--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/fncache.rs	Mon Jul 29 20:28:42 2024 +0200
@@ -0,0 +1,26 @@
+use std::path::Path;
+
+use dyn_clone::DynClone;
+
+/// The FnCache stores the list of most files contained in the store and is
+/// used for stream/copy clones.
+///
+/// It keeps track of the name of "all" indexes and data files for all revlogs.
+/// The names are relative to the store roots and are stored before any
+/// encoding or path compression.
+///
+/// Despite its name, the FnCache is *NOT* a cache, it keep tracks of
+/// information that is not easily available elsewhere. It has no mechanism
+/// for detecting isn't up to date, and de-synchronization with the actual
+/// contents of the repository will lead to a corrupted clone and possibly
+/// other corruption during maintenance operations.
+/// Strictly speaking, it could be recomputed by looking at the contents of all
+/// manifests AND actual store files on disk, however that is a
+/// prohibitively expensive operation.
+pub trait FnCache: Sync + Send + DynClone {
+    /// Whether the fncache was loaded from disk
+    fn is_loaded(&self) -> bool;
+    /// Add a path to be tracked in the fncache
+    fn add(&self, path: &Path);
+    // TODO add more methods once we start doing more with the FnCache
+}