[2.0.0-rc.18] renderToStream: one value seroval cannot serialize leaves every value in its hydration batch unresolved #4242

Summary

On the server, a Loading boundary whose async memo resolves to another Loading boundary (an Element) streams a document in which none of the hydration batch's boundaries ever resolves on rc.18. On rc.17 only the outer memo's own value was left unresolved, and the boundaries themselves completed.

The outer memo's value is the inner boundary's function ($lhSkip), which seroval cannot serialize, and renderToStream logs SerovalUnsupportedTypeError on both versions. On rc.17 that error drops just that key. On rc.18 the same error drops every value written in the same $B batch, including both boundaries' completion flags (_fr) and the inner memo's value, which serialized fine. The client then waits forever on those boundaries.

seroval 1.7.1 forced through overrides behaves like rc.18's pinned 1.7.0, so the change looks like how rc.18's writer batches values, with seroval 1.7 ending a write at its first failing value.

Reproduction

In a fresh directory, install one version pair:

npm install --save-exact solid-js@2.0.0-rc.17 @solidjs/web@2.0.0-rc.17
# Then repeat in a separate directory with both packages at 2.0.0-rc.18.
node --conditions=node repro.mjs

repro.mjs:

import { createComponent, createMemo, Loading } from "solid-js";
import { renderToStream } from "@solidjs/web";

// A Loading boundary whose async memo resolves to a value.
function slot(read, fallback) {
  return createComponent(Loading, {
    fallback,
    get children() {
      const value = createMemo(async () => read());
      return [() => value()];
    }
  });
}

const flat = () => slot(async () => "flat-41", "outer");
// The outer memo resolves to an inner Loading boundary (an Element).
const nested = () =>
  slot(() => Promise.resolve(1).then(v => slot(async () => `nested-${v * 41}`, "inner")), "outer");

function render(app) {
  return new Promise(resolve => {
    let text = "";
    renderToStream(app).pipe({ write: c => void (text += c), end: () => resolve(text) });
  });
}

for (const [name, app] of [["flat", flat], ["nested", nested]]) {
  const text = await render(app);
  const keys = [...text.matchAll(/"?([\w]+)"?:\$R\[(\d+)\]=\(\$R\[(\d+)\]=/g)].map(m => [m[1], m[3]]);
  const settled = new Set([...text.matchAll(/\(\$R\[(\d+)\],/g)].map(m => m[1]));
  console.log(`${name}: body ${JSON.stringify(text.slice(0, text.indexOf("<script>")))}`);
  for (const [key, ref] of keys) console.log(`  ${key}: ${settled.has(ref) ? "resolved" : "NEVER RESOLVED"}`);
}

Output

rc.17:

flat: body "flat-41"
  0_fr: resolved
  0000: resolved
SerovalUnsupportedTypeError: Seroval Error (specific: 1)
  value: [Function (anonymous)] { '$lhSkip': true }
nested: body "nested-41"
  0_fr: resolved
  0000: NEVER RESOLVED
  undefined_fr: resolved
  undefined000: resolved

rc.18 (and rc.18 with seroval 1.7.1):

flat: body "flat-41"
  0_fr: resolved
  0000: resolved
SerovalUnsupportedTypeError: Seroval Error (specific: 1)
  value: [Function (anonymous)] { '$lhSkip': true }
nested: body "nested-41"
  0_fr: NEVER RESOLVED
  0000: NEVER RESOLVED
  undefined_fr: NEVER RESOLVED
  undefined000: NEVER RESOLVED

The raw rc.18 tail is ..._$HY.r["$B"]=($R[0]={"0_fr":...,"0000":...,undefined_fr:...,undefined000:...});(b=>{...})(_$HY.r["$B"]);</script> with no resolver calls after it, where rc.17 ends with ($R[9],"nested-41");$R[11]($R[2],!0);$R[11]($R[7],!0);.

The same rule reaches values an app writes itself. Two writes in one tick, one resolving to a function, drop the other on rc.18 and stream it on rc.17:

import { renderToStream, getHydrationWriter } from "@solidjs/web";

const app = () => {
  const writer = getHydrationWriter();
  writer.write("bad", Promise.resolve(() => 10));
  writer.write("fine", Promise.resolve("still here"));
  return "odd";
};
const text = await new Promise(resolve => {
  let t = "";
  renderToStream(app).pipe({ write: c => void (t += c), end: () => resolve(t) });
});
console.log(`"still here" streamed: ${text.includes("still here")}`);

This prints true on rc.17 and false on rc.18, after the same SerovalUnsupportedTypeError on both.

Expected

A value that cannot be serialized fails only its own key, as on rc.17, and the boundaries in the same batch still complete. Separately, the inner boundary's keys read undefined_fr and undefined000 on both versions, which looks like a missing id for a boundary created inside a resolved memo.

Environment

Node 26.10.0, solid-js and @solidjs/web 2.0.0-rc.17 and 2.0.0-rc.18 (seroval 1.6.7 and 1.7.0 as each pins), Linux.