http: emit 'drain' after flushing coalesced chunks

lib/internal/http/outgoing.js

@@ -376,6 +376,14 @@ OutgoingMessage.prototype.uncork = function uncork() {
       for (const callback of callbacks) {
         callback(err);
       }
+      // write() returned false based on the byte length of the coalesced
+      // chunks, but net.Socket counts string chunks in UTF-16 code units and
+      // may stay below its own highWaterMark, so it never emits 'drain'.
+      // Re-check once the flushed data has been written.
+      if (!err && this.writableNeedDrain && !this.destroyed && this.writableLength === 0) {
+        this[kNeedDrain] = false;
+        this.emit('drain');
+      }
     });
 
     this[kChunkedBuffer].length = 0;

test/parallel/test-http-outgoing-chunked-drain-multibyte.js

@@ -0,0 +1,78 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+const { Duplex } = require('stream');
+
+// Refs: https://github.com/nodejs/node/issues/66781
+// A chunked response written with multi-byte string chunks must emit 'drain'
+// after write() returned false. write() decides backpressure from the byte
+// length of the auto-corked chunks, while net.Socket counts string chunks in
+// UTF-16 code units, so the socket may stay below its own highWaterMark and
+// never emit 'drain' itself once the coalesced chunks are flushed.
+
+// Behaves like net.Socket for this purpose: strings are counted in UTF-16
+// code units and writes complete asynchronously, so flushed data is still in
+// flight when uncork() returns. The high water mark is pinned because the
+// default is platform dependent (16 KiB on Windows), and the coalesced
+// payloads below must exceed it in bytes while staying under it in code
+// units, so that the socket never emits 'drain' on its own.
+function createSocket() {
+  const pending = [];
+  const socket = new Duplex({
+    decodeStrings: false,
+    writableHighWaterMark: 64 * 1024,
+    read() {},
+    write(chunk, encoding, callback) {
+      pending.push(callback);
+    },
+  });
+  // Complete one write per turn of the event loop.
+  (function flush() {
+    pending.shift()?.();
+    if (!socket.destroyed) setImmediate(flush);
+  })();
+  return socket;
+}
+
+const row = 'a'.repeat(100) + 'ع'.repeat(150) + '\n'; // 401 bytes
+
+{
+  const ROWS = 2000;
+  const socket = createSocket();
+  const server = http.createServer(common.mustCall(async (req, res) => {
+    let drains = 0;
+    for (let i = 0; i < ROWS; i++) {
+      if (!res.write(row)) {
+        assert.strictEqual(res.writableNeedDrain, true);
+        await new Promise((resolve) => res.once('drain', resolve));
+        assert.strictEqual(res.writableLength, 0);
+        assert.strictEqual(res.writableNeedDrain, false);
+        drains++;
+      }
+    }
+    assert.ok(drains > 0);
+    res.end(common.mustCall(() => socket.destroy()));
+  }));
+
+  server.emit('connection', socket);
+  socket.push('GET / HTTP/1.1\r\nHost: localhost\r\nConnection: close\r\n\r\n');
+}
+
+// 'drain' must not be emitted once the response has been destroyed from a
+// write callback. The socket stays below its own high water mark, so the
+// only candidate emitter is the message itself.
+{
+  const socket = createSocket();
+  socket.on('drain', common.mustNotCall());
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.on('drain', common.mustNotCall());
+    const ret = res.write(row.repeat(200), common.mustCall(() => {
+      res.destroy();
+    }));
+    assert.strictEqual(ret, false);
+  }));
+
+  server.emit('connection', socket);
+  socket.push('GET / HTTP/1.1\r\nHost: localhost\r\n\r\n');
+}