feat: introduce CacheCodec for serializing index cache entries

rust/lance-core/src/cache/backend.rs

@@ -17,6 +17,8 @@ use futures::Future;
 
 use crate::Result;
 
+use super::ErasedCodec;
+
 /// A type-erased cache entry.
 pub type CacheEntry = Arc<dyn Any + Send + Sync>;
 
@@ -74,10 +76,26 @@ impl InternalCacheKey {
 #[async_trait]
 pub trait CacheBackend: Send + Sync + std::fmt::Debug {
     /// Look up an entry by its key.
-    async fn get(&self, key: &InternalCacheKey) -> Option<CacheEntry>;
+    ///
+    /// `codec` is provided so that backends with persistent storage can
+    /// deserialize the entry. In-memory backends can ignore it.
+    async fn get(
+        &self,
+        key: &InternalCacheKey,
+        codec: Option<&Arc<ErasedCodec>>,
+    ) -> Option<CacheEntry>;
 
     /// Store an entry. `size_bytes` is used for eviction accounting.
-    async fn insert(&self, key: &InternalCacheKey, entry: CacheEntry, size_bytes: usize);
+    ///
+    /// `codec` is provided so that backends with persistent storage can
+    /// serialize the entry. In-memory backends can ignore it.
+    async fn insert(
+        &self,
+        key: &InternalCacheKey,
+        entry: CacheEntry,
+        size_bytes: usize,
+        codec: Option<&Arc<ErasedCodec>>,
+    );
 
     /// Get an existing entry or compute it from `loader`.
     ///
@@ -86,10 +104,13 @@ pub trait CacheBackend: Send + Sync + std::fmt::Debug {
     ///
     /// Returns `(entry, was_cached)` where `was_cached` is `true` if the entry
     /// was already present in the cache (the loader was not invoked).
+    ///
+    /// `codec` is provided for backends that need to serialize/deserialize.
     async fn get_or_insert<'a>(
         &self,
         key: &InternalCacheKey,
         loader: Pin<Box<dyn Future<Output = Result<(CacheEntry, usize)>> + Send + 'a>>,
+        codec: Option<&Arc<ErasedCodec>>,
     ) -> Result<(CacheEntry, bool)>;
 
     /// Remove all entries whose prefix starts with the given string.