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‎test/parallel/test-http-generic-streams.js‎
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‎test/parallel/test-http-insecure-parser-per-stream.js‎
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@@ -45,8 +45,11 @@ There are four fundamental stream types within Node.js:
   is written and read (for example, [`zlib.createDeflate()`][]).
 
 Additionally, this module includes the utility functions
-[`stream.pipeline()`][], [`stream.finished()`][], [`stream.Readable.from()`][]
-and [`stream.addAbortSignal()`][].
+[`stream.duplexPair()`][],
+[`stream.pipeline()`][],
+[`stream.finished()`][]
+[`stream.Readable.from()`][], and
+[`stream.addAbortSignal()`][].
 
 ### Streams Promises API
 
@@ -2675,6 +2678,30 @@ unless `emitClose` is set in false.
 Once `destroy()` has been called, any further calls will be a no-op and no
 further errors except from `_destroy()` may be emitted as `'error'`.
 
+#### `stream.duplexPair([options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object} A value to pass to both [`Duplex`][] constructors,
+  to set options such as buffering.
+* Returns: {Array} of two [`Duplex`][] instances.
+
+The utility function `duplexPair` returns an Array with two items,
+each being a `Duplex` stream connected to the other side:
+
+```js
+const [ sideA, sideB ] = duplexPair();
+```
+
+Whatever is written to one stream is made readable on the other. It provides
+behavior analogous to a network connection, where the data written by the client
+becomes readable by the server, and vice-versa.
+
+The Duplex streams are symmetrical; one or the other may be used without any
+difference in behavior.
+
 ### `stream.finished(stream[, options], callback)`
 
 <!-- YAML
@@ -4865,6 +4892,7 @@ contain multi-byte characters.
 [`stream.addAbortSignal()`]: #streamaddabortsignalsignal-stream
 [`stream.compose`]: #streamcomposestreams
 [`stream.cork()`]: #writablecork
+[`stream.duplexPair()`]: #streamduplexpairoptions
 [`stream.finished()`]: #streamfinishedstream-options-callback
 [`stream.pipe()`]: #readablepipedestination-options
 [`stream.pipeline()`]: #streampipelinesource-transforms-destination-callback
 
@@ -0,0 +1,62 @@
+'use strict';
+const {
+  Symbol,
+} = primordials;
+
+const { Duplex } = require('stream');
+const assert = require('internal/assert');
+
+const kCallback = Symbol('Callback');
+const kInitOtherSide = Symbol('InitOtherSide');
+
+class DuplexSide extends Duplex {
+  #otherSide = null;
+
+  constructor(options) {
+    super(options);
+    this[kCallback] = null;
+    this.#otherSide = null;
+  }
+
+  [kInitOtherSide](otherSide) {
+    // Ensure this can only be set once, to enforce encapsulation.
+    if (this.#otherSide === null) {
+      this.#otherSide = otherSide;
+    } else {
+      assert(this.#otherSide === null);
+    }
+  }
+
+  _read() {
+    const callback = this[kCallback];
+    if (callback) {
+      this[kCallback] = null;
+      callback();
+    }
+  }
+
+  _write(chunk, encoding, callback) {
+    assert(this.#otherSide !== null);
+    assert(this.#otherSide[kCallback] === null);
+    if (chunk.length === 0) {
+      process.nextTick(callback);
+    } else {
+      this.#otherSide.push(chunk);
+      this.#otherSide[kCallback] = callback;
+    }
+  }
+
+  _final(callback) {
+    this.#otherSide.on('end', callback);
+    this.#otherSide.push(null);
+  }
+}
+
+function duplexPair(options) {
+  const side0 = new DuplexSide(options);
+  const side1 = new DuplexSide(options);
+  side0[kInitOtherSide](side1);
+  side1[kInitOtherSide](side0);
+  return [ side0, side1 ];
+}
+module.exports = duplexPair;
@@ -101,6 +101,7 @@ Stream.Writable = require('internal/streams/writable');
 Stream.Duplex = require('internal/streams/duplex');
 Stream.Transform = require('internal/streams/transform');
 Stream.PassThrough = require('internal/streams/passthrough');
+Stream.duplexPair = require('internal/streams/duplexpair');
 Stream.pipeline = pipeline;
 const { addAbortSignal } = require('internal/streams/add-abort-signal');
 Stream.addAbortSignal = addAbortSignal;
 
@@ -12,7 +12,6 @@ This directory contains modules used to test the Node.js implementation.
 * [CPU Profiler module](#cpu-profiler-module)
 * [Debugger module](#debugger-module)
 * [DNS module](#dns-module)
-* [Duplex pair helper](#duplex-pair-helper)
 * [Environment variables](#environment-variables)
 * [Fixtures module](#fixtures-module)
 * [Heap dump checker module](#heap-dump-checker-module)
@@ -669,14 +668,6 @@ Reads a Domain String and returns a Buffer containing the domain.
 Takes in a parsed Object and writes its fields to a DNS packet as a Buffer
 object.
 
-## Duplex pair helper
-
-The `common/duplexpair` module exports a single function `makeDuplexPair`,
-which returns an object `{ clientSide, serverSide }` where each side is a
-`Duplex` stream connected to the other side.
-
-There is no difference between client or server side beyond their names.
-
 ## Environment variables
 
 The behavior of the Node.js test suite can be altered using the following
 
@@ -128,6 +128,7 @@ if (common.isMainThread) {
     'NativeModule internal/streams/compose',
     'NativeModule internal/streams/destroy',
     'NativeModule internal/streams/duplex',
+    'NativeModule internal/streams/duplexpair',
     'NativeModule internal/streams/end-of-stream',
     'NativeModule internal/streams/from',
     'NativeModule internal/streams/legacy',
 
@@ -8,7 +8,7 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
-const makeDuplexPair = require('../common/duplexpair');
+const { duplexPair } = require('stream');
 const onGC = require('../common/ongc');
 const assert = require('assert');
 const tls = require('tls');
@@ -37,7 +37,7 @@ function connect() {
     return;
   }
 
-  const { clientSide, serverSide } = makeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
 
   const tlsSocket = tls.connect({ socket: clientSide });
   tlsSocket.on('error', common.mustCall(connect));
 
@@ -3,7 +3,7 @@
 const common = require('../common');
 const http = require('http');
 const async_hooks = require('async_hooks');
-const makeDuplexPair = require('../common/duplexpair');
+const { duplexPair } = require('stream');
 
 // Regression test for https://github.com/nodejs/node/issues/30122
 // When a domain is attached to an http Agent’s ReusedHandle object, that
@@ -36,7 +36,7 @@ async_hooks.createHook({
 // attached to too many objects that use strong references (timers, the network
 // socket handle, etc.) and wrap the client side in a JSStreamSocket so we don’t
 // have to implement the whole _handle API ourselves.
-const { serverSide, clientSide } = makeDuplexPair();
+const [ serverSide, clientSide ] = duplexPair();
 const JSStreamSocket = require('internal/js_stream_socket');
 const wrappedClientSide = new JSStreamSocket(clientSide);
 
 
@@ -2,7 +2,7 @@
 const common = require('../common');
 const assert = require('assert');
 const http = require('http');
-const MakeDuplexPair = require('../common/duplexpair');
+const { duplexPair } = require('stream');
 
 // Test 1: Simple HTTP test, no keep-alive.
 {
@@ -13,7 +13,7 @@ const MakeDuplexPair = require('../common/duplexpair');
     res.end(testData);
   }));
 
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
   server.emit('connection', serverSide);
 
   const req = http.request({
@@ -37,7 +37,7 @@ const MakeDuplexPair = require('../common/duplexpair');
     res.end(testData);
   }, 2));
 
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
   server.emit('connection', serverSide);
 
   function doRequest(cb) {
@@ -77,7 +77,7 @@ const MakeDuplexPair = require('../common/duplexpair');
     });
   }));
 
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
   server.emit('connection', serverSide);
   clientSide.on('end', common.mustCall());
   serverSide.on('end', common.mustCall());
@@ -117,7 +117,7 @@ const MakeDuplexPair = require('../common/duplexpair');
 
   }));
 
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
   server.emit('connection', serverSide);
   clientSide.on('end', common.mustCall());
   serverSide.on('end', common.mustCall());
@@ -143,7 +143,7 @@ const MakeDuplexPair = require('../common/duplexpair');
 {
   const server = http.createServer(common.mustNotCall());
 
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
   server.emit('connection', serverSide);
 
   server.on('clientError', common.mustCall());
 
@@ -2,13 +2,13 @@
 const common = require('../common');
 const assert = require('assert');
 const http = require('http');
-const MakeDuplexPair = require('../common/duplexpair');
+const { duplexPair } = require('stream');
 
 // Test that setting the `maxHeaderSize` option works on a per-stream-basis.
 
 // Test 1: The server sends an invalid header.
 {
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
 
   const req = http.request({
     createConnection: common.mustCall(() => clientSide),
@@ -30,7 +30,7 @@ const MakeDuplexPair = require('../common/duplexpair');
 
 // Test 2: The same as Test 1 except without the option, to make sure it fails.
 {
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
 
   const req = http.request({
     createConnection: common.mustCall(() => clientSide)
@@ -59,7 +59,7 @@ const MakeDuplexPair = require('../common/duplexpair');
 
   server.on('clientError', common.mustNotCall());
 
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
   serverSide.server = server;
   server.emit('connection', serverSide);
 
@@ -75,7 +75,7 @@ const MakeDuplexPair = require('../common/duplexpair');
 
   server.on('clientError', common.mustCall());
 
-  const { clientSide, serverSide } = MakeDuplexPair();
+  const [ clientSide, serverSide ] = duplexPair();
   serverSide.server = server;
   server.emit('connection', serverSide);