add missing file

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/accessor/cache.rs

@@ -0,0 +1,16 @@
+//! Cache helper for random access pxar accesses.
+
+use std::sync::Arc;
+
+/// A simple asynchronous caching trait to help speed up random access for pxar archives.
+pub trait Cache<K, V: ?Sized> {
+    /// Fetch a value from this cache store.
+    fn fetch(&self, key: K) -> Option<Arc<V>>;
+
+    /// Update the cache with a value.
+    ///
+    /// It is up to the implementation to choose whether or not to store the value. Note that since
+    /// all values are `Arc`s, the values will remain alive as long as they're in use even if
+    /// they're not cached.
+    fn insert(&self, key: K, value: Arc<V>);
+}