Feature request: unified App handle for live + emulator backends #33

Use case

I'm building a backend service that needs to support both live Firebase (production) and the Auth emulator (local dev/CI), selected at startup from environment config. The service holds a long-lived auth client and uses two App capabilities:

1. auth() — user management (create_user, get_user, etc.)
2. id_token_verifier() — verify incoming ID tokens on each request

In other ecosystems (e.g. TypeScript), this is straightforward: hold a reference to a single App interface and call the same methods regardless of backend. I'd like the Rust SDK to support a similar ergonomics story without every consumer re-implementing the same adapter layer.

Current friction

App<EmulatorCredentials> and App<AccessTokenCredentials> are separate types with different method signatures:

This makes it impossible to write something as simple as:

fn build_client(app: ???) -> FirebaseAuthClient { ... }

without either generics (FirebaseAuthClient<C>) or a consumer-side enum.

Workaround we had to build

We ended up with ~70 lines of boilerplate in our app just to paper over the API differences:

pub enum FirebaseAppHandle {
    Live {
        app: App<AccessTokenCredentials>,
        project_id: String,  // duplicated — App::project_id is private
    },
    Emulated {
        app: App<EmulatorCredentials>,
        emulator_url: String,
    },
}

impl FirebaseAppHandle {
    pub fn auth(&self) -> FirebaseAuth<ReqwestApiClient> { /* match */ }

    pub fn id_token_validator(&self) -> Result<IdTokenValidator, AuthError> {
        match self {
            // Cannot use app.id_token_verifier() — see below
            Self::Live { project_id, .. } => Ok(IdTokenValidator::Live(
                LiveValidator::new_jwt_validator(project_id.clone())?,
            )),
            Self::Emulated { .. } => Ok(IdTokenValidator::Emulator(EmulatorValidator)),
        }
    }
}

pub enum IdTokenValidator {
    Live(LiveValidator),
    Emulator(EmulatorValidator),
}

Specific pain points

1. impl TokenValidator return type is not usable at call sites that need concrete types

App::id_token_verifier() returns impl TokenValidator, but our enum variant expects LiveValidator:

// Does not compile:
Ok(IdTokenValidator::Live(
    app.id_token_verifier()?.  // error: expected LiveValidator, found impl TokenValidator
))

We had to bypass App::id_token_verifier() entirely and call LiveValidator::new_jwt_validator() directly — which requires duplicating project_id because App::project_id is private.

2. Inconsistent error handling between live and emulator

Live id_token_verifier() returns Result<...>, emulator returns the validator directly. Consumers normalizing both paths need extra match arms for no functional reason.

3. Emulator URL is not part of App state

auth(emulator_url) requires the URL on every call (or the consumer must store it alongside App). For a long-lived client, the emulator host is configuration — it would be more natural to set it once at construction (e.g. from FIREBASE_AUTH_EMULATOR_HOST).

4. No shared abstraction for "live or emulated"

The phantom type parameter App<C> is a nice compile-time distinction, but most application code wants runtime selection. A first-party enum or trait would save every consumer from writing the same adapter.

Suggested improvements

Any of these would significantly improve the developer experience. Ordered roughly by impact:

Option A: First-party AppMode enum (highest impact)

pub enum App {
    Live { /* ... */ },
    Emulated { emulator_url: String, /* ... */ },
}

impl App {
    pub async fn live() -> Result<Self, ...> { ... }
    pub fn emulated(emulator_url: impl Into<String>) -> Self { ... }

    pub fn project_id(&self) -> &str { ... }
    pub fn auth(&self) -> FirebaseAuth<ReqwestApiClient> { ... }
    pub fn id_token_verifier(&self) -> Result<IdTokenVerifier, ...> { ... }
}

pub enum IdTokenVerifier {
    Live(LiveValidator),
    Emulator(EmulatorValidator),
}

impl TokenValidator for IdTokenVerifier { ... }

This gives consumers a single type to store and pass around, with normalized method signatures.

Option B: Normalize the existing impl blocks

Smaller changes that would still help:

• Unify auth(): e.g. auth(&self) -> FirebaseAuth<...> on both variants, with emulator URL set at App::emulated(url) construction time.
• Unify id_token_verifier(): same return type on both paths — Result<IdTokenVerifier, _> or return concrete enums instead of impl TokenValidator.
• Expose project_id() as a public accessor on both App variants.
• Return concrete types from id_token_verifier() (LiveValidator / EmulatorValidator) rather than impl TokenValidator, or provide an SDK-owned enum that implements TokenValidator.

Option C: Object-safe TokenValidator

If TokenValidator were object-safe (e.g. via async_trait or boxed futures), consumers could use Box<dyn TokenValidator>. Currently the impl Future return on validate() prevents this, forcing the concrete-type enum workaround.

Environment

• rs-firebase-admin-sdk (latest from crates.io)
• Rust 2024 edition
• Use case: Axum HTTP server holding Arc<FirebaseAuthClient> across request handlers

Happy to contribute

If any of these directions align with your design goals, I'm happy to open a PR. Just let me know which approach you'd prefer.