Function createDebugChannel

packages/next/src/client/dev/debug-channel.ts:128–175

(
  requestHeaders: Record<string, string> | undefined
)

}

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.`
      )
    }
  }

  // Only attempt to restore the sessionStorage debug channel entry for the
  // initial document load (no request headers). Client-side navigations pass
  // request headers and should always use the WebSocket-backed debug channel.
  if (!requestHeaders && wasServedFromCache()) {
    const readable = restoreDebugChannelFromSessionStorage(requestId)

    if (readable) {
      return { readable }
    }

    // Debug channel can't be restored — debug deps would block hydration.
    // Force a fresh page load from the server. Return a never-closing stream
    // so the Flight client stays parked until the reload tears the document
    // down, instead of synchronously erroring with "Connection closed.".
    location.reload()
    return { readable: new ReadableStream() }
  }

  const { readable } = getOrCreateDebugChannelReadableWriterPair(requestId)

  return { readable }
}