next.js/packages/next/src/server/body-streams.ts

import type { IncomingMessage } from 'http'
import type { Readable } from 'stream'
import { PassThrough } from 'stream'

export function requestToBodyStream(
  context: { ReadableStream: typeof ReadableStream },
  KUint8Array: typeof Uint8Array,
  stream: Readable
) {
  return new context.ReadableStream({
    start: async (controller) => {
      for await (const chunk of stream) {
        controller.enqueue(new KUint8Array(chunk))
      }
      controller.close()
    },
  })
}

function replaceRequestBody<T extends IncomingMessage>(
  base: T,
  stream: Readable
): T {
  for (const key in stream) {
    let v = stream[key as keyof Readable] as any
    if (typeof v === 'function') {
      v = v.bind(base)
    }
    base[key as keyof T] = v
  }

  return base
}

export interface CloneableBody {
  finalize(): Promise<void>
  cloneBodyStream(): Readable
}

export function getCloneableBody<T extends IncomingMessage>(
  readable: T
): CloneableBody {
  let buffered: Readable | null = null

  const endPromise = new Promise<void | { error?: unknown }>(
    (resolve, reject) => {
      readable.on('end', resolve)
      readable.on('error', reject)
    }
  ).catch((error) => {
    return { error }
  })

  return {
    /**
     * Replaces the original request body if necessary.
     * This is done because once we read the body from the original request,
     * we can't read it again.
     */
    async finalize(): Promise<void> {
      if (buffered) {
        const res = await endPromise

        if (res && typeof res === 'object' && res.error) {
          throw res.error
        }
        replaceRequestBody(readable, buffered)
        buffered = readable
      }
    },
    /**
     * Clones the body stream
     * to pass into a middleware
     */
    cloneBodyStream() {
      const input = buffered ?? readable
      const p1 = new PassThrough()
      const p2 = new PassThrough()
      input.on('data', (chunk) => {
        p1.push(chunk)
        p2.push(chunk)
      })
      input.on('end', () => {
        p1.push(null)
        p2.push(null)
      })
      buffered = p2
      return p1
    },
  }
}