Add timeout and abort middleware and processing halts (#18177)

* Add middleware to timeout requests after a period

* Add halt-on-dropped-connection middleware to stop the middleware processing stack if the connection was already dropped

* Add a few strategic bail-out spots for dropped connections during the render-page middleware

* Handle 404s and HEAD requests earlier in the page rendering flow

* Add a few more strategic bail-out spots for dropped connections during the render-page middleware

* Add middleware to notice aborted requests

* Add a check for aborted requests into the isConnectionDropped logic

* Reformat comment for consistency

* Handle aborted requests correctly in the error handler

* Explicit returns for consistency

middleware/handle-errors.js

@@ -6,7 +6,9 @@ const loadSiteData = require('../lib/site-data')
 function shouldLogException (error) {
   const IGNORED_ERRORS = [
     // avoid sending CSRF token errors (from bad-actor POST requests)
-    'EBADCSRFTOKEN'
+    'EBADCSRFTOKEN',
+    // Client connected aborted
+    'ECONNRESET'
   ]
 
   if (IGNORED_ERRORS.includes(error.code)) {
@@ -26,8 +28,8 @@ async function logException (error, req) {
 }
 
 module.exports = async function handleError (error, req, res, next) {
-  // If the headers have already been sent...
-  if (res.headersSent) {
+  // If the headers have already been sent or the request was aborted...
+  if (res.headersSent || req.aborted) {
     // Report to Failbot
     await logException(error, req)