[KVEvent] User request.block_hash for parent block_hash (#30544)

Signed-off-by: Chen Zhang <zhangch99@outlook.com>
Signed-off-by: Yifan Qiao <yifanqiao@berkeley.edu>
Co-authored-by: Yifan Qiao <yifanqiao@berkeley.edu>

tests/v1/core/test_prefix_caching.py

@@ -1356,6 +1356,69 @@ def test_kv_cache_events(blocks_to_cache: int):
     assert len(manager.block_pool.cached_block_hash_to_block) == 0
 
 
+def test_null_parent_block_hash():
+    block_size = 1
+    num_cached_blocks = 2
+    num_full_blocks = 4
+
+    pool = BlockPool(
+        num_gpu_blocks=8,
+        enable_caching=True,
+        hash_block_size=block_size,
+        enable_kv_cache_events=True,
+    )
+
+    req = make_request(
+        "req_null_parent",
+        prompt_token_ids=[10, 11, 12, 13],
+        block_size=block_size,
+        hash_fn=sha256,
+    )
+    assert len(req.block_hashes) == num_full_blocks
+
+    # Physical parent is `null_block` (no hash), while the logical parent hash
+    # still exists in `request.block_hashes[num_cached_blocks - 1]`.
+    assert pool.null_block.block_hash is None
+    new_blocks = pool.get_new_blocks(num_full_blocks - 1)
+    blocks = [
+        new_blocks[: num_cached_blocks - 1],
+        pool.null_block,  # physical parent
+        *new_blocks[num_cached_blocks - 1 :],
+    ]
+
+    pool.cache_full_blocks(
+        request=req,
+        blocks=blocks,
+        num_cached_blocks=num_cached_blocks,
+        num_full_blocks=num_full_blocks,
+        block_size=block_size,
+        kv_cache_group_id=0,
+    )
+
+    events = pool.take_events()
+    assert len(events) == 1
+    event = events[0]
+    assert isinstance(event, BlockStored)
+
+    expected_parent = kv_cache_utils.maybe_convert_block_hash(
+        req.block_hashes[num_cached_blocks - 1]
+    )
+    assert event.parent_block_hash == expected_parent
+    assert event.parent_block_hash is not None
+
+    expected_new_hashes = [
+        kv_cache_utils.maybe_convert_block_hash(h)
+        for h in req.block_hashes[num_cached_blocks:num_full_blocks]
+    ]
+    assert event.block_hashes == expected_new_hashes
+
+    # Ensure we didn't accidentally assign a hash to the null block.
+    assert pool.null_block.block_hash is None
+    # Sanity check: newly cached physical blocks should have hashes assigned.
+    assert blocks[num_cached_blocks].block_hash is not None
+    assert blocks[num_full_blocks - 1].block_hash is not None
+
+
 @pytest.mark.parametrize("blocks_to_cache", [2, 3, 10])
 def test_kv_cache_events_with_lora(blocks_to_cache: int):
     """Test BlockStored events contain correct lora_id when using LoRA requests."""

vllm/v1/core/block_pool.py

@@ -270,10 +270,8 @@ class BlockPool:
             if num_cached_blocks == 0:
                 parent_block_hash: ExternalBlockHash | None = None
             else:
-                parent_block = blocks[num_cached_blocks - 1]
-                assert parent_block.block_hash is not None
                 parent_block_hash = maybe_convert_block_hash(
-                    get_block_hash(parent_block.block_hash)
+                    block_hashes[num_cached_blocks - 1]
                 )
 
             self.kv_event_queue.append(