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>

1 files changed, 12 insertions, 3 deletions

diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
index 9f79a07a6f..7c6671fcd0 100644
--- a/lib/_stream_readable.js
+++ b/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);
 }