stream: make stream/iter toWritable apply backpressure

Signed-off-by: James M Snell <jasnell@gmail.com>
Assisted-by: Opencode
PR-URL: #66079
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: jasnell

doc/api/stream_iter.md

@@ -1757,9 +1757,9 @@ non-Error reason is wrapped in an `ERR_FALSY_VALUE_REJECTION` or
 `ERR_OPERATION_FAILED` error before it is passed to the callback. The error's
 `reason` property contains the original value.
 
-The Writable's `highWaterMark` is set to `Number.MAX_SAFE_INTEGER` to
-effectively disable its internal buffering, allowing the underlying Writer
-to manage backpressure directly.
+The Writable uses the default classic stream `highWaterMark`. Classic stream
+backpressure bounds writes waiting to reach the underlying Writer, while the
+Writer controls completion of the active `_write()` or `_writev()` operation.
 
 ```mjs
 import { push, toWritable } from 'node:stream/iter';

lib/internal/streams/iter/classic.js

@@ -14,7 +14,6 @@
 const {
   ArrayPrototypePush,
   FunctionPrototypeCall,
-  NumberMAX_SAFE_INTEGER,
   Promise,
   PromisePrototypeThen,
   PromiseReject,
@@ -937,11 +936,6 @@ function toWritable(writer) {
 
   const writableOptions = {
     __proto__: null,
-    // Use MAX_SAFE_INTEGER to effectively disable the Writable's
-    // internal buffering. The underlying stream/iter Writer has its
-    // own backpressure handling; we want _write to be called
-    // immediately so the Writer can manage flow control directly.
-    highWaterMark: NumberMAX_SAFE_INTEGER,
     write: _write,
     final: _final,
     destroy: _destroy,

test/parallel/test-stream-iter-writable-from.js

@@ -6,6 +6,7 @@
 
 const common = require('../common');
 const assert = require('assert');
+const { once } = require('events');
 const { setImmediate, setTimeout } = require('timers/promises');
 const {
   push,
@@ -603,18 +604,33 @@ async function testDestroyWithoutFail() {
 }
 
 // =============================================================================
-// Custom highWaterMark option
+// Classic Writable backpressure
 // =============================================================================
 
-function testHighWaterMarkIsMaxSafeInt() {
+function testUsesBoundedHighWaterMark() {
   const writer = {
     write(chunk) { return Promise.resolve(); },
   };
 
-  // HWM is set to MAX_SAFE_INTEGER to disable Writable's internal
-  // buffering. The underlying Writer manages backpressure directly.
   const writable = toWritable(writer);
-  assert.strictEqual(writable.writableHighWaterMark, Number.MAX_SAFE_INTEGER);
+  assert.ok(writable.writableHighWaterMark > 0);
+  assert.ok(writable.writableHighWaterMark < Number.MAX_SAFE_INTEGER);
+}
+
+async function testAppliesClassicBackpressure() {
+  let resolveWrite;
+  const writable = toWritable({
+    write: common.mustCall(() => new Promise((resolve) => {
+      resolveWrite = resolve;
+    })),
+  });
+  const chunk = Buffer.alloc(writable.writableHighWaterMark);
+
+  assert.strictEqual(writable.write(chunk), false);
+  const finished = once(writable, 'finish');
+  resolveWrite();
+  writable.end();
+  await finished;
 }
 
 // =============================================================================
@@ -700,10 +716,11 @@ async function testEndThrowsSyncPropagation() {
 
 testInvalidWriterThrows();
 testNoWritevWithoutWriterWritev();
-testHighWaterMarkIsMaxSafeInt();
+testUsesBoundedHighWaterMark();
 
 Promise.all([
   testBasicWrite(),
+  testAppliesClassicBackpressure(),
   testFalsyWriterRejectionBecomesClassicError(),
   testClassicWrapperReusePreservesErrorIdentity(),
   testWriteDelegatesToWriter(),