How to spawn pre-loaded mojolicious applications from a mojolicious application? - perl

Question

I work on a fairly large Mojolicious application, which takes many seconds to compile.
Parts of the test suite of that application are written using playwright, which currently sets up a pristine database for each test case, and spins up an instance of the mojolicious application using #mojolicious/server-starter.
The compile-time of the application is starting to make it impractical to significantly expand the playwright test suite, and I'd like to address that without having to give up on the isolation that separate test databases and mojolicious application instances currently afford me.
In order to achieve that, the idea I'm currently pursuing is to have a small perl application that can pre-load the larger Mojolicious application, and which can be asked by the playwright test-suite to spawn a new instance of the larger application with a pristine database on an open port.
I'd like to communicate with that small perl application using HTTP, mainly out of convenience, and I'd like that small perl application to use Mojolicious to perform the HTTP communication, because that's also convenient and consistent with the rest of the code-base.
I've tried some naive approaches to implementing this idea, which looked roughly like this:
use TheBigApp;
$app->routes->post('/spawn-child')->to(cb => sub ($c) {
my ($sock, $port) = new_listen_socket();
if (my $pid = fork()) {
# record pid to later be able to shut it down or whatever
$c->render(json => { url => "http://localhost:$port" })
} else {
my $bapp = TheBigApp->new($c->req->json);
my $s = Mojo::Server::Daemon->new(listen => ...);
$s->app($bapp);
$bapp->start;
return;
}
});
All of the implementations I've tried along these lines seemed to run into issues due to the various singletons, such as the IOLoop, and even when overriding IOLoop->singleton to return a new instance with a new reactor within the sub-process, it appeared as if the forked-off child processes were still listening on the same socket as the parent-process that spawned them.
Are there perhaps lower-level Mojolicious APIs that could make this use-case work? Would it perhaps be simpler to implement the small parent process without Mojolicious to sidestep the issue entirely?
Thanks!

Answer 1

Digging a little in the Mojo Server/Daemon code, it always(?) sets up 'ReusePort' on the IO::Socket.
On linux (not macOS or BSD, windows I have no idea) that means that TCP connections to the same IP and port combination are 'load balanced' across multiple server instances by the kernel.
It is unclear from you post if the spawned processes are listening on the same port or not.
Assuming you are running linux, changing the listening port for each spawned child might help.