Iterating cancelled ReadableStream

[ronag]

This is kind of weird behavior I'm not sure what to think about. Iterating a cancelled readable stream will not throw...

const { ReadableStream } = require('stream/web')

const stream = new ReadableStream({
  async start (controller) {
  },
  async pull () {
  },
  async cancel (reason) {
  }
})

async function main () {
  for await (const chunk of stream) {
  }
  assert(false) // ouch
}

stream.cancel(new Error())

main().catch(err => {
  console.log(err)
})

This means that e.g. helpers from #39594 can succeeded without any data.

[ronag]

@jasnell

[ronag]

Also as far as I know there is no introspection here where one could determine if the stream is cancelled before iterating it.

[jasnell]

Yeah, this is expected behavior. The semantics of cancel() are essentially "I'm not interested in the data any more", so iterating over a canceled stream will just result in no data. Changing that behavior would require a change to the spec. I've been thinking about opening a proposal for the spec to add an api that allows introspection of the state.

[ronag]

Hm, this makes it difficult to implement fetch. Right now if the stream is cancelled we get a false success :/

[jasnell]

The way fetch is currently defined, it's able to access the internal state of the ReadableStream. We'll likely have to do the same unless we're able to get a couple new public APIs added.

[ronag]

Actually fetch spec says to "error" the stream and not "cancel", so I assume it refers to controller.error which does error the iterator.