http: support readable hwm in IncomingMessage

doc/api/http.md

@@ -1667,6 +1667,10 @@ the request body should be sent.
 ## Class: http.IncomingMessage
 <!-- YAML
 added: v0.1.17
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/30135
+    description: The `readableHighWaterMark` value mirrors that of the socket.
 -->
 
 * Extends: {stream.Readable}

lib/_http_incoming.js

@@ -37,7 +37,15 @@ function readStop(socket) {
 
 /* Abstract base class for ServerRequest and ClientResponse. */
 function IncomingMessage(socket) {
-  Stream.Readable.call(this);
+  let streamOptions;
+
+  if (socket) {
+    streamOptions = {
+      highWaterMark: socket.readableHighWaterMark
+    };
+  }
+
+  Stream.Readable.call(this, streamOptions);
 
   this._readableState.readingMore = true;
 

test/parallel/test-http-incoming-message-options.js

@@ -0,0 +1,24 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+const net = require('net');
+const readableHighWaterMark = 1024;
+const server = http.createServer((req, res) => { res.end(); });
+
+server.listen(0, common.mustCall(() => {
+  const req = http.request({
+    port: server.address().port,
+    createConnection(options) {
+      options.readableHighWaterMark = readableHighWaterMark;
+      return net.createConnection(options);
+    }
+  }, common.mustCall((res) => {
+    assert.strictEqual(res.socket, req.socket);
+    assert.strictEqual(res.socket.readableHighWaterMark, readableHighWaterMark);
+    assert.strictEqual(res.readableHighWaterMark, readableHighWaterMark);
+    server.close();
+  }));
+
+  req.end();
+}));