http: include Content-Length in HEAD responses for keep-alive

[erdinccurebal]

Summary

HEAD responses from http.createServer do not include a Content-Length header by default when res.end() is called without explicit headers. This breaks HTTP keep-alive for HEAD requests because the client-side HTTP parser cannot determine message boundaries.

GET responses already include Content-Length: 0 by default. This change applies the same behavior to HEAD (and other bodyless) responses.

Before

GET / → headers: { 'content-length': '0', connection: 'keep-alive', ... }
HEAD / → headers: { connection: 'keep-alive', ... }  // missing content-length!

After

GET / → headers: { 'content-length': '0', connection: 'keep-alive', ... }
HEAD / → headers: { 'content-length': '0', connection: 'keep-alive', ... }

Changes

• lib/_http_outgoing.js: In _storeHeader(), when the response has no body (!this._hasBody), also emit Content-Length header if the content length is known and was not explicitly removed by the user.
• test/parallel/test-http-head-response-content-length-keep-alive.js: New test verifying that both GET and HEAD responses include Content-Length: 0 when res.end() is called.

Reproduction

const http = require('http');
const server = http.createServer((req, res) => res.end());
server.listen(2333);

const agent = new http.Agent({ keepAlive: true });
http.request({ port: 2333, method: 'HEAD', agent }, (res) => {
  console.log(res.headers); // no content-length → keep-alive broken
}).end();

Fixes: #28438

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net