stream: fix backpressure when multiple sync

PR-URL: https://github.com/nodejs/node/pull/19613
Fixes: https://github.com/nodejs/node/issues/19601
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/_stream_readable.js

@@ -310,8 +310,7 @@ function chunkInvalid(state, chunk) {
 // 'readable' event will be triggered.
 function needMoreData(state) {
   return !state.ended &&
-         (state.needReadable ||
+         (state.length < state.highWaterMark ||
-          state.length < state.highWaterMark ||
           state.length === 0);
 }
 
@@ -536,7 +535,17 @@ function emitReadable_(stream) {
   if (!state.destroyed && (state.length || state.ended)) {
     stream.emit('readable');
   }
-  state.needReadable = !state.flowing && !state.ended;
+
+  // The stream needs another readable event if
+  // 1. It is not flowing, as the flow mechanism will take
+  //    care of it.
+  // 2. It is not ended.
+  // 3. It is below the highWaterMark, so we can schedule
+  //    another readable later.
+  state.needReadable =
+    !state.flowing &&
+    !state.ended &&
+    state.length <= state.highWaterMark;
   flow(stream);
 }
 

test/parallel/test-stream-backpressure.js

@@ -0,0 +1,39 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const stream = require('stream');
+
+let pushes = 0;
+const total = 65500 + 40 * 1024;
+const rs = new stream.Readable({
+  read: common.mustCall(function() {
+    if (pushes++ === 10) {
+      this.push(null);
+      return;
+    }
+
+    const length = this._readableState.length;
+
+    // We are at most doing two full runs of _reads
+    // before stopping, because Readable is greedy
+    // to keep its buffer full
+    assert(length <= total);
+
+    this.push(Buffer.alloc(65500));
+    for (let i = 0; i < 40; i++) {
+      this.push(Buffer.alloc(1024));
+    }
+
+    // We will be over highWaterMark at this point
+    // but a new call to _read is scheduled anyway.
+  }, 11)
+});
+
+const ws = stream.Writable({
+  write: common.mustCall(function(data, enc, cb) {
+    setImmediate(cb);
+  }, 41 * 10)
+});
+
+rs.pipe(ws);