to investigate why this is failing. Multiple assertions are coming up false.

Everything else passed 🎉