[2.0 rc.14 / next] A synchronous prepareRequest hook makes every hydrated server-function read refetch in the browser #3959

Summary

With any prepareRequest configured, even an identity hook init => init, a page whose data comes from server functions fetches that data twice: once during SSR, and again from the browser right after hydration, although the serialized value was adopted. Without the hook there is no client request.

Reproduces on 2.0.0-rc.14 and on next at 358159c.

Reproduction

// client entry, before render
import { configureServerFunctionsClient } from "@solidjs/web/server-functions/client";
configureServerFunctionsClient({ prepareRequest: init => init });

// a page reading a "use server" function through a memo or derived store
const value = createMemo(() => readThing());
return <p>{value()}</p>;

Load the page directly (SSR, then hydrate) and watch the network:

The DOM is adopted correctly in both cases; the extra request is wasted (and, for private reads, doubles server work on every page load).

Cause

Hydration runs a serialized source's compute once, synchronously, inside subFetch (packages/solid/src/client/hydration.ts), with window.fetch swapped for a never-settling mock, then restores it. The server-function transport's createRequest (packages/web/server-functions/src/client.ts) does:

if (config.prepareRequest) {
  const prepared = await config.prepareRequest(init, { id, meta });

The await runs even when the hook returns synchronously, so send(base, init) happens a microtask later, after subFetch has restored the real fetch. The call goes to the network instead of the mock.

Fix

Await only when the hook returns a promise:

-    const prepared = await config.prepareRequest(init, { id, meta });
+    let prepared = config.prepareRequest(init, { id, meta });
+    if (typeof (prepared as PromiseLike<RequestInit>)?.then === "function") prepared = await prepared;

A synchronous hook then sends inside the hydration window like the default transport. An async hook still awaits, so it keeps the current behaviour (an async hook will always miss the synchronous window; that may be worth documenting). With this change the table above shows 0 client requests for both rows, for memo and store sources, sync and async. @solidjs/web's client, server and hydration test suites pass unchanged. I can open a PR.

(submitted by Claude Opus 5.5 on behalf of rvlzzr)