Allow null Session in SessionCallback when there is an error.

[kylerush]

It looks like the existing code support an error being thrown on store.get, however, the type definition SessionCallback for the callback function on store.get requires a Session type variable for the session argument. This change would not cause the type check to fail when you call the store.get callback function like so:

callback(new Error("Some error"), null)

This change also exports the SessionCallback type on the FastifySession type so that we can specify that our store.get callback function is of type FastifySession.SessionCallback.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[kylerush]

Thanks @climba03003! I'm going to commit your suggestion and then would like to propose one more change to export the SessionCallback type. Will add that change shortly.

[mcollina]

lgtm

[Uzlopak]

@climba03003 PTAL

[kylerush]

@climba03003 this new type definition seems to not be working, or at least I don't understand how it should work.

With the suggestion change, typescript tells me:

1. Argument of type 'null' is not assignable to parameter of type 'Error'.
2. Argument of type 'Session' is not assignable to parameter of type 'undefined'.

I am not quite sure how to solve this problem. Is seems this union type definition doesn't quite work for the use case. Any idea?

I tried to change it to an overloaded function, but that caused the arguments in the callback function accepts to lose their types:

[mcollina]

CI is not happy

[kylerush]

Yeah it needs a little more work. The new tuple type seems to be causing issues as currently implemented. Will try to knock this out tomorrow.

[jsardev]

Hey folks 👋 I see this one didn't get finished and I'd like to have it fixed as well.

I took some time to tinker around with CallbackSession type and found out that we don't really have a flawless solution for this case. Here are a few implementations that I tested out:

Intersection

type Callback = ( ((err: Error) => void) & ((err: null, result: number) => void) );

const getSession = (callback: Callback) => {
    callback(new Error()); // correct
    callback(new Error(), 5); // throws error, correct
    callback(null, 5); // correct
}

getSession((error) => {
    // parameters properly typed, TS compiles
    return;
})

getSession((error, result) => {
    // parameters properly typed, but TS does not compile
    return;
})

Function overload

type Callback = {
    (err: Error): void;
    (err: null, result: number): void;
}

const getSession = (callback: Callback) => {
    callback(new Error()); // correct
    callback(new Error(), 5); // throws error, correct
    callback(null, 5); // correct
}

getSession((error) => {
    // parameters properly typed, TS compiles
    return;
})

getSession((error, result) => {
    // parameters properly typed, but TS does not compile
    return;
})

You can see within the comments that those do not work as intended due to how TypeScript works. I think that the most optimal way we could go is this:

type Callback = {
    (err: Error, result?: undefined): void;
    (err: null, result: number): void;
}

const getSession = (callback: Callback) => {
    callback(new Error()); // correct
    callback(new Error(), 5); // throws error, correct
    callback(null, 5); // correct
}

getSession((error) => {
    // parameters properly typed but are not fully precise, TS compiles
    return;
})

getSession((error, result) => {
    // parameters properly typed but are not fully precise, TS compiles
    return;
})

This way we lose the preciseness of the arguments of the callback that we pass to the get function, but at least callback calls work as intended.

@kylerush Let me know if you have time to finish this off. If not, please let me know - I'll create a separate PR with the fix.

Also, let me know what you all folks think!

[mcollina]

@sarneeh go ahead and open a fresh PR!