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Rust Google Oauth2 Client Implementation

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gauth
=====

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The library supports the following Google Auth flows:

* [OAuth2 for installed apps](https://developers.google.com/identity/protocols/oauth2#installed)
* [Service Accounts](https://developers.google.com/identity/protocols/oauth2/service-account)

```toml
[dependencies]
gauth = "0.10"
```

#### OAuth2

1. Create your application in [Google API Console](https://console.developers.google.com/apis/credentials)
   a. `Credentials` > `Create credentials` > `OAuth client ID`
   b. Set application type to `Desktop app` (Google retired the `Other` type — `Desktop app` is the modern equivalent for installed-app flows)
   c. Enter your application name
   d. `Download JSON` configuration of the newly created application

**Client implementation with defaults**

```rust,no_run
use gauth::app::Auth;

#[tokio::main]
async fn main() {
    let auth_client = Auth::from_file(
        "my_credentials.json",
        vec!["https://www.googleapis.com/auth/drive"],
    )
    .unwrap();

let token = auth_client.access_token().await.unwrap();
    println!("access token: {}", token);
}
```

It is also possible to make a **blocking call** to retrieve an access token. This may be helpful if we want to wrap the logic into a closure.

```toml
[dependencies]
gauth = { version = "0.10", features = ["app-blocking"] }
```

```rust,no_run
use gauth::app::Auth;

#[tokio::main]
async fn main() {
    let ga = Auth::from_file(
        "client_secret.json",
        vec!["https://www.googleapis.com/auth/drive"]
    ).unwrap();

let closure = move || {
        // add some logic here
        ga.access_token_blocking()
    };

let token = closure().unwrap();
    println!("token from closure: {}", token);
}
```

**Custom app name and handler**: access token will be stored in `$HOME/.{app_name}/access_token.json`

To assign a custom directory as access token caching, set env var value: `GAUTH_TOKEN_DIR`

```rust,no_run
use gauth::app::Auth;
use anyhow::Error as AnyError;

#[tokio::main]
async fn main() {
    let auth_handler = |consent_uri: String| -> Result<String, AnyError> {
        // business logic
        Ok("auth_code".to_owned())
    };

let auth_client = Auth::from_file(
        "my_credentials.json",
        vec!["https://www.googleapis.com/auth/drive"],
    )
    .unwrap()
    .app_name("new_name")
    .handler(auth_handler);
    let token = auth_client.access_token().await.unwrap();
    println!("access token: {}", token);
}
```

#### Service Account

Follow instructions for [creating a service account](https://developers.google.com/identity/protocols/oauth2/service-account#creatinganaccount). After a service account key has been created,
it can be used to obtain an access token.

```rust,no_run
use gauth::serv_account::ServiceAccount;

#[tokio::main]
async fn access_token() {
    let scopes = vec!["https://www.googleapis.com/auth/drive"];
    let key_path = "test_fixtures/service-account-key.json";

let mut service_account = ServiceAccount::from_file(key_path, scopes);
    let access_token = service_account.access_token().await.unwrap();

println!("access token {}:", access_token);
}
```

**Loading the key from memory** — useful when the credentials live in a database, an environment variable, or are fetched at runtime (no disk write needed):

```rust,no_run
use gauth::serv_account::ServiceAccount;

#[tokio::main]
async fn access_token_from_bytes(key_json: &[u8]) {
    let scopes = vec!["https://www.googleapis.com/auth/drive"];
    let mut service_account = ServiceAccount::from_bytes(key_json, scopes);
    let access_token = service_account.access_token().await.unwrap();

println!("access token: {}", access_token);
}
```

`JwtToken::from_bytes` is also available if you only need the signed JWT and want to drive the token exchange yourself.

### Token sources with expiry-aware caching (recommended)

The `source` feature provides the v2 token API: a `TokenSource` fetches
one fresh token, and `CachingTokenSource` adds expiry-aware caching with
single-flight refresh — no background task, no empty-string cold reads,
and no runtime coupling (works on any executor).

```toml
[dependencies]
gauth = { version = "0.11", features = ["source"] }
```

```rust,no_run
use gauth::serv_account::ServiceAccount;
use gauth::source::CachingTokenSource;

let account = ServiceAccount::from_file(
    "/path/to/key.json",
    vec!["https://www.googleapis.com/auth/pubsub"],
);
let source = CachingTokenSource::new(account);

// Cached while fresh; refreshed inline (single-flight) near expiry.
let token = source.token().await?;
let header = token.authorization_header(); // "Bearer …"
```

#### Workload Identity / metadata server

On Google compute platforms (GCE, GKE with Workload Identity, Cloud Run)
the metadata server issues tokens for the workload's service account —
no key file involved:

```rust,no_run
use gauth::metadata::MetadataTokenSource;
use gauth::source::CachingTokenSource;

let source = CachingTokenSource::new(
    MetadataTokenSource::new()
        .with_scopes(["https://www.googleapis.com/auth/pubsub"]),
);

let token = source.token().await?;
```

The metadata host honors the `GCE_METADATA_HOST` environment variable.

### Bridging sync and async code (deprecated)

> **Deprecated since 0.11:** prefer the `source` API above.
> `AsyncTokenProvider` returns an empty string before its first
> successful refresh and refreshes on a wall-clock interval rather than
> token expiry. It will be removed in 0.12.

The default implementation for acquiring the access token in this library is asynchronous. However, there are scenarios where a synchronous call is necessary. For instance, asynchronous signatures can be cumbersome when used with [tonic middlewares](https://docs.rs/tonic/latest/tonic/service/trait.Interceptor.html). The difficulties of integrating synchronous and asynchronous code are outlined in this [GitHub issue](https://github.com/hyperium/tonic/issues/870).

To resolve this, we adopted an experimental approach by developing a `token_provider` package. This package includes a `Watcher` trait, which has been implemented for both the `app` and `serv_account` packages. Each implementation of this trait spawns a daemon that periodically polls for and caches token updates at specified intervals. As a result, tokens are consistently refreshed through an asynchronous process. The retrieval of tokens is simplified to a synchronous function that reads from the internal cache.

```toml
[dependencies]
gauth = { version = "0.10", features = ["token-watcher"] }
```

```rust,no_run
let service_account = ServiceAccount::from_file(&keypath, vec!["https://www.googleapis.com/auth/pubsub"]);

let tp = AsyncTokenProvider::new(service_account).with_interval(5);

// the token is updated every 5 seconds
// and cached in AsyncTokenProvider
tp.watch_updates().await;

// sync call to get the access token
let access_token = tp.access_token()?;
```

The full example can be found [here](./examples/async_token_provider.rs)

## License

License under either or:

* [MIT](LICENSE-MIT)
* [Apache License, Version 2.0](LICENSE-APACHE)

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