http: allow Content-Length header for 304 responses

Fixes: #31037

PR-URL: #34835
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/_http_client.js

@@ -643,6 +643,11 @@ function parserOnIncomingClient(res, shouldKeepAlive) {
   if (method === 'HEAD')
     return 1;  // Skip body but don't treat as Upgrade.
 
+  if (res.statusCode === 304) {
+    res.complete = true;
+    return 1; // Skip body as there won't be any
+  }
+
   return 0;  // No special treatment.
 }
 

test/parallel/test-http-allow-content-length-304.js

@@ -0,0 +1,32 @@
+'use strict';
+const common = require('../common');
+
+// This test ensures that the http-parser doesn't expect a body when
+// a 304 Not Modified response has a non-zero Content-Length header
+
+const assert = require('assert');
+const http = require('http');
+
+const server = http.createServer(common.mustCall((req, res) => {
+  res.setHeader('Content-Length', 11);
+  res.statusCode = 304;
+  res.end(null);
+}));
+
+server.listen(0, () => {
+  const request = http.request({
+    port: server.address().port,
+  });
+
+  request.on('response', common.mustCall((response) => {
+    response.on('data', common.mustNotCall());
+    response.on('aborted', common.mustNotCall());
+    response.on('end', common.mustCall(() => {
+      assert.strictEqual(response.headers['content-length'], '11');
+      assert.strictEqual(response.statusCode, 304);
+      server.close();
+    }));
+  }));
+
+  request.end(null);
+});