http: send implicit headers on HEAD with no body

If we respond to a HEAD request with a body, we ignore all writes.
However, we must still include all implicit headers.

Fixes a regressions introduced in
nodejs#47732.

Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: nodejs#48108
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

lib/_http_outgoing.js

@@ -883,17 +883,6 @@ function write_(msg, chunk, encoding, callback, fromEnd) {
     err = new ERR_STREAM_DESTROYED('write');
   }
 
-  if (!msg._hasBody) {
-    if (msg[kRejectNonStandardBodyWrites]) {
-      throw new ERR_HTTP_BODY_NOT_ALLOWED();
-    } else {
-      debug('This type of response MUST NOT have a body. ' +
-        'Ignoring write() calls.');
-      process.nextTick(callback);
-      return true;
-    }
-  }
-
   if (err) {
     if (!msg.destroyed) {
       onError(msg, err, callback);
@@ -926,6 +915,17 @@ function write_(msg, chunk, encoding, callback, fromEnd) {
     msg._implicitHeader();
   }
 
+  if (!msg._hasBody) {
+    if (msg[kRejectNonStandardBodyWrites]) {
+      throw new ERR_HTTP_BODY_NOT_ALLOWED();
+    } else {
+      debug('This type of response MUST NOT have a body. ' +
+        'Ignoring write() calls.');
+      process.nextTick(callback);
+      return true;
+    }
+  }
+
   if (!fromEnd && msg.socket && !msg.socket.writableCorked) {
     msg.socket.cork();
     process.nextTick(connectionCorkNT, msg.socket);

test/parallel/test-http-head-response-has-no-body-end-implicit-headers.js

@@ -0,0 +1,27 @@
+'use strict';
+const common = require('../common');
+const http = require('http');
+
+// This test is to make sure that when the HTTP server
+// responds to a HEAD request with data to res.end,
+// it does not send any body but the response is sent
+// anyway.
+
+const server = http.createServer(function(req, res) {
+  res.end('FAIL'); // broken: sends FAIL from hot path.
+});
+server.listen(0);
+
+server.on('listening', common.mustCall(function() {
+  const req = http.request({
+    port: this.address().port,
+    method: 'HEAD',
+    path: '/'
+  }, common.mustCall(function(res) {
+    res.on('end', common.mustCall(function() {
+      server.close();
+    }));
+    res.resume();
+  }));
+  req.end();
+}));

test/parallel/test-http-head-response-has-no-body-end.js

@@ -29,7 +29,7 @@ const http = require('http');
 
 const server = http.createServer(function(req, res) {
   res.writeHead(200);
-  res.end();
+  res.end('FAIL'); // broken: sends FAIL from hot path.
 });
 server.listen(0);