nodejs · QuantumBreakz · Sep 15, 2026 · Sep 15, 2026
diff --git a/lib/_http_outgoing.js b/lib/_http_outgoing.js
@@ -414,6 +414,13 @@ OutgoingMessage.prototype._send = function _send(data, encoding, callback, byteL
     if (typeof data === 'string' &&
         (encoding === 'utf8' || encoding === 'latin1' || !encoding)) {
       data = this._header + data;
+      if (byteLength !== undefined) {
+        // `data` now carries the header as well, so a byte length measured by
+        // the caller for the body chunk alone no longer describes it. Header
+        // values are restricted to the latin1 range, so no surrogate pair can
+        // straddle the join and the two lengths are additive.
+        byteLength += Buffer.byteLength(this._header, encoding);
+      }
     } else {
       const header = this._header;
       this.outputData.unshift({
@@ -453,8 +460,19 @@ function _writeRaw(data, encoding, callback, size) {
   }
   // Buffer, as long as we're not destroyed.
   this.outputData.push({ data, encoding, callback });
-  this.outputSize += data.length;
-  this._onPendingData(data.length);
+  // `outputSize` and the pending data counter track how many *bytes* are
+  // queued, so string chunks have to be measured accordingly: `.length` is a
+  // count of UTF-16 code units and undercounts any multi-byte character.
+  // Callers that already computed the byte length hand it over as `size` so
+  // that the string is not measured twice.
+  // Single byte encodings, which is what every internal write here uses,
+  // have a byte length equal to the string length; only utf8 has to be
+  // measured.
+  const len = size ?? (typeof data !== 'string' || encoding === 'latin1' ||
+    encoding === 'ascii' || encoding === 'binary' ?
+    data.length : Buffer.byteLength(data, encoding));
+  this.outputSize += len;
+  this._onPendingData(len);
   return this.outputSize < this[kHighWaterMark];
 }
 

diff --git a/test/parallel/test-http-outgoing-buffer-bytelength.js b/test/parallel/test-http-outgoing-buffer-bytelength.js
@@ -0,0 +1,193 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+const { OutgoingMessage } = http;
+
+// `outputSize`, and the per-connection counter fed by `_onPendingData()`,
+// track how many *bytes* are buffered on an outgoing message. They are used
+// to decide when to apply backpressure, so measuring a string by its number
+// of UTF-16 code units instead of its byte length made Node under-account any
+// multi-byte body and apply backpressure too late.
+// Refs: https://github.com/nodejs/node/issues/57985
+
+// An OutgoingMessage without a socket buffers everything it is handed, which
+// is the path that does the accounting.
+function createMessage(options) {
+  const msg = new OutgoingMessage(options);
+  msg._implicitHeader = function() {};
+  return msg;
+}
+
+// A two byte character is counted as two bytes, not as one character.
+{
+  const msg = createMessage();
+  assert.strictEqual(msg.write('é'.repeat(100)), true);
+  assert.strictEqual(msg.outputSize, 200);
+}
+
+// Characters outside the BMP are a surrogate pair in UTF-16 (two code units)
+// and four bytes in UTF-8.
+{
+  const msg = createMessage();
+  msg.write('😀'.repeat(10));
+  assert.strictEqual(msg.outputSize, 40);
+}
+
+// Plain ASCII is unaffected: one code unit is one byte.
+{
+  const msg = createMessage();
+  msg.write('a'.repeat(100));
+  assert.strictEqual(msg.outputSize, 100);
+}
+
+// Buffers and other views already report a byte length.
+{
+  const msg = createMessage();
+  const chunk = Buffer.from('é'.repeat(100), 'utf8');
+  assert.strictEqual(chunk.length, 200);
+  msg.write(chunk);
+  assert.strictEqual(msg.outputSize, 200);
+}
+
+// The declared encoding decides the byte length: the same string is 100 bytes
+// as latin1 and 200 bytes as utf8.
+{
+  const msg = createMessage();
+  msg.write('é'.repeat(100), 'latin1');
+  assert.strictEqual(msg.outputSize, 100);
+}
+
+// Encodings that decode to fewer bytes than the string has characters are
+// counted by what they decode to, not by the length of the source string.
+{
+  const msg = createMessage();
+  msg.write('deadbeef', 'hex');
+  assert.strictEqual(msg.outputSize, 4);
+}
+
+{
+  const msg = createMessage();
+  msg.write('AAAAAA==', 'base64');
+  assert.strictEqual(msg.outputSize, Buffer.byteLength('AAAAAA==', 'base64'));
+}
+
+// The per-connection counter is fed the same byte length.
+{
+  const msg = createMessage();
+  const deltas = [];
+  msg._onPendingData = (delta) => deltas.push(delta);
+  msg.write('é'.repeat(100));
+  assert.deepStrictEqual(deltas, [200]);
+}
+
+// The header is prepended to the first string chunk, and has to be accounted
+// for along with it.
+{
+  const header = 'HTTP/1.1 200 OK\r\n\r\n';
+  const msg = createMessage();
+  msg._implicitHeader = function() { this._header = header; };
+  msg.write('é'.repeat(100));
+  assert.strictEqual(msg.outputSize, Buffer.byteLength(header) + 200);
+}
+
+// The header is also accounted for when the caller already knows the byte
+// length of the body, as is the case for chunked encoding. The bytes it
+// contributes must not be dropped in favour of the body length alone.
+{
+  const header = 'HTTP/1.1 200 OK\r\n\r\n';
+  const msg = createMessage();
+  msg._implicitHeader = function() { this._header = header; };
+  msg.chunkedEncoding = true;
+  msg.write('é'.repeat(100));
+  // Header + "c8" + CRLF + 200 bytes of body + CRLF.
+  assert.strictEqual(msg.outputSize,
+                     Buffer.byteLength(header) + 2 + 2 + 200 + 2);
+}
+
+// Chunked encoding without a header: the byte length handed to `_send()` is
+// the one that gets used, rather than the body being measured again.
+{
+  const msg = createMessage();
+  msg.chunkedEncoding = true;
+  msg.write('é'.repeat(100));
+  assert.strictEqual(msg.outputSize, 2 + 2 + 200 + 2);
+}
+
+// Backpressure kicks in once the buffered *bytes* reach the high water mark.
+// 50 two-byte characters are exactly 100 bytes, so the message is full.
+{
+  const msg = createMessage({ highWaterMark: 100 });
+  assert.strictEqual(msg.writableHighWaterMark, 100);
+  const ret = msg.write('é'.repeat(50));
+  assert.strictEqual(msg.outputSize, 100);
+  assert.strictEqual(ret, false);
+  assert.strictEqual(msg.writableNeedDrain, true);
+}
+
+// The same number of single-byte characters is only half as much data, so it
+// still fits.
+{
+  const msg = createMessage({ highWaterMark: 100 });
+  const ret = msg.write('a'.repeat(50));
+  assert.strictEqual(msg.outputSize, 50);
+  assert.strictEqual(ret, true);
+  assert.strictEqual(msg.writableNeedDrain, false);
+}
+
+// `writableLength` reports the buffered byte count.
+{
+  const msg = createMessage();
+  msg.write('é'.repeat(100));
+  assert.strictEqual(msg.writableLength, 200);
+}
+
+// Flushing hands back exactly what was accounted for, leaving the counters at
+// zero rather than drifting.
+{
+  const msg = createMessage();
+  let pending = 0;
+  msg._onPendingData = (delta) => { pending += delta; };
+  msg.write('é'.repeat(100));
+  msg.write('😀'.repeat(10));
+  assert.strictEqual(pending, 240);
+  assert.strictEqual(msg.outputSize, 240);
+
+  const written = [];
+  msg._flushOutput({
+    cork() {},
+    uncork() {},
+    write(data, encoding) { written.push([data, encoding]); },
+  });
+  assert.strictEqual(msg.outputSize, 0);
+  assert.strictEqual(pending, 0);
+  assert.strictEqual(written.length, 2);
+}
+
+// End to end: correcting the accounting must not change what is put on the
+// wire. A chunked multi-byte body goes through the corked path, where the
+// chunk is handed to `_send()` without a precomputed length, so this also
+// exercises measuring the string inside `_writeRaw()`.
+{
+  const body = '😀é漢字'.repeat(2000);
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.setHeader('Content-Type', 'text/plain; charset=utf-8');
+    // Without a content-length the response is chunked.
+    res.write(body);
+    res.end();
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    http.get({ port: server.address().port }, common.mustCall((res) => {
+      assert.strictEqual(res.headers['transfer-encoding'], 'chunked');
+      const chunks = [];
+      res.on('data', (chunk) => chunks.push(chunk));
+      res.on('end', common.mustCall(() => {
+        const received = Buffer.concat(chunks);
+        assert.strictEqual(received.length, Buffer.byteLength(body));
+        assert.strictEqual(received.toString('utf8'), body);
+        server.close();
+      }));
+    }));
+  }));
+}