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

use std::sync::Arc;

use crate::proto::PathInfo;
use futures::stream::BoxStream;
use nix_compat::nixbase32;
use tonic::async_trait;
use tracing::{debug, instrument};
use tvix_castore::composition::{CompositionContext, ServiceBuilder};
use tvix_castore::Error;

use super::PathInfoService;

/// Asks near first, if not found, asks far.
/// If found in there, returns it, and *inserts* it into
/// near.
/// There is no negative cache.
/// Inserts and listings are not implemented for now.
pub struct Cache<PS1, PS2> {
    near: PS1,
    far: PS2,
}

impl<PS1, PS2> Cache<PS1, PS2> {
    pub fn new(near: PS1, far: PS2) -> Self {
        Self { near, far }
    }
}

#[async_trait]
impl<PS1, PS2> PathInfoService for Cache<PS1, PS2>
where
    PS1: PathInfoService,
    PS2: PathInfoService,
{
    #[instrument(level = "trace", skip_all, fields(path_info.digest = nixbase32::encode(&digest)))]
    async fn get(&self, digest: [u8; 20]) -> Result<Option<PathInfo>, Error> {
        match self.near.get(digest).await? {
            Some(path_info) => {
                debug!("serving from cache");
                Ok(Some(path_info))
            }
            None => {
                debug!("not found in near, asking remote…");
                match self.far.get(digest).await? {
                    None => Ok(None),
                    Some(path_info) => {
                        debug!("found in remote, adding to cache");
                        self.near.put(path_info.clone()).await?;
                        Ok(Some(path_info))
                    }
                }
            }
        }
    }

    async fn put(&self, _path_info: PathInfo) -> Result<PathInfo, Error> {
        Err(Error::StorageError("unimplemented".to_string()))
    }

    fn list(&self) -> BoxStream<'static, Result<PathInfo, Error>> {
        Box::pin(tokio_stream::once(Err(Error::StorageError(
            "unimplemented".to_string(),
        ))))
    }
}

#[derive(serde::Deserialize)]
pub struct CacheConfig {
    pub near: String,
    pub far: String,
}

impl TryFrom<url::Url> for CacheConfig {
    type Error = Box<dyn std::error::Error + Send + Sync>;
    fn try_from(_url: url::Url) -> Result<Self, Self::Error> {
        Err(Error::StorageError(
            "Instantiating a CombinedPathInfoService from a url is not supported".into(),
        )
        .into())
    }
}

#[async_trait]
impl ServiceBuilder for CacheConfig {
    type Output = dyn PathInfoService;
    async fn build<'a>(
        &'a self,
        _instance_name: &str,
        context: &CompositionContext,
    ) -> Result<Arc<dyn PathInfoService>, Box<dyn std::error::Error + Send + Sync + 'static>> {
        let (near, far) = futures::join!(
            context.resolve(self.near.clone()),
            context.resolve(self.far.clone())
        );
        Ok(Arc::new(Cache {
            near: near?,
            far: far?,
        }))
    }
}

#[cfg(test)]
mod test {
    use std::num::NonZeroUsize;

    use crate::{
        pathinfoservice::{LruPathInfoService, MemoryPathInfoService, PathInfoService},
        tests::fixtures::PATH_INFO_WITH_NARINFO,
    };

    const PATH_INFO_DIGEST: [u8; 20] = [0; 20];

    /// Helper function setting up an instance of a "far" and "near"
    /// PathInfoService.
    async fn create_pathinfoservice() -> super::Cache<LruPathInfoService, MemoryPathInfoService> {
        // Create an instance of a "far" PathInfoService.
        let far = MemoryPathInfoService::default();

        // … and an instance of a "near" PathInfoService.
        let near = LruPathInfoService::with_capacity(NonZeroUsize::new(1).unwrap());

        // create a Pathinfoservice combining the two and return it.
        super::Cache::new(near, far)
    }

    /// Getting from the far backend is gonna insert it into the near one.
    #[tokio::test]
    async fn test_populate_cache() {
        let svc = create_pathinfoservice().await;

        // query the PathInfo, things should not be there.
        assert!(svc.get(PATH_INFO_DIGEST).await.unwrap().is_none());

        // insert it into the far one.
        svc.far.put(PATH_INFO_WITH_NARINFO.clone()).await.unwrap();

        // now try getting it again, it should succeed.
        assert_eq!(
            Some(PATH_INFO_WITH_NARINFO.clone()),
            svc.get(PATH_INFO_DIGEST).await.unwrap()
        );

        // peek near, it should now be there.
        assert_eq!(
            Some(PATH_INFO_WITH_NARINFO.clone()),
            svc.near.get(PATH_INFO_DIGEST).await.unwrap()
        );
    }
}