fix: radix cache memory accounting (#10637)

Signed-off-by: Alex Chi Z <iskyzh@gmail.com>

python/sglang/srt/mem_cache/radix_cache.py

@@ -267,7 +267,7 @@ class RadixCache(BasePrefixCache):
         """
         key.token_ids = self.key_convert_fn(key.token_ids)
 
-        if self.disable or len(key) == 0:
+        def empty_match_result():
             return MatchResult(
                 device_indices=torch.empty(
                     (0,),
@@ -278,10 +278,16 @@ class RadixCache(BasePrefixCache):
                 last_host_node=self.root_node,
             )
 
+        if self.disable or len(key) == 0:
+            return empty_match_result()
+
         if self.page_size != 1:
             page_aligned_len = len(key) // self.page_size * self.page_size
             key = key[:page_aligned_len]
 
+        if len(key) == 0:
+            return empty_match_result()
+
         value, last_node = self._match_prefix_helper(self.root_node, key)
         if value:
             value = torch.cat(value)
@@ -475,9 +481,9 @@ class RadixCache(BasePrefixCache):
         delta = 0
         while node != self.root_node:
             if node.lock_ref == 0:
-                self.evictable_size_ -= len(node.value)
-                self.protected_size_ += len(node.value)
-                delta -= len(node.value)
+                self.evictable_size_ -= len(node.key)
+                self.protected_size_ += len(node.key)
+                delta -= len(node.key)
             node.lock_ref += 1
             node = node.parent
         return delta
@@ -489,9 +495,9 @@ class RadixCache(BasePrefixCache):
         delta = 0
         while node != self.root_node:
             if node.lock_ref == 1:
-                self.evictable_size_ += len(node.value)
-                self.protected_size_ -= len(node.value)
-                delta += len(node.value)
+                self.evictable_size_ += len(node.key)
+                self.protected_size_ -= len(node.key)
+                delta += len(node.key)
             node.lock_ref -= 1
             node = node.parent
         return delta
@@ -589,7 +595,7 @@ class RadixCache(BasePrefixCache):
             new_node.key = key
             new_node.value = value
             node.children[child_key] = new_node
-            self.evictable_size_ += len(value)
+            self.evictable_size_ += len(key)
             self._record_store_event(new_node)
         return total_prefix_length