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latest() of an unread identity memo returns undefined when first read in the flush that re-asks its source

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难度
4/5
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3-5 天
新手友好度
52/100
Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
活跃
技术栈
typescript
领域
frontend

调研方向

Start with the issue’s reproducer and trace the first read of latest(id) alongside isPending(id) when the Loading boundary mounts. Compare that case with creating the identity memo inside the mounted content, where the bug does not occur. Done when the reproducer no longer yields latest(id) === undefined after m has answered, while pending behavior remains correct.

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描述

On next (e49b0c769), latest() can return undefined, which D-22 rules out ("It returns T, never undefined").

Setup:

  • m is an async memo, and id = createMemo(() => m()) is an identity memo created at the root. Nothing reads id yet.
  • m's first answer (2) lands in its own flush.
  • A write then re-asks m. In that same flush, a Loading boundary mounts content whose render effect reads latest(id) and isPending(id).

The effect's first run sees latest(id) === undefined and isPending(id) === true. Because m has been answered, latest(id) should be 2. If id is created inside the mounted content instead, the bug does not happen.

const asks: { v: number; resolve: (v: number) => void }[] = [];
const [s, setS] = createSignal(1);
const m = createMemo(() => { const v = 1 + s(); return new Promise<number>(resolve => asks.push({ v, resolve })); });
const id = createMemo(() => m());
const leaves: string[] = [];
createRoot(() => {
  const b = createLoadingBoundary(() => {
    const gate = createMemo(() => s() > 1);
    return createMemo(() => {
      if (!gate()) return "off";
      createRenderEffect(() => `l:${latest(id)} p:${isPending(id)}`, v => void leaves.push(v));
      return "on";
    });
  }, () => "fallback");
  createRenderEffect(() => { let v: any = b; while (typeof v === "function") v = v(); return v; }, () => {});
});
flush(); await task(); flush();
const first = asks.shift()!; first.resolve(first.v);
await task(); flush();          // m's answer (2) lands on its own
setS(2); await task(); flush(); // re-ask m and mount the content
// leaves: ["l:undefined p:true", ...]

Found while reviewing #4459; #4459 does not change it.

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  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

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