http: don't write error to socket

The state of the connection is unknown at this point and
writing to it can corrupt client state before it is aware
of an error.

PR-URL: #34465
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/http.md

@@ -1071,8 +1071,8 @@ type other than {net.Socket}.
 
 Default behavior is to try close the socket with a HTTP '400 Bad Request',
 or a HTTP '431 Request Header Fields Too Large' in the case of a
-[`HPE_HEADER_OVERFLOW`][] error. If the socket is not writable it is
-immediately destroyed.
+[`HPE_HEADER_OVERFLOW`][] error. If the socket is not writable or has already
+written data it is immediately destroyed.
 
 `socket` is the [`net.Socket`][] object that the error originated from.
 

lib/_http_server.js

@@ -599,7 +599,7 @@ function socketOnError(e) {
   this.on('error', noop);
 
   if (!this.server.emit('clientError', e, this)) {
-    if (this.writable) {
+    if (this.writable && this.bytesWritten === 0) {
       const response = e.code === 'HPE_HEADER_OVERFLOW' ?
         requestHeaderFieldsTooLargeResponse : badRequestResponse;
       this.write(response);

test/parallel/test-http-header-badrequest.js

@@ -0,0 +1,31 @@
+'use strict';
+const { mustCall } = require('../common');
+const assert = require('assert');
+const { createServer } = require('http');
+const { createConnection } = require('net');
+
+const server = createServer();
+
+server.on('request', mustCall((req, res) => {
+  res.write('asd', () => {
+    res.socket.emit('error', new Error('kaboom'));
+  });
+}));
+
+server.listen(0, mustCall(() => {
+  const c = createConnection(server.address().port);
+  let received = '';
+
+  c.on('connect', mustCall(() => {
+    c.write('GET /blah HTTP/1.1\r\n\r\n');
+  }));
+  c.on('data', mustCall((data) => {
+    received += data.toString();
+  }));
+  c.on('end', mustCall(() => {
+    // Should not include anything else after asd.
+    assert.strictEqual(received.indexOf('asd\r\n'), received.length - 5);
+    c.end();
+  }));
+  c.on('close', mustCall(() => server.close()));
+}));