stream: preserve toReadableSync batch after backpressure

trivikr · aduh95 · commit b568649f6fa4 · 2026-05-19T14:08:22.000+02:00
Keep the current batch and index across _read() calls so chunks that remain after push() returns false are emitted on later reads. Fixes: #63275 Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com> Assisted-by: openai:gpt-5.5 PR-URL: #63276 Fixes: #63275 Reviewed-By: Matteo Collina <matteo.collina@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com>
diff --git a/lib/internal/streams/iter/classic.js b/lib/internal/streams/iter/classic.js
@@ -363,23 +363,37 @@ function toReadableSync(source, options = kNullPrototype) {
 
 const ReadableCtor = lazyReadable();
 const iterator = source[SymbolIterator]();
+ let hasBatch = false;
+ let batch;
+ let batchIndex = 0;
 
 return new ReadableCtor({
 __proto__: null,
 highWaterMark,
 read() {
 for (;;) {
- const { value: batch, done } = iterator.next();
+ if (hasBatch) {
+ while (batchIndex < batch.length) {
+ if (!this.push(batch[batchIndex++])) return;
+ }
+ batch = undefined;
+ hasBatch = false;
+ batchIndex = 0;
+ }
+
+ const result = iterator.next();
+ const { done } = result;
 if (done) {
 this.push(null);
 return;
 }
- for (let i = 0; i < batch.length; i++) {
- if (!this.push(batch[i])) return;
- }
+ batch = result.value;
+ hasBatch = true;
 }
 },
 destroy(err, cb) {
+ batch = undefined;
+ hasBatch = false;
 if (typeof iterator.return === 'function') iterator.return();
 cb(err);
 },
diff --git a/test/parallel/test-stream-iter-to-readable.js b/test/parallel/test-stream-iter-to-readable.js
@@ -439,6 +439,21 @@ async function testBackpressureSync() {
 assert.strictEqual(chunks.length, 10);
 }
 
+// =============================================================================
+// fromStreamIterSync: backpressure within a batch
+// =============================================================================
+
+async function testBackpressureSyncMultiChunkBatch() {
+ function* gen() {
+ yield [Buffer.from('a'), Buffer.from('b'), Buffer.from('c')];
+ }
+
+ const readable = toReadableSync(gen(), { highWaterMark: 1 });
+ const result = await collect(readable);
+
+ assert.strictEqual(result.toString(), 'abc');
+}
+
 // =============================================================================
 // fromStreamIterSync: source error
 // =============================================================================
@@ -613,6 +628,7 @@ Promise.all([
 testWithTransformAsync(),
 testBasicSync(),
 testBackpressureSync(),
+ testBackpressureSyncMultiChunkBatch(),
 testErrorSync(),
 testDestroySync(),
 testRoundTrip(),

-Original file line number
+Diff line change
 constReadableCtor=lazyReadable();
 constiterator=source[SymbolIterator]();
 +lethasBatch=false;
 +letbatch;
 +letbatchIndex=0;
 returnnewReadableCtor({
 __proto__: null,
  highWaterMark,
 read(){
 for(;;){
 -const{value: batch, done }=iterator.next();
 +if(hasBatch){
 +while(batchIndex<batch.length){
 +if(!this.push(batch[batchIndex++]))return;
 +}
 +batch=undefined;
 +hasBatch=false;
 +batchIndex=0;
 +}
++
 +constresult=iterator.next();
 +const{ done }=result;
 if(done){
 this.push(null);
 return;
+}
 -for(leti=0;i<batch.length;i++){
 -if(!this.push(batch[i]))return;
 -}
 +batch=result.value;
 +hasBatch=true;
+}
 },
 destroy(err,cb){
 +batch=undefined;
 +hasBatch=false;
 if(typeofiterator.return==='function')iterator.return();
 cb(err);
 },