fix: SGLANG_RADIX_FORCE_MISS chunk-cache passthrough (#24950)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

python/sglang/srt/mem_cache/base_prefix_cache.py

@@ -152,13 +152,10 @@ class MatchResult(NamedTuple):
 
 
 def zero_match_result(tree_cache, match_result: "MatchResult") -> "MatchResult":
-    root = getattr(tree_cache, "root_node", None)
-    if root is None:
-        raise RuntimeError(
-            f"SGLANG_RADIX_FORCE_MISS is not supported by {type(tree_cache).__name__} "
-            "(no `root_node` attribute). Disable the flag or use a cache backend "
-            "that exposes a tree root."
-        )
+    if tree_cache.is_chunk_cache():
+        # Chunk caches' match_prefix already returns a miss; no root_node to walk back to.
+        return match_result
+    root = tree_cache.root_node
     return match_result._replace(
         # [:0] keeps dtype and device of the original tensor (e.g. CUDA int64)
         # without allocating a fresh empty tensor.

test/registered/unit/mem_cache/test_radix_force_miss.py

@@ -55,20 +55,18 @@ class TestZeroMatchResult(unittest.TestCase):
         self.assertEqual(zeroed.device_indices.dtype, match.device_indices.dtype)
         self.assertEqual(zeroed.device_indices.device, match.device_indices.device)
 
-    def test_no_root_node_raises(self):
-        # tree_cache without a root_node: must raise loudly rather than silently
-        # leak cache hits past the gate.
-        class _NoRoot:
-            pass
+    def test_chunk_cache_is_passthrough(self):
+        class _StubChunkCache:
+            def is_chunk_cache(self) -> bool:
+                return True
 
         original = MatchResult(
-            device_indices=torch.tensor([7, 8, 9], dtype=torch.int64),
-            last_device_node="sentinel-device",
-            last_host_node="sentinel-host",
-            host_hit_length=4,
+            device_indices=torch.empty((0,), dtype=torch.int64),
+            last_device_node=None,
+            last_host_node=None,
+            host_hit_length=0,
         )
-        with self.assertRaisesRegex(RuntimeError, "SGLANG_RADIX_FORCE_MISS"):
-            zero_match_result(_NoRoot(), original)
+        self.assertIs(zero_match_result(_StubChunkCache(), original), original)
 
 
 class TestMatchPrefixForReqForceMiss(unittest.TestCase):