packages/next/src/client/dev/debug-channel.ts

import { NEXT_REQUEST_ID_HEADER } from '../components/app-router-headers'
import { InvariantError } from '../../shared/lib/invariant-error'

export interface DebugChannelReadableWriterPair {
  readonly readable: ReadableStream<Uint8Array>
  readonly writer: WritableStreamDefaultWriter<Uint8Array>
}

const pairs = new Map<string, DebugChannelReadableWriterPair>()

export function getOrCreateDebugChannelReadableWriterPair(
  requestId: string
): DebugChannelReadableWriterPair {
  let pair = pairs.get(requestId)

  if (!pair) {
    const { readable, writable } = new TransformStream<Uint8Array, Uint8Array>()
    pair = { readable, writer: writable.getWriter() }
    pairs.set(requestId, pair)
    pair.writer.closed.finally(() => pairs.delete(requestId))
  }

  return pair
}

export function createDebugChannel(
  requestHeaders: Record<string, string> | undefined
): {
  writable?: WritableStream
  readable?: ReadableStream
} {
  let requestId: string | undefined

  if (requestHeaders) {
    requestId = requestHeaders[NEXT_REQUEST_ID_HEADER] ?? undefined

    if (!requestId) {
      throw new InvariantError(
        `Expected a ${JSON.stringify(NEXT_REQUEST_ID_HEADER)} request header.`
      )
    }
  } else {
    requestId = self.__next_r

    if (!requestId) {
      throw new InvariantError(
        `Expected a request ID to be defined for the document via self.__next_r.`
      )
    }
  }

  const { readable } = getOrCreateDebugChannelReadableWriterPair(requestId)

  return { readable }
}