Fix(flags): False positive when variant rule is an async function

[vitoUwu]

There are scenarios where we have a multivariant (e.g Pages with variants) and variant.rule can return a Promise even if the matcher module returns a synchronous function, this occurs when I have a matcher in my project and an (asynchronous) middleware in my app site.ts , making variant.rule asynchronous as well. It doesn't impact matchers that are from other apps like website app that have a lot of matchers, but only the local ones

Illustration of my problem.

- Page "Home" 
   |- variants.0.home => Promise<boolean> -> when resolved it returns "false"
   \- variants.1.home => boolean -> always "true"

// looping through the flags.variants
index 0 -> variants.rule(ctx) -> Promise<false> -> true // because Promise is a truly value
early return with wrong variant match

The variant.rule check is always true since Promise is a truly value, even if the final value of the promise is false.

This PR makes it possible for us to detect asynchronous functions in the flags block, where the problem was occurring, and if necessary we make sure the promise is resolved so that we can check if the variant.rule returns true

- Page "Home" 
   |- variants.0.home => Promise<boolean> -> when resolved it returns "false"
   \- variants.1.home => boolean -> always "true"

// looping through the flags.variants
index 0 -> variants.rule(ctx) -> Promise<false> -> await for the result -> don't match
index 1 -> variants.rule(ctx) -> return as it's returning "true"
return with the correct match

[github-actions]

Tagging Options

Should a new tag be published when this PR is merged?

• 👍 for Patch 1.108.1 update
• 🎉 for Minor 1.109.0 update
• 🚀 for Major 2.0.0 update

[mcandeia]

this will impact tracings as only async functions are included on tracing. This shouldn't be though but we should keep an eye on it.

[mcandeia]

in the case you provided (when site.ts has an app middleware) this code runs on serial (one await for each matcher) instead of parallel since all matchers will returns a promise, right? Given that, you should instead, parallelize the rule evaluation.

[vitoUwu]

not at all, matchers coming from "website" app continue as sync functions, but my local ones, that depends on my async middleware, turns into async function

[mcandeia]

I mean, anyways introducing a middleware will cause all your matchers to be async, so it worth to make parallel instead, wdyt?

[vitoUwu]

make sense, your proposal is to always use Promise.all on this list, right?

Promise.all(flags.variants.map((variant) => variant.rule(ctx)).find((result) => result)