batchMiddleware crashes when a single request rejects

[matt-dalton]

When batchMiddleware flushes a queue containing only one request and that request’s fetch rejects (server unavailable, timeout, etc.), the middleware throws RRNLBatchMiddlewareError: Server does not return response for request at index 0. Response should have an array with 1 item(s).

import {
  RelayNetworkLayer,
  batchMiddleware,
  urlMiddleware,
  authMiddleware,
  retryMiddleware,
} from 'react-relay-network-modern/es'
import { Environment, RecordSource, Store } from 'relay-runtime'

const network = new RelayNetworkLayer([
  urlMiddleware({ url: () => Promise.resolve('https://example.com/graphql') }),
  batchMiddleware({
    batchUrl: () => Promise.resolve('https://example.com/graphql'),
    batchTimeout: 15,
  }),
  authMiddleware({ token: () => Promise.resolve('token') })
])

export const environment = new Environment({
  network,
  store: new Store(new RecordSource()),
})

For us this causes a problem if the server is down, for example

Root cause

In sendRequests(), the single-request branch awaits next(wrapper.req) without a try/catch:

if (requestList.length === 1) {
  const wrapper = requestList[0]
  const res = await next(wrapper.req) // <-- rejection bubbles up
  wrapper.completeOk(res)
  return res
}

Because the rejection isn’t caught, wrapper.done never flips to true, and later finalizeUncompleted() throws the RRNLBatchMiddlewareError.

Expected

The single-request branch should mirror the batched branch: catch the rejection, call wrapper.completeErr(err) (and do the same for duplicates), then rethrow so callers receive the original network error instead of the batch middleware crash.