Discard auto-allocated BYOB request on enqueue

As discovered in #1170 (comment): if a BYOB request was auto-allocated, but then you call controller.enqueue() instead of byobRequest.respond(), then the auto-allocated pull-into descriptor stays in the queue. If you later on switch from a default reader to a BYOB reader, you run into trouble because the list of read-into requests no longer matches up with the list of pending pull-into descriptors.

This PR fixes it by making enqueue() discard that auto-allocated BYOB request.

Author: MattiasBuelens

index.bs

@@ -3137,10 +3137,17 @@ The following abstract operations support the implementation of the
  1. Perform ! [$ReadableByteStreamControllerInvalidateBYOBRequest$](|controller|).
  1. If ! [$ReadableStreamHasDefaultReader$](|stream|) is true
   1. If ! [$ReadableStreamGetNumReadRequests$](|stream|) is 0,
+   1. Assert: |controller|.[=ReadableByteStreamController/[[pendingPullIntos]]=] is
+      [=list/is empty|empty=].
    1. Perform ! [$ReadableByteStreamControllerEnqueueChunkToQueue$](|controller|,
       |transferredBuffer|, |byteOffset|, |byteLength|).
   1. Otherwise,
    1. Assert: |controller|.[=ReadableByteStreamController/[[queue]]=] [=list/is empty=].
+   1. If |controller|.[=ReadableByteStreamController/[[pendingPullIntos]]=] is not
+      [=list/is empty|empty=],
+    1. Assert: |controller|.[=ReadableByteStreamController/[[pendingPullIntos]]=][0]'s [=pull-into
+       descriptor/reader type=] is "`default`".
+    1. Perform ! [$ReadableByteStreamControllerShiftPendingPullInto$](|controller|).
    1. Let |transferredView| be ! [$Construct$]({{%Uint8Array%}}, « |transferredBuffer|,
       |byteOffset|, |byteLength| »).
    1. Perform ! [$ReadableStreamFulfillReadRequest$](|stream|, |transferredView|, false).

reference-implementation/lib/abstract-ops/readable-streams.js

@@ -1289,10 +1289,14 @@ function ReadableByteStreamControllerEnqueue(controller, chunk) {
 
   if (ReadableStreamHasDefaultReader(stream) === true) {
     if (ReadableStreamGetNumReadRequests(stream) === 0) {
+      assert(controller._pendingPullIntos.length === 0);
       ReadableByteStreamControllerEnqueueChunkToQueue(controller, transferredBuffer, byteOffset, byteLength);
     } else {
       assert(controller._queue.length === 0);
-
+      if (controller._pendingPullIntos.length > 0) {
+        assert(controller._pendingPullIntos[0].readerType === 'default');
+        ReadableByteStreamControllerShiftPendingPullInto(controller);
+      }
       const transferredView = new Uint8Array(transferredBuffer, byteOffset, byteLength);
       ReadableStreamFulfillReadRequest(stream, transferredView, false);
     }