Fix dynamic size errors for MetaType (#121)

Author: Schamper

dissect/cstruct/compiler.py

@@ -21,6 +21,7 @@
     Structure,
     Union,
     Void,
+    VoidArray,
     Wchar,
     WcharArray,
 )
@@ -48,6 +49,7 @@
     Void,
     Wchar,
     WcharArray,
+    VoidArray,
 )
 
 log = logging.getLogger(__name__)
@@ -365,7 +367,7 @@ def _generate_struct_info(cs: cstruct, fields: list[Field], align: bool = False)
         read_type = _get_read_type(cs, field.type)
 
         # Drop voids
-        if issubclass(read_type, Void):
+        if issubclass(read_type, (Void, VoidArray)):
             continue
 
         # Array of more complex types are handled elsewhere

dissect/cstruct/types/__init__.py

@@ -7,7 +7,7 @@
 from dissect.cstruct.types.packed import Packed
 from dissect.cstruct.types.pointer import Pointer
 from dissect.cstruct.types.structure import Field, Structure, Union
-from dissect.cstruct.types.void import Void
+from dissect.cstruct.types.void import Void, VoidArray
 from dissect.cstruct.types.wchar import Wchar, WcharArray
 
 __all__ = [
@@ -27,6 +27,7 @@
     "Structure",
     "Union",
     "Void",
+    "VoidArray",
     "Wchar",
     "WcharArray",
 ]

dissect/cstruct/types/base.py

@@ -54,6 +54,10 @@ def __getitem__(cls, num_entries: int | Expression | None) -> type[BaseArray]:
         """Create a new array with the given number of entries."""
         return cls.cs._make_array(cls, num_entries)
 
+    def __bool__(cls) -> bool:
+        """Type class is always truthy."""
+        return True
+
     def __len__(cls) -> int:
         """Return the byte size of the type."""
         # Python 3.9 compat thing for bound type vars

dissect/cstruct/types/void.py

@@ -1,27 +1,44 @@
 from __future__ import annotations
 
-from typing import Any, BinaryIO
+from typing import TYPE_CHECKING, Any, BinaryIO
 
-from dissect.cstruct.types.base import BaseType
+from dissect.cstruct.types.base import BaseArray, BaseType
+
+if TYPE_CHECKING:
+    from typing_extensions import Self
+
+
+class VoidArray(list, BaseArray):
+    """Array type representing void elements, primarily used for no-op reading and writing operations."""
+
+    @classmethod
+    def __default__(cls) -> Self:
+        return cls()
+
+    @classmethod
+    def _read(cls, stream: BinaryIO, context: dict[str, Any] | None = None) -> Self:
+        return cls()
+
+    @classmethod
+    def _write(cls, stream: BinaryIO, data: bytes) -> int:
+        return 0
 
 
 class Void(BaseType):
     """Void type."""
 
+    ArrayType = VoidArray
+
     def __bool__(self) -> bool:
         return False
 
     def __eq__(self, value: object) -> bool:
         return isinstance(value, Void)
 
     @classmethod
-    def _read(cls, stream: BinaryIO, context: dict[str, Any] | None = None) -> Void:
+    def _read(cls, stream: BinaryIO, context: dict[str, Any] | None = None) -> Self:
         return cls.__new__(cls)
 
-    @classmethod
-    def _read_0(cls, stream: BinaryIO, context: dict[str, Any] | None = None) -> Void:
-        return [cls.__new__(cls)]
-
     @classmethod
     def _write(cls, stream: BinaryIO, data: Void) -> int:
         return 0

tests/test_types_base.py

@@ -137,3 +137,12 @@ def _read(cls, stream: BinaryIO, context: dict | None = None) -> CustomType:
     assert isinstance(result.b, CustomType)
     assert result.a.value == b"ASDF"
     assert result.b.value == b"asdf"
+
+
+def test_truthy_type(cs: cstruct) -> None:
+    static_type = cs.uint32
+    dynamic_type = cs.uint32[None]
+
+    assert static_type
+    # Should not raise a TypeError: Dynamic size
+    assert dynamic_type

tests/test_types_void.py

@@ -10,7 +10,9 @@
 
 
 def test_void_read(cs: cstruct) -> None:
-    assert not cs.void
+    # The type itself is truthy, but an instance is not
+    assert cs.void
+    assert not cs.void()
 
     stream = io.BytesIO(b"AAAA")
     assert not cs.void(stream)
@@ -23,11 +25,11 @@ def test_void_write(cs: cstruct) -> None:
 
 
 def test_void_array_read(cs: cstruct) -> None:
-    assert not cs.void[4]
+    assert not cs.void[4]()
 
     stream = io.BytesIO(b"AAAA")
-    assert not any(cs.void[4](stream))
-    assert not any(cs.void[None](stream))
+    assert not cs.void[4](stream)
+    assert not cs.void[None](stream)
     assert stream.tell() == 0
 
 
@@ -41,7 +43,7 @@ def test_void_default(cs: cstruct) -> None:
     assert not cs.void()
     assert not cs.void.__default__()
 
-    assert cs.void[1].__default__() == [cs.void()]
+    assert cs.void[1].__default__() == []
     assert cs.void[None].__default__() == []
 
 
@@ -61,8 +63,8 @@ def test_void_struct(cs: cstruct, compiled: bool) -> None:
 
     obj = cs.test(stream)
     assert not obj.a
-    assert not any(obj.b)
-    assert not any(obj.c)
+    assert not obj.b
+    assert not obj.c
 
     assert stream.tell() == 0