nodejs
diff --git a/‎lib/internal/streams/iter/pull.js‎
Lines changed: 32 additions & 0 deletions b/‎lib/internal/streams/iter/pull.js‎
Lines changed: 32 additions & 0 deletions
diff --git a/‎lib/internal/streams/iter/push.js‎
Lines changed: 9 additions & 2 deletions b/‎lib/internal/streams/iter/push.js‎
Lines changed: 9 additions & 2 deletions
diff --git a/‎lib/internal/streams/iter/types.js‎
Lines changed: 3 additions & 0 deletions b/‎lib/internal/streams/iter/types.js‎
Lines changed: 3 additions & 0 deletions
diff --git a/‎test/parallel/test-stream-iter-pipeto-writev.js‎
Lines changed: 32 additions & 1 deletion b/‎test/parallel/test-stream-iter-pipeto-writev.js‎
Lines changed: 32 additions & 1 deletion
@@ -51,6 +51,8 @@ const {
 } = require('internal/streams/iter/utils');
 
 const {
+  drainableProtocol,
+  kSyncWriteAcceptedOnFalse,
   kValidatedTransform,
 } = require('internal/streams/iter/types');
 
@@ -828,13 +830,33 @@ async function pipeTo(source, ...args) {
   const hasWriteSync = typeof writer.writeSync === 'function';
   const hasWritevSync = typeof writer.writevSync === 'function';
   const hasEndSync = typeof writer.endSync === 'function';
+  const syncFalseCanBeAccepted = writer[kSyncWriteAcceptedOnFalse] === true;
+
+  function syncFalseWasAccepted() {
+    return syncFalseCanBeAccepted && writer.desiredSize === 0;
+  }
+
+  function waitForSyncBackpressure() {
+    const ondrain = writer[drainableProtocol];
+    return ondrain?.call(writer);
+  }
+
+  async function writeBatchAfterAcceptedBackpressure(batch, startIndex) {
+    await waitForSyncBackpressure();
+    await writeBatchAsyncFallback(batch, startIndex);
+  }
+
   // Async fallback for writeBatch when sync write fails partway through.
   // Continues writing from batch[startIndex] using async write().
   async function writeBatchAsyncFallback(batch, startIndex) {
     for (let i = startIndex; i < batch.length; i++) {
       const chunk = batch[i];
       if (hasWriteSync && writer.writeSync(chunk)) {
         // Sync retry succeeded
+      } else if (syncFalseWasAccepted()) {
+        totalBytes += TypedArrayPrototypeGetByteLength(chunk);
+        await waitForSyncBackpressure();
+        continue;
       } else {
         const result = writer.write(
           chunk, signal ? { __proto__: null, signal } : undefined);
@@ -852,6 +874,12 @@ async function pipeTo(source, ...args) {
   function writeBatch(batch) {
     if (hasWritev && batch.length > 1) {
       if (!hasWritevSync || !writer.writevSync(batch)) {
+        if (hasWritevSync && syncFalseWasAccepted()) {
+          for (let i = 0; i < batch.length; i++) {
+            totalBytes += TypedArrayPrototypeGetByteLength(batch[i]);
+          }
+          return waitForSyncBackpressure();
+        }
         const opts = signal ? { __proto__: null, signal } : undefined;
         return PromisePrototypeThen(writer.writev(batch, opts), () => {
           for (let i = 0; i < batch.length; i++) {
@@ -867,6 +895,10 @@ async function pipeTo(source, ...args) {
     for (let i = 0; i < batch.length; i++) {
       const chunk = batch[i];
       if (!hasWriteSync || !writer.writeSync(chunk)) {
+        if (hasWriteSync && syncFalseWasAccepted()) {
+          totalBytes += TypedArrayPrototypeGetByteLength(chunk);
+          return writeBatchAfterAcceptedBackpressure(batch, i + 1);
+        }
         // Sync path failed at index i - fall back to async for the rest.
         // Count bytes for chunks already written synchronously (0..i-1).
         return writeBatchAsyncFallback(batch, i);
 
@@ -32,6 +32,7 @@ const {
 
 const {
   drainableProtocol,
+  kSyncWriteAcceptedOnFalse,
 } = require('internal/streams/iter/types');
 
 const {
@@ -560,6 +561,10 @@ class PushWriter {
     return this.#queue.desiredSize;
   }
 
+  get [kSyncWriteAcceptedOnFalse]() {
+    return this.#queue.backpressurePolicy === 'block';
+  }
+
   write(chunk, options) {
     if (!options?.signal && this.#queue.canWriteSync()) {
       const bytes = toUint8Array(chunk);
@@ -586,7 +591,8 @@ class PushWriter {
   writeSync(chunk) {
     const bytes = toUint8Array(chunk);
     const result = this.#queue.writeSync([bytes]);
-    if (!result && this.#queue.backpressurePolicy === 'block') {
+    if (!result && this.#queue.backpressurePolicy === 'block' &&
+        this.#queue.desiredSize === 0) {
       // Block policy: force-enqueue and return false as backpressure signal.
       // Data IS accepted; false tells caller to slow down.
       this.#queue.forceEnqueue([bytes]);
@@ -601,7 +607,8 @@ class PushWriter {
     }
     const bytes = convertChunks(chunks);
     const result = this.#queue.writeSync(bytes);
-    if (!result && this.#queue.backpressurePolicy === 'block') {
+    if (!result && this.#queue.backpressurePolicy === 'block' &&
+        this.#queue.desiredSize === 0) {
       this.#queue.forceEnqueue(bytes);
       return false;
     }
 
@@ -64,9 +64,12 @@ const kValidatedTransform = Symbol('kValidatedTransform');
  */
 const kValidatedSource = Symbol('kValidatedSource');
 
+const kSyncWriteAcceptedOnFalse = Symbol('kSyncWriteAcceptedOnFalse');
+
 module.exports = {
   broadcastProtocol,
   drainableProtocol,
+  kSyncWriteAcceptedOnFalse,
   kValidatedSource,
   kValidatedTransform,
   shareProtocol,
 
@@ -5,7 +5,8 @@
 
 const common = require('../common');
 const assert = require('assert');
-const { pipeTo, pipeToSync } = require('stream/iter');
+const { setImmediate: setImmediatePromise } = require('timers/promises');
+const { pipeTo, pipeToSync, push, text } = require('stream/iter');
 
 // Multi-chunk batch with writevSync (sync success path)
 async function testWritevSyncSuccess() {
@@ -104,6 +105,35 @@ async function testWriteSyncAlwaysFails() {
   assert.strictEqual(total, 2);
 }
 
+// PushWriter block mode accepts sync writes even when returning false for
+// backpressure. pipeTo must wait for drain, not retry the same write.
+async function assertPushWriterBlockPipeTo(source, expected, expectedTotal) {
+  const { writer, readable } = push({
+    highWaterMark: 1,
+    backpressure: 'block',
+  });
+
+  const pipe = pipeTo(source, writer);
+  await setImmediatePromise();
+  const data = await text(readable);
+  const total = await pipe;
+
+  assert.strictEqual(data, expected);
+  assert.strictEqual(total, expectedTotal);
+}
+
+async function testPushWriterBlockSyncFalseAccepted() {
+  await assertPushWriterBlockPipeTo((async function*() {
+    yield [new Uint8Array([97])];
+    yield [new Uint8Array([98])];
+  })(), 'ab', 2);
+
+  await assertPushWriterBlockPipeTo((async function*() {
+    yield [new Uint8Array([97, 98])];
+    yield [new Uint8Array([99]), new Uint8Array([100])];
+  })(), 'abcd', 4);
+}
+
 // pipeToSync with writevSync
 async function testPipeToSyncWritev() {
   const batches = [];
@@ -142,6 +172,7 @@ Promise.all([
   testWritevSyncFails(),
   testWriteSyncFailsMidBatch(),
   testWriteSyncAlwaysFails(),
+  testPushWriterBlockSyncFalseAccepted(),
   testPipeToSyncWritev(),
   testPipeToSyncWriteFallback(),
 ]).then(common.mustCall());