[Functions] Address more Swift 6 warnings

[ncooke3]

Note: module is not yet 100% Swift 6 compliant.

1. This was the first error I sought to fix.

2. I added @MainActor so the compiler enforces that the completion handler can only be called on the main actor. This address the error in step 1.

3. The new @MainActor attribute needed to bubble up to signature of a function that called the first one.

4. Next, the compiler enforces the completion is only called on main actor.

5. That's fixable by wrapping in a main queue dispatch block.

6. The next appears back in the first function that was changed. The source of the error is that line 487 returns a HTTPSCallableResult which is not Sendable, so sending it could cause data races.

7. It doesn't seem safe to mark HTTPSCallableResult as Sendable (it would have to be unchecked) because it retains an Any. While our SDK could be audited to verify that it doesn't access the Any reference, there'd be no way to enforce that it isn't accessed unsafely in SDK or client code.

8. To fix the error, we can specify that the returned HTTPSCallableResult is sending which indicates that it isn't accessed after being returned to the calling context.

9. The new Task or actor isolated value cannot be sent error in the above screenshot is tricky. The following fixes appear to address it. I think the issue is that we need to indicate that the Any value is only owned in the current context. To do that, we mark responseDataJSON to return a sending Any. Marking FunctionsSerializer as Sendable is the final piece. I'm less sure why it resolves it though? Previously, I added sending to FunctionsSerializer's decodeAPI but that led to errors down a rabbit hole that I ultimately couldn't solve simply.

#no-changelog

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[paulb777]

Thanks for the step-by-step thorough explanation!

[morganchen12]

LGTM

I wonder if we could make HTTPCallableResult a generic instead of Any and then add Sendable conformance for parameterized types that are Sendable.

[ncooke3]

LGTM

I wonder if we could make HTTPCallableResult a generic instead of Any and then add Sendable conformance for parameterized types that are Sendable.

Yes, I like that approach. It would be breaking to add the generic and I am curious how the ObjC API would be affected.

[paulb777]

It looks like this might have broken the Objective-C API. See the nightly failure in https://github.com/firebase/firebase-ios-sdk/actions/runs/14679321280/job/41206125399. @ncooke3 PTAL