How to use Middleware to catch exceptions, instead of Kelp?

[rawleyfowler]

I have a large existing Plack monolith that uses Plack::Builder to compose and mount a variety of Plack applications. We have an existing Middleware that translates exceptions into HTTP codes, logs, them and adds them to a tracing platform.

We have a new small Plack application that is working as a POC for using Kelp, that is being mounted, and we'd like to have our existing Plack exception handler (a Plack middleware), handle exceptions thrown during the runtime of our new Kelp application, is this possible? If not would this feature be wanted? Thanks.

[bbrtj]

I don't think this is currently possible, because Kelp catches errors and renders them, does not contain any code that may re-throw them: https://metacpan.org/dist/Kelp/source/lib/Kelp.pm#L344-370

This isn't wrong because Kelp has its own request/response handling, error logging and finalization hooks, and re-throwing the exception would make it lose all this context while reverting to pure Plack state. So I think your approach is pretty custom, but a sane upgrade path for a Plack application.

The best way to make it possible would probably be moving the code I highlighted above to a separate method, for example _handle_exception. Kelp subclass could then completely override that method and do anything it wants with the exception.

[rawleyfowler]

For now, we've overwritten the psgi method on Kelp. Kind of a huge hack, but it does work...

[bbrtj]

I will eventually move the catch code to a separate method, so that you will only have to override that method. But I am not doing Kelp work currently, and this is low severity in my eyes, so can't give an ETA on it. Should probably wait for more stuff to be included before releasing the next version anyway.

If you could share your modified psgi method then I could fully understand what exactly you need for your error handling to work. That may affect the way I implement it.

[rawleyfowler]

@bbrtj

Basically just removing the try/catch.

BEGIN {
  # Kelp's psgi method needs to be overridden to handle exceptions properly
  *{'Kelp::psgi'} = sub {
    my ($self, $env) = @_;

    $self->context->clear;
    my $req = $self->req($self->build_request($env));
    my $res = $self->res($self->build_response);

    my $match = $self->routes->match($req->path, $req->method);

    if (!@$match) {
        $res->render_404;
        return $self->finalize;
    }

    $env->{'kelp.execution_chain'} = [
        (grep { defined } map { $_->psgi_middleware } @$match),
        sub { $self->_psgi_internal($match) },
    ];

    return Kelp->NEXT_APP->($env);
  };
}

# configurations

builder {
   #... Plack applications
}

I could implement a method on Kelp that sets a flag to use the default error handler or not.