next/dist/esm/server/lib/clone-response.js

The React Framework

const noop = ()=>{};
let registry;
if (globalThis.FinalizationRegistry) {
    registry = new FinalizationRegistry((weakRef)=>{
        const stream = weakRef.deref();
        if (stream && !stream.locked) {
            stream.cancel('Response object has been garbage collected').then(noop);
        }
    });
}
/**
 * Clones a response by teeing the body so we can return two independent
 * ReadableStreams from it. This avoids the bug in the undici library around
 * response cloning.
 *
 * After cloning, the original response's body will be consumed and closed.
 *
 * @see https://github.com/vercel/next.js/pull/73274
 *
 * @param original - The original response to clone.
 * @returns A tuple containing two independent clones of the original response.
 */ export function cloneResponse(original) {
    // If the response has no body, then we can just return the original response
    // twice because it's immutable.
    if (!original.body) {
        return [
            original,
            original
        ];
    }
    const [body1, body2] = original.body.tee();
    const cloned1 = new Response(body1, {
        status: original.status,
        statusText: original.statusText,
        headers: original.headers
    });
    Object.defineProperty(cloned1, 'url', {
        value: original.url,
        // How the original response.url behaves
        configurable: true,
        enumerable: true,
        writable: false
    });
    // The Fetch Standard allows users to skip consuming the response body by
    // relying on garbage collection to release connection resources.
    // https://github.com/nodejs/undici?tab=readme-ov-file#garbage-collection
    //
    // To cancel the stream you then need to cancel both resulting branches.
    // Teeing a stream will generally lock it for the duration, preventing other
    // readers from locking it.
    // https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/tee
    // cloned2 is stored in a react cache and cloned for subsequent requests.
    // It is the original request, and is is garbage collected by a
    // FinalizationRegistry in Undici, but since we're tee-ing the stream
    // ourselves, we need to cancel clone1's stream (the response returned from
    // our dedupe fetch) when clone1 is reclaimed, otherwise we leak memory.
    if (registry && cloned1.body) {
        registry.register(cloned1, new WeakRef(cloned1.body));
    }
    const cloned2 = new Response(body2, {
        status: original.status,
        statusText: original.statusText,
        headers: original.headers
    });
    Object.defineProperty(cloned2, 'url', {
        value: original.url,
        // How the original response.url behaves
        configurable: true,
        enumerable: true,
        writable: false
    });
    return [
        cloned1,
        cloned2
    ];
}

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