src/client/remote_chunk_reader.rs: implement simple caching

src/client/remote_chunk_reader.rs

@@ -1,6 +1,7 @@
 use failure::*;
 use futures::future::Future;
 use std::sync::Arc;
+use std::collections::HashMap;
 
 use super::BackupReader;
 use crate::backup::{ReadChunk, DataChunk, CryptConfig};
@@ -9,12 +10,22 @@ use crate::backup::{ReadChunk, DataChunk, CryptConfig};
 pub struct RemoteChunkReader {
     client: Arc<BackupReader>,
     crypt_config: Option<Arc<CryptConfig>>,
+    cache_hint: HashMap<[u8; 32], usize>,
+    cache:  HashMap<[u8; 32], Vec<u8>>,
 }
 
 impl RemoteChunkReader {
 
-    pub fn new(client: Arc<BackupReader>, crypt_config: Option<Arc<CryptConfig>>) -> Self {
-        Self { client, crypt_config }
+    /// Create a new instance.
+    ///
+    /// Chunks listed in ``cache_hint`` are cached and kept in RAM.
+    pub fn new(
+        client: Arc<BackupReader>,
+        crypt_config: Option<Arc<CryptConfig>>,
+        cache_hint: HashMap<[u8; 32], usize>,
+    ) -> Self {
+
+        Self { client, crypt_config, cache_hint, cache: HashMap::new() }
     }
 }
 
@@ -24,6 +35,12 @@ impl ReadChunk for RemoteChunkReader {
 
         let writer = Vec::with_capacity(4*1024*1024);
 
+        if let Some(raw_data) = self.cache.get(digest) {
+            return Ok(raw_data.to_vec());
+        }
+
+        let use_cache = self.cache_hint.contains_key(digest);
+
         let chunk_data = self.client.download_chunk(&digest, writer).wait()?;
 
         let chunk = DataChunk::from_raw(chunk_data, *digest)?;
@@ -34,6 +51,10 @@ impl ReadChunk for RemoteChunkReader {
             None => chunk.decode(None)?,
         };
 
+        if use_cache {
+            self.cache.insert(*digest, raw_data.to_vec());
+        }
+
         Ok(raw_data)
     }
 }