Zero-argument server function calls 400 behind hosts that convert Node requests (empty POST body stream parses as an argument)

[ryansolid]

Describe the bug

A zero-argument server function call fails with 400 Malformed server function arguments when the request reaches handleServerFunctionRequest through a host that converts a Node IncomingMessage into a web Request.

The client transport sends zero-argument calls as a bodyless POST (initializeResponse: args.length === 0 → createRequest with no body, no body-format tag), and the browser adds Content-Length: 0. The runtime detects "no arguments" via request.body === null:

// packages/web/server-functions/src/server.ts:1171 (parseArguments)
if (request.method === "POST" && request.body !== null) {
  const decoded = await extractBody(request.clone(), codec);
  ...
  if (decoded === undefined) {
    throw new TypeError("Server function body carries no usable encoding"); // :1234
  }

But Node-to-web conversions (Astro's dev/prod request handling, and the same pattern in every connect/Node adapter ecosystem) attach the socket stream as the body of every non-GET/HEAD request — including empty ones. So the empty POST arrives with request.body !== null, extractBody finds no format tag and no content type, answers undefined, and the guard from #3130 refuses the call: 400 on every zero-argument invocation.

Functions with arguments are unaffected (their bodies carry a format tag). The plugin's own dev middleware path doesn't hit it, which is why this only surfaces behind meta-framework hosts — found while building the Astro @astrojs/solid-js v8 adapter, where every zero-arg call through the injected endpoint route 400'd.

Reproduction

import { handleServerFunctionRequest, registerServerFunction } from '@solidjs/web/server-functions/server';

registerServerFunction('whoami-00000000', async () => 'me');

// What a Node-converted bodyless POST looks like: empty stream, Content-Length: 0.
const response = await handleServerFunctionRequest(
  new Request('http://localhost/_server/data/whoami-00000000', {
    method: 'POST',
    headers: { 'content-length': '0' },
    body: new ReadableStream({ start(c) { c.close(); } }),
    duplex: 'half',
  }),
);
// 400 — expected: 200 with the encoded 'me'

Same request with body: null answers 200.

Expected behavior

An empty POST body with no body-format tag is a zero-argument call, not a malformed one. parseArguments (or the dispatch ahead of it) could treat Content-Length: 0 as bodyless — or buffer the untagged/typeless body and skip the undefined-decode refusal when it is actually empty. The #3130 guard stays intact for tagged bodies and non-empty untagged ones.

Hosts can work around it by rebuilding the request when content-length is 0 (the Astro adapter now does), but every Node-based host conversion produces this shape, so the runtime accepting it would fix the whole class.

@solidjs/web 2.0.0-rc.5 / solid-js 2.0.0-rc.5.

[ryansolid]

Fixed on next by ee73e05. Empty untagged POST streams are now treated as zero-argument calls after inspecting the actual body bytes; non-empty and invalidly tagged bodies remain refused.