Re: [whatwg/streams] ReadableStream.from(asyncIterable) (#1083)

@MattiasBuelens commented on this pull request.



> @@ -1456,6 +1456,6 @@ function ReadableStreamFromIterable(asyncIterable) {
     return promiseResolvedWith(returnResult);
   }
 
-  stream = CreateReadableStream(startAlgorithm, pullAlgorithm, cancelAlgorithm);
+  stream = CreateReadableStream(startAlgorithm, pullAlgorithm, cancelAlgorithm, 0);

Oh right, I didn't update the spec text yet to pass a zero high-water mark. But I'll first wait until we decide what that high-water mark should be. 😉

> Transform streams are a bit different, as we want introducing them to be more of a no-op and not cause a bunch of chunks to sit in intermediate transform-stream queues, unless explicitly requested.

Hmm... it doesn't look like that's working as intended? 😕 

By default, a `TransformStream` is indeed created with a readable HWM of 0, but it still has a **writable HWM** of 1. So piping a stream into it *will* pull at least one chunk, to fill the writable end's queue.
```javascript
var rs1 = new ReadableStream({
  pull(c) {
    console.log('pull');
    c.enqueue('a');
  }
}, { highWaterMark: 0 });
// no "pull" is logged yet
var rs2 = rs1.pipeThrough(new TransformStream());
// logs "pull" once
```

There's also no good way to fix this. You can't set both the readable and writable HWM to 0, since then the pipe will stall:
```javascript
var rs1 = new ReadableStream({
  pull(c) {
    console.log('pull');
    c.enqueue('a');
  }
}, { highWaterMark: 0 });
var rs2 = rs.pipeThrough(new TransformStream({}, { highWaterMark: 0 }));
var r = rs.getReader();
await r.read(); // never resolves
```

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