stream: add null push transform in async_iterator

when the readable side of a transform ends any for await
loop on that transform stream should also complete. This
fix prevents for await loop on a transform stream
from hanging indefinitely.
PR-URL: #28566
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: David Mark Clements

lib/internal/streams/async_iterator.js

@@ -155,7 +155,7 @@ const createReadableStreamAsyncIterator = (stream) => {
 });
 iterator[kLastPromise]=null;
 
-finished(stream,(err)=>{
+finished(stream,{writable: false},(err)=>{
 if(err&&err.code!=='ERR_STREAM_PREMATURE_CLOSE'){
 constreject=iterator[kLastReject];
 // Reject if we are waiting for data in the Promise returned by next() and

test/parallel/test-stream-readable-async-iterators.js

@@ -1,7 +1,7 @@
 'use strict';
 
 constcommon=require('../common');
-const{ Readable, PassThrough, pipeline }=require('stream');
+const{ Readable,Transform,PassThrough, pipeline }=require('stream');
 constassert=require('assert');
 
 asyncfunctiontests(){
@@ -396,6 +396,30 @@ async function tests() {
 }
 }
 
+{
+console.log('readable side of a transform stream pushes null');
+consttransform=newTransform({
+objectMode: true,
+transform: (chunk,enc,cb)=>{cb(null,chunk);}
+});
+transform.push(0);
+transform.push(1);
+process.nextTick(()=>{
+transform.push(null);
+});
+
+constmustReach=[common.mustCall(),common.mustCall()];
+
+constiter=transform[Symbol.asyncIterator]();
+assert.strictEqual((awaititer.next()).value,0);
+
+forawait(constdofiter){
+assert.strictEqual(d,1);
+mustReach[0]();
+}
+mustReach[1]();
+}
+
 {
 console.log('all next promises must be resolved on end');
 constr=newReadable({