python: Generate missing constraint validation for traversed children

The following pattern is missing constraint validation in the generated
code:

```
packet A {
  a: 8,
  b: 8,
  _payload_,
}

packet B : A (a = 1) {
  _payload_,
}

packet C : B (b = 1) {}
packet D : B (b = 2) {}
```

The constraint a == 1 is not validated in either C or D

pdl-compiler/scripts/generate_python_backend.py

@@ -904,9 +904,10 @@ def generate_packet_parser(packet: ast.Declaration) -> List[str]:
 
     # Convert the packet constraints to a boolean expression.
     validation = []
-    if packet.constraints:
+    constraints = core.get_all_packet_constraints(packet)
+    if constraints:
         cond = []
-        for c in packet.constraints:
+        for c in constraints:
             if c.value is not None:
                 cond.append(f"fields['{c.id}'] != {hex(c.value)}")
             else:
@@ -989,11 +990,7 @@ def generate_packet_post_init(decl: ast.Declaration) -> List[str]:
     """Generate __post_init__ function to set constraint field values."""
 
     # Gather all constraints from parent packets.
-    constraints = []
-    current = decl
-    while current.parent_id:
-        constraints.extend(current.constraints)
-        current = current.parent
+    constraints = core.get_all_packet_constraints(decl)
 
     if constraints:
         code = []

pdl-compiler/scripts/pdl/core.py

@@ -182,6 +182,19 @@ def get_derived_packets(
     return children
 
 
+def get_all_packet_constraints(
+    decl: Union[PacketDeclaration, StructDeclaration]
+) -> List[Constraint]:
+    """Return the list of constraints defined in the selected packet and
+    its parent declarations."""
+
+    constraints = []
+    while decl.parent_id:
+        constraints.extend(decl.constraints)
+        decl = decl.parent
+    return constraints
+
+
 def get_field_size(field: Field, skip_payload: bool = False) -> Optional[int]:
     """Determine the size of a field in bits, if possible.
     If the field is dynamically sized (e.g. unsized array or payload field),