fix off-by-one in dynamic array length calculation (#2821)

one slot too few was allocated to dynamic arrays, allowing a write to
the end of the dynamic array to clobber the storage variable allocated
immediately after it.

tests/cli/outputs/test_storage_layout.py

@@ -21,6 +21,8 @@ def public_foo2():
 def _bar():
     pass
 
+arr: DynArray[uint256, 3]
+
 # mix it up a little
 baz: Bytes[65]
 bar: uint256
@@ -49,6 +51,11 @@ def public_foo3():
             "location": "storage",
             "slot": 2,
         },
-        "baz": {"type": "Bytes[65]", "location": "storage", "slot": 3},
-        "bar": {"type": "uint256", "location": "storage", "slot": 7},
+        "arr": {
+            "type": "DynArray[uint256, 3]",
+            "location": "storage",
+            "slot": 3,
+        },
+        "baz": {"type": "Bytes[65]", "location": "storage", "slot": 7},
+        "bar": {"type": "uint256", "location": "storage", "slot": 11},
     }

tests/functional/context/types/test_size_in_bytes.py

@@ -27,6 +27,16 @@ def test_array_value_types(build_node, type_str, location, length, size):
     assert type_definition.size_in_bytes == size
 
 
+@pytest.mark.parametrize("type_str", BASE_TYPES)
+@pytest.mark.parametrize("location", LOCATIONS)
+@pytest.mark.parametrize("length", range(1, 4))
+def test_dynamic_array_lengths(build_node, type_str, location, length):
+    node = build_node(f"DynArray[{type_str}, {length}]")
+    type_definition = get_type_from_annotation(node, location)
+
+    assert type_definition.size_in_bytes == 32 + length * 32
+
+
 @pytest.mark.parametrize("type_str", BASE_TYPES)
 @pytest.mark.parametrize("location", LOCATIONS)
 @pytest.mark.parametrize("length", range(1, 4))

tests/functional/test_storage_slots.py

@@ -14,6 +14,7 @@
 c: public(Bytes[32])
 d: public(int128[4])
 foo: public(HashMap[uint256, uint256[3]])
+dyn_array: DynArray[uint256, 3]
 e: public(String[47])
 f: public(int256[1])
 g: public(StructTwo[2])
@@ -36,6 +37,7 @@ def __init__():
             c: "whatifthisstringtakesuptheentirelengthwouldthatbesobadidothinkso"
         })
     ]
+    self.dyn_array = [1, 2, 3]
     self.h =  [123456789]
     self.foo[0] = [987, 654, 321]
     self.foo[1] = [123, 456, 789]

vyper/semantics/types/indexable/sequence.py

@@ -162,7 +162,8 @@ def is_dynamic_size(self):
     # TODO rename me to memory_bytes_required
     @property
     def size_in_bytes(self):
-        return self.value_type.size_in_bytes * self.length
+        # one length word + size of the array items
+        return 32 + self.value_type.size_in_bytes * self.length
 
     def validate_index_type(self, node):
         if isinstance(node, vy_ast.Int):