fix(error_handler): fix the bug that error handler can't recognize st…

…atus code 494. (#12114)

* fix(error_handler): fix the bug that error handler can't recognize
status code 494.

There is a dedicated response body for 494 defined in error_handler.
However, based on the current configuration for `error_page` in
nginx-kong.conf, 494 will not be treated correctly wihout reserving it
by the `=response` option in `error_page` directive.
In this PR, a `error_page` configuration is added for 494 separately, so
that it can be recognized in error handler, and it will be replaced
with 400 finally.

FTI-5374

changelog/unreleased/kong/error_handler_494.yml

@@ -0,0 +1,3 @@
+message: Fix a bug that the error_handler can not provide the meaningful response body when the internal error code 494 is triggered.
+type: bugfix
+scope: Core

kong/error_handlers.lua

@@ -59,6 +59,13 @@ return function(ctx)
   local status = kong.response.get_status()
   local message = get_body(status)
 
+  -- Nginx 494 status code is used internally when the client sends
+  -- too large or invalid HTTP headers. Kong is obliged to convert
+  -- it back to `400 Bad Request`.
+  if status == 494 then
+    status = 400
+  end
+
   local headers
   if find(accept_header, TYPE_GRPC, nil, true) == 1 then
     message = { message = message }

kong/templates/nginx_kong.lua

@@ -81,7 +81,8 @@ server {
     listen $(entry.listener);
 > end
 
-    error_page 400 404 405 408 411 412 413 414 417 494 /kong_error_handler;
+    error_page 400 404 405 408 411 412 413 414 417 /kong_error_handler;
+    error_page 494 =494                            /kong_error_handler;
     error_page 500 502 503 504                     /kong_error_handler;
 
     # Append the kong request id to the error log

spec/02-integration/05-proxy/13-error_handlers_spec.lua

@@ -36,7 +36,7 @@ describe("Proxy error handlers", function()
     assert.res_status(400, res)
     local body = res:read_body()
     assert.matches("kong/", res.headers.server, nil, true)
-    assert.matches("Bad request\nrequest_id: %x+\n", body)
+    assert.matches("Request header or cookie too large", body)
   end)
 
   it("Request For Routers With Trace Method Not Allowed", function ()