Add rationale explaining why we do the header modifications in the filter

2021-03-01 18:01:44 +00:00
parent 510c2b90be
commit 316b7bff61
1 changed files with 5 additions and 0 deletions
@@ -102,6 +102,11 @@ export function buildMiddleware(
  // Use the custom middleware filter to do two things:
  //  1. Remove any headers not in the allow list to stop them being forwarded
  //  2. Only permit the allowed HTTP methods if configured
+  //
+  // We are filtering the proxy request headers here rather than in
+  // `onProxyReq` becuase when global-agent is enabled then `onProxyReq`
+  // fires _after_ the agent has already sent the headers to the proxy
+  // target, causing a ERR_HTTP_HEADERS_SENT crash
  const filter = (_pathname: string, req: http.IncomingMessage): boolean => {
    const headerNames = Object.keys(req.headers);
    headerNames.forEach(h => {