1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166

use axum::{
    async_trait,
    extract::{Extension, FromRequestParts},
};
use http::request::Parts;

/// Cache results of other extractors.
///
/// `Cached` wraps another extractor and caches its result in [request extensions].
///
/// This is useful if you have a tree of extractors that share common sub-extractors that
/// you only want to run once, perhaps because they're expensive.
///
/// The cache purely type based so you can only cache one value of each type. The cache is also
/// local to the current request and not reused across requests.
///
/// # Example
///
/// ```rust
/// use axum_extra::extract::Cached;
/// use axum::{
///     async_trait,
///     extract::FromRequestParts,
///     response::{IntoResponse, Response},
///     http::{StatusCode, request::Parts},
/// };
///
/// #[derive(Clone)]
/// struct Session { /* ... */ }
///
/// #[async_trait]
/// impl<S> FromRequestParts<S> for Session
/// where
///     S: Send + Sync,
/// {
///     type Rejection = (StatusCode, String);
///
///     async fn from_request_parts(parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
///         // load session...
///         # unimplemented!()
///     }
/// }
///
/// struct CurrentUser { /* ... */ }
///
/// #[async_trait]
/// impl<S> FromRequestParts<S> for CurrentUser
/// where
///     S: Send + Sync,
/// {
///     type Rejection = Response;
///
///     async fn from_request_parts(parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
///         // loading a `CurrentUser` requires first loading the `Session`
///         //
///         // by using `Cached<Session>` we avoid extracting the session more than
///         // once, in case other extractors for the same request also loads the session
///         let session: Session = Cached::<Session>::from_request_parts(parts, state)
///             .await
///             .map_err(|err| err.into_response())?
///             .0;
///
///         // load user from session...
///         # unimplemented!()
///     }
/// }
///
/// // handler that extracts the current user and the session
/// //
/// // the session will only be loaded once, even though `CurrentUser`
/// // also loads it
/// async fn handler(
///     current_user: CurrentUser,
///     // we have to use `Cached<Session>` here otherwise the
///     // cached session would not be used
///     Cached(session): Cached<Session>,
/// ) {
///     // ...
/// }
/// ```
///
/// [request extensions]: http::Extensions
#[derive(Debug, Clone, Default)]
pub struct Cached<T>(pub T);

#[derive(Clone)]
struct CachedEntry<T>(T);

#[async_trait]
impl<S, T> FromRequestParts<S> for Cached<T>
where
    S: Send + Sync,
    T: FromRequestParts<S> + Clone + Send + Sync + 'static,
{
    type Rejection = T::Rejection;

    async fn from_request_parts(parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
        match Extension::<CachedEntry<T>>::from_request_parts(parts, state).await {
            Ok(Extension(CachedEntry(value))) => Ok(Self(value)),
            Err(_) => {
                let value = T::from_request_parts(parts, state).await?;
                parts.extensions.insert(CachedEntry(value.clone()));
                Ok(Self(value))
            }
        }
    }
}

axum_core::__impl_deref!(Cached);

#[cfg(test)]
mod tests {
    use super::*;
    use axum::{http::Request, routing::get, Router};
    use std::{
        convert::Infallible,
        sync::atomic::{AtomicU32, Ordering},
        time::Instant,
    };

    #[tokio::test]
    async fn works() {
        static COUNTER: AtomicU32 = AtomicU32::new(0);

        #[derive(Clone, Debug, PartialEq, Eq)]
        struct Extractor(Instant);

        #[async_trait]
        impl<S> FromRequestParts<S> for Extractor
        where
            S: Send + Sync,
        {
            type Rejection = Infallible;

            async fn from_request_parts(
                _parts: &mut Parts,
                _state: &S,
            ) -> Result<Self, Self::Rejection> {
                COUNTER.fetch_add(1, Ordering::SeqCst);
                Ok(Self(Instant::now()))
            }
        }

        let (mut parts, _) = Request::new(()).into_parts();

        let first = Cached::<Extractor>::from_request_parts(&mut parts, &())
            .await
            .unwrap()
            .0;
        assert_eq!(COUNTER.load(Ordering::SeqCst), 1);

        let second = Cached::<Extractor>::from_request_parts(&mut parts, &())
            .await
            .unwrap()
            .0;
        assert_eq!(COUNTER.load(Ordering::SeqCst), 1);

        assert_eq!(first, second);
    }

    // Not a #[test], we just want to know this compiles
    async fn _last_handler_argument() {
        async fn handler(_: http::Method, _: Cached<http::HeaderMap>) {}
        let _r: Router = Router::new().route("/", get(handler));
    }
}