jmt(tree_cache): conservative guards for version mismatches

src/tree_cache.rs

@@ -72,7 +72,7 @@ use hashbrown::{hash_map::Entry, HashMap, HashSet};
 #[cfg(feature = "std")]
 use std::collections::{hash_map::Entry, HashMap, HashSet};
 
-use anyhow::{bail, Result};
+use anyhow::{bail, ensure, Result};
 
 use crate::{
     node_type::{Node, NodeKey},
@@ -189,8 +189,23 @@ where
     }
 
     #[cfg(feature = "migration")]
+    /// Instantiate a [`TreeCache`] over the a [`TreeReader`] that is defined
+    /// against a root node key at version `current_version`.
+    ///
+    /// # Usage
+    /// This method is used to perform incremental addition to a tree without
+    /// increasing the tree version's number.
     pub fn new_overwrite(reader: &'a R, current_version: Version) -> Result<Self> {
         let node_cache = HashMap::new();
+        let Some((node_key, _)) = reader.get_rightmost_leaf()? else {
+            bail!("creating an overwrite cache for an empty tree is not supported")
+        };
+
+        ensure!(
+            node_key.version() == current_version,
+            "the supplied version is not the latest version of the tree"
+        );
+
         let root_node_key = NodeKey::new_empty_path(current_version);
         Ok(Self {
             node_cache,