radix trie: add assertions (#2491)

These should all be cheap assertions.

Also:

* Fixup some comments.
* Delete a `remove` that was done unnecessarily twice.

backends/v3/src/radix.rs

@@ -73,14 +73,13 @@ impl Allocator for RadixAllocator {
             let node_id = self
                 .cache_blocks
                 .find(prefill_tokens.as_slice(), &mut blocks);
-            // Even if this allocation fails below, we need to increase he
-            // refcount to ensure that the prefix that was found is not evicted.
-
             node_id
         } else {
             self.cache_blocks.root_id()
         };
 
+        // Even if this allocation fails below, we need to increase he
+        // refcount to ensure that the prefix that was found is not evicted.
         self.cache_blocks
             .incref(prefix_node)
             .expect("Failed to increment refcount");
@@ -303,6 +302,11 @@ impl RadixTrie {
 
         node.ref_count -= 1;
         if node.ref_count == 0 {
+            assert!(
+                node.children.is_empty(),
+                "Nodes with children must have refcount > 0"
+            );
+
             self.leaves.insert((node.last_accessed, node_id));
         }
 
@@ -345,6 +349,12 @@ impl RadixTrie {
             let blocks_needed = n_blocks - evicted.len();
 
             let node = self.nodes.get(node_id).expect("Leave does not exist");
+            assert_eq!(
+                node.ref_count, 0,
+                "Leaf must have refcount of 0, got {}",
+                node.ref_count
+            );
+
             if blocks_needed >= node.blocks.len() {
                 // We need to evict the whole node if we need more blocks than it has.
                 let node = self.remove_node(node_id);
@@ -500,12 +510,16 @@ impl RadixTrie {
     fn remove_node(&mut self, node_id: NodeId) -> TrieNode {
         // Unwrap here, passing in an unknown id is a programming error.
         let node = self.nodes.remove(node_id).expect("Unknown node");
+        assert!(
+            node.children.is_empty(),
+            "Tried to remove a node with {} children",
+            node.children.len()
+        );
         let parent_id = node.parent.expect("Attempted to remove root node");
         let parent = self.nodes.get_mut(parent_id).expect("Unknown parent node");
         parent.children.remove(&node.key[0]);
         self.decref(parent_id)
             .expect("Failed to decrease parent refcount");
-        self.nodes.remove(node_id);
         node
     }
 
@@ -579,6 +593,9 @@ impl TrieNode {
 
 fn shared_prefix(left: &[u32], right: &[u32], block_size: usize) -> usize {
     let full = left.iter().zip(right).take_while(|(a, b)| a == b).count();
+    // NOTE: this is the case because the child node was chosen based on
+    //       matching the first character of the key/prefix.
+    assert!(full > 0, "Prefixes must at least share 1 token");
     (full / block_size) * block_size
 }