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

When an HTTP server responds to a HEAD request with res.end(),
the response does not include a Content-Length header by default.
This causes the HTTP parser on the client side to be unable to
determine message boundaries, which breaks keep-alive connections
for HEAD requests.

GET responses already include Content-Length: 0 by default when
res.end() is called without data. This change applies the same
behavior to HEAD responses (and other bodyless responses) by
adding Content-Length when known and not explicitly removed.

Fixes: #28438

Author: erdinccurebal

lib/_http_outgoing.js

@@ -493,6 +493,12 @@ function _storeHeader(firstLine, headers) {
     if (!this._hasBody) {
       // Make sure we don't end the 0\r\n\r\n at the end of the message.
       this.chunkedEncoding = false;
+      // For HEAD responses, include Content-Length if known so that
+      // the client can determine message boundaries for keep-alive.
+      if (!this._removedContLen &&
+          typeof this._contentLength === 'number') {
+        header += 'Content-Length: ' + this._contentLength + '\r\n';
+      }
     } else if (!this.useChunkedEncodingByDefault) {
       this._last = true;
     } else if (!state.trailer &&

test/parallel/test-http-head-response-content-length-keep-alive.js

@@ -0,0 +1,35 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+// This test ensures that HEAD responses include a Content-Length header
+// when the server calls res.end() without explicit headers, so that
+// HTTP keep-alive works correctly for HEAD requests.
+// Ref: https://github.com/nodejs/node/issues/28438
+
+const server = http.createServer(common.mustCall(function(req, res) {
+  res.end();
+}, 2));
+
+server.listen(0, common.mustCall(function() {
+  const port = this.address().port;
+  const agent = new http.Agent({ keepAlive: true });
+
+  // First: verify GET response includes Content-Length
+  const getReq = http.request({ port, method: 'GET', agent }, common.mustCall(function(res) {
+    assert.strictEqual(res.headers['content-length'], '0');
+
+    // Second: verify HEAD response also includes Content-Length
+    const headReq = http.request({ port, method: 'HEAD', agent }, common.mustCall(function(res) {
+      assert.strictEqual(res.headers['content-length'], '0',
+        'HEAD response should include Content-Length for keep-alive');
+      agent.destroy();
+      server.close();
+    }));
+    headReq.end();
+
+    res.resume();
+  }));
+  getReq.end();
+}));