GAE how to serve 404 pages - google-app-engine

Question

I have app engine an app with some static pages.
I want to serve my static content, and a script. Finally, I want to serve 404 for everything else.
My app.yaml file looks something like
runtime: go113
handlers:
- url: /myscripy
script: auto
- url: /$
static_files: public/index.html
secure: always
upload: public/index.html
- url: /(.*/)?$
static_files: public/\1index.html
secure: always
upload: public/(.*/)?index.html
http_headers:
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Frame-Options: SAMEORIGIN
OK, that's working except a few things
it's unclear to me why the - url: /$ is necessary, why is it not subsumed by - url: /(.*/)?$, but that's just a side note.
main problem: any URL that matches nothing will be rendered as empty file with a 200 status code instead of a 404.
I can force 404s by adding something like
- url: /.*
static_files: public/some_non_existing_file.html
secure: always
upload: public/some_non_existing_file.html
at the end of the app.yaml file, but that triggers an ugly 404 last ditch page that is not very appealing.
I can serve nice 404 custom pages like
- url: /.*
secure: always
static_files: public/404.html
upload: public/404.html
http_headers:
Status: 404
where public/404.html exists, however the Status: 200 header is sent before the Status: 404 header, making the whole response a 200.
I had hopes for
error_handlers:
- file: 404.html
to do something for me, but no, it seems to not do anything at all.
I can of course add a custom script to create the 404 response, but it feels a bit silly to me. Isn't there a better way ?
I'm a bit stumped - I can't believe that a long standing industry tool like GAE has no way to statically define a custom 404 page, or even render 404 status code for nonexisting pages.
Really appreciate any advice, thanks in advance!

Answer 1

Indeed, there is not easier way for you to achieve the custom 404 page, then adding a custom script to create the 404 response. App Engine, unfortunately, still doesn't work very well with handling some errors, which includes the error 404. As you can check in this official documentation here, there is no custom handlers from this error on Java.
This is just an example that a custom script is the viable solution.Otherwise, the solutions won't return the error 404 - as you well mentioned - or you would need to use a catch-all handler, which is not very recommended as well, since it's too broad of a solution.
In case you are using Python, there is a very good example of how to configure the trigger and example of script here.
Let me know if the information helped you!