more explicit about merkleization limit/pad

test_libs/pyspec/eth2spec/utils/merkle_minimal.py

@@ -37,12 +37,17 @@ def get_merkle_proof(tree, item_index):
     return proof
 
 
-def merkleize_chunks(chunks, pad_to: int=1):
-    if pad_to == 0:
+def merkleize_chunks(chunks, limit=None):
+    # If no limit is defined, we are just merkleizing chunks (e.g. SSZ container).
+    if limit is None:
+        limit = len(chunks)
+    if limit == 0:
         return zerohashes[0]
-    count = min(len(chunks), pad_to)
+    # Limit strictly. Makes no sense to merkleize objects above the intended padding.
+    # And illegal to exceed list limits, just as with serialization.
+    count = min(len(chunks), limit)
     depth = max(count - 1, 0).bit_length()
-    max_depth = (pad_to - 1).bit_length()
+    max_depth = (limit - 1).bit_length()
     tmp = [None for _ in range(max_depth + 1)]
 
     def merge(h, i):

test_libs/pyspec/eth2spec/utils/ssz/ssz_impl.py

@@ -126,6 +126,7 @@ def item_length(typ: SSZType) -> int:
 
 
 def chunk_count(typ: SSZType) -> int:
+    # note that for lists, .length *on the type* describes the list limit.
     if isinstance(typ, BasicType):
         return 1
     elif issubclass(typ, Bits):
@@ -150,7 +151,7 @@ def hash_tree_root(obj: SSZValue):
         raise Exception(f"Type not supported: {type(obj)}")
 
     if isinstance(obj, (List, Bytes, Bitlist)):
-        return mix_in_length(merkleize_chunks(leaves, pad_to=chunk_count(obj.type())), len(obj))
+        return mix_in_length(merkleize_chunks(leaves, limit=chunk_count(obj.type())), len(obj))
     else:
         return merkleize_chunks(leaves)
 

test_libs/pyspec/eth2spec/utils/test_merkle_minimal.py

@@ -61,10 +61,10 @@ cases = [
 
 
 @pytest.mark.parametrize(
-    'count,pad_to,value',
+    'count,limit,value',
     cases,
 )
-def test_merkleize_chunks_and_get_merkle_root(count, pad_to, value):
+def test_merkleize_chunks_and_get_merkle_root(count, limit, value):
     chunks = [e(i) for i in range(count)]
-    assert merkleize_chunks(chunks, pad_to=pad_to) == value
-    assert get_merkle_root(chunks, pad_to=pad_to) == value
+    assert merkleize_chunks(chunks, limit=limit) == value
+    assert get_merkle_root(chunks, pad_to=limit) == value