package Mojolicious::Plugin::Google::Cloud::UserAgent;
use Mojo::Base 'Mojolicious::Plugin';
use Mojo::JWT::Google;

our $VERSION = '0.02';

has oauth_url     => 'https://www.googleapis.com/oauth2/v4/token';
has grant_type    => 'urn:ietf:params:oauth:grant-type:jwt-bearer';
has web_token     => undef;
has web_token_enc => undef;

sub register {
    my ($self, $app, $config) = @_;

    $app->helper(
        jwt => sub {
            my $c = shift;
            my $scopes = shift // $config->{scopes};

            Mojo::JWT::Google->new(
                from_json  => $config->{gcp_auth_file},
                target     => $config->{oauth_url} // $self->oauth_url,
                scopes     => Mojo::Collection->new($scopes)->flatten,
                issue_at   => time,
                expires_in => $config->{duration} // 3600
            );
        }
    );

    $app->helper(
        gcp_ua => sub {
            my $c      = shift;
            my $err_cb = pop;
            my $cb     = pop;
            my @args   = @_;

            if (!$self->web_token
                or ($self->web_token->issue_at + $self->web_token->expires_in) < time)
            {
                $self->web_token($c->jwt);
                $self->web_token_enc($self->web_token->encode);
            }

            Mojo::IOLoop::Delay->new->steps(
                sub {
                    my $d = shift;
                    $c->ua->post(
                        $self->oauth_url,
                        form => {
                            grant_type => $config->{grant_type} // $self->grant_type,
                            assertion => $self->web_token_enc
                        },
                        $d->begin
                    );
                },

                sub {
                    my $d  = shift;
                    my $tx = pop;

                    if ($tx->res->json('/access_token')) {
                        $d->pass($tx->res->json);
                    }

                    else {
                        $d->remaining([$err_cb]);
                        $d->pass($tx);
                    }
                },

                sub {
                    my $d     = shift;
                    my $token = pop;

                    my $tx = $c->ua->transactor->tx(@args);
                    $tx->req->headers->header(Authorization => $token->{token_type} . ' ' . $token->{access_token});
                    $c->ua->start($tx, $d->begin);
                },

                $cb
            );
        }
    );
}

1;
__END__

=encoding utf8

=head1 NAME

Mojolicious::Plugin::Google::Cloud::UserAgent - A user agent for GCP

=head1 SYNOPSIS

  # Mojolicious
  $self->plugin('Google::Cloud::UserAgent');

  # Mojolicious::Lite
  plugin 'Google::Cloud::UserAgent' => {
    gcp_auth_file => $ENV{GCP_AUTH_FILE},
    scopes        => ['https://www.googleapis.com/auth/pubsub'],
    duration      => 3600
  };

  # in a controller
  get '/' => sub {
    my $c = shift;
    $c->render_later;

    $c->gcp_ua(GET => "https://pubsub.googleapis.com/v1/projects/$ENV{GCP_PROJECT}/topics",
               sub {  ## response handler
                   my $tx = pop;
                   $c->render(json => $tx->res->json, status => $tx->res->code);
               },
               sub {  ## error sub
                   my $tx = pop;
                   $c->render(json => $tx->res->json, status => $tx->res->code);
               }
    );
  };

=head1 DESCRIPTION

L<Mojolicious::Plugin::Google::Cloud::UserAgent> is a L<Mojolicious>
plugin for interacting with Google Cloud Platform. It performs
2-legged OAuth described here:
L<https://cloud.google.com/docs/authentication>.

First, create a service account key:

L<https://console.cloud.google.com/apis/credentials?project=your-project>

Download this JSON file; you may hard-code the path to the JSON
service account key file when you load this plugin, or set an
environment variable, etc.

Next, determine the proper scopes for the user agent. Each GCP API has
a section in its documentation describing the scopes required to
invoke the API. For example, here is the Logging API scopes:

L<https://cloud.google.com/logging/docs/api/tasks/authorization>

Once you have these two items, pass them to the plugin as
configuration:

   plugin 'Google::Cloud::UserAgent' => {
     gcp_auth_file => $ENV{GCP_AUTH_FILE},
     scopes        => ['https://www.googleapis.com/auth/logging.write',
                       'https://www.googleapis.com/auth/logging.read'],
   };

=head1 METHODS

L<Mojolicious::Plugin::Google::Cloud::UserAgent> inherits all methods from
L<Mojolicious::Plugin> and implements the following new ones.

=head2 register

  $plugin->register(Mojolicious->new);

Register plugin in L<Mojolicious> application.

=head2 gcp_ua

  $c->gcp_ua(METHOD => $gcp_url, $headers, $type => $payload, $cb, $error_cb);

Makes a non-blocking HTTP request of the given method to the given GCP
url. Like L<Mojo::UserAgent>'s HTTP methods, C<$headers>, C<$type>,
and C<$payload> are optional. If the OAuth step succeeds, the C<$cb>
callback will be invoked, otherwise, the C<$error_cb> will be invoked.

The success callback will be passed a delay object and a transaction
object. Note that while the OAuth succeeded if this callback is
invoked, the API method itself may have failed, so you would check
that like this:

  $c->gcp_ua(GET => $api,
             sub {
                 my $tx = pop;
                 unless ($tx->success) {
                     $c->render(json => { error => "Had a boo-boo" },
                                status => 503);
                 }

                 ## everything is ok
                 ...
            },
            sub { }  ## error cb
            );

The second (error) callback is invoked if the OAuth attempt fails. It
will be passed a controller object and a transaction object.

  $c->gcp_ua(GET => $api,
             sub { },  ## success cb
             sub {
               my $tx = pop;
               $c->render(json => { error => $tx->res->body },
                          status => 403);
             });

Here is a more complete example to demonstrate headers and using a
JSON generator:

  $c->gcp_ua(POST => $api,
             { 'Accept' => 'application/json' },
             json => { message => "the payload" },
             sub {
               my $tx = pop;
               $c->render(json => $tx->res->json,
                          status => $tx->res->code);
             },
             sub {
               my $tx = pop;
               $c->app->log->warn("OAuth failed: " . $tx->res->body);
               $c->render(json => { error => "Could not auth" },
                          status => 403);
             });

=head1 SEE ALSO

L<Mojo::UserAgent>, L<Mojo::JWT::Google>

=cut
