Fix IndexError in CacheOrder.pop() on empty cache

mlx_lm/server.py

@@ -196,6 +196,8 @@ def remove(self, model, tokens):
                 self._lru_checkpoints.remove((model, tokens))
 
         def pop(self):
+            if not self._lru and not self._lru_checkpoints:
+                raise IndexError("pop from empty CacheOrder")
             if len(self._lru) >= len(self._lru_checkpoints):
                 return self._lru.popleft()
             else:
@@ -344,9 +346,13 @@ def trim_to(
         while len(self._lru) > n_sequences:
             model, tokens = self._lru.pop()
             self._delete(model, tokens)
-        while self._n_bytes > n_bytes:
+        while self._n_bytes > n_bytes and len(self._lru) > 0:
             model, tokens = self._lru.pop()
             self._delete(model, tokens)
+        if self._n_bytes > n_bytes:
+            raise RuntimeError(
+                "LRUPromptCache byte accounting drifted out of sync with cache order"
+            )
 
     def log_cache_stats(self):
         ncaches, nbytes = len(self), self.nbytes

tests/test_server.py

@@ -560,6 +560,30 @@ def test_lru_bytes(self):
         self.assertEqual(c, None)
         self.assertEqual(t, [3, 4])
 
+    def test_trim_to_zero_bytes_on_empty_cache(self):
+        cache = LRUPromptCache(max_size=10)
+        # Should not raise IndexError on empty cache
+        cache.trim_to(n_bytes=0)
+        self.assertEqual(len(cache), 0)
+
+    def test_trim_to_raises_on_inconsistent_byte_accounting(self):
+        cache = LRUPromptCache(max_size=10)
+        cache._n_bytes = 1
+
+        with self.assertRaisesRegex(RuntimeError, "byte accounting"):
+            cache.trim_to(n_bytes=0)
+
+    def test_trim_to_zero_bytes_evicts_all(self):
+        cache = LRUPromptCache(max_size=10)
+        model = ("test", None, None)
+        cache.insert_cache(model, [1, 2], [MockCache("aaa")])
+        cache.insert_cache(model, [3, 4], [MockCache("bbb")])
+        self.assertEqual(len(cache), 2)
+
+        cache.trim_to(n_bytes=0)
+        self.assertEqual(len(cache), 0)
+        self.assertEqual(cache.nbytes, 0)
+
 
 if __name__ == "__main__":
     unittest.main()