Implement getField CEL function

I'm proposing this as an eventual replacement (before 1.0) of our hack around the fact that the `in` identifier is reserved in CEL. This is especially urgent for protovalidate-cc which is currently carrying patches to the CEL implementation in order to enable it, since cel-cpp doesn't allow this sort of functionality to be added in at runtime.

Author: John Chadwick

protovalidate/internal/constraints.py

@@ -40,7 +40,7 @@ def make_timestamp(msg: message.Message) -> celtypes.TimestampType:
 
 
 def unwrap(msg: message.Message) -> celtypes.Value:
-    return _field_to_cel(msg, msg.DESCRIPTOR.fields_by_name["value"])
+    return field_to_cel(msg, msg.DESCRIPTOR.fields_by_name["value"])
 
 
 _MSG_TYPE_URL_TO_CTOR: dict[str, typing.Callable[..., celtypes.Value]] = {
@@ -70,7 +70,7 @@ def __init__(self, msg: message.Message):
         for field in self.desc.fields:
             if field.containing_oneof is not None and not self.msg.HasField(field.name):
                 continue
-            self[field.name] = _field_to_cel(self.msg, field)
+            self[field.name] = field_to_cel(self.msg, field)
 
     def __getitem__(self, name):
         field = self.desc.fields_by_name[name]
@@ -175,7 +175,7 @@ def _map_field_to_cel(msg: message.Message, field: descriptor.FieldDescriptor) -
     return _map_field_value_to_cel(_proto_message_get_field(msg, field), field)
 
 
-def _field_to_cel(msg: message.Message, field: descriptor.FieldDescriptor) -> celtypes.Value:
+def field_to_cel(msg: message.Message, field: descriptor.FieldDescriptor) -> celtypes.Value:
     if field.label == descriptor.FieldDescriptor.LABEL_REPEATED:
         return _repeated_field_to_cel(msg, field)
     elif field.message_type is not None and not _proto_message_has_field(msg, field):
@@ -374,7 +374,7 @@ def add_rule(
         rule_cel = None
         if rule_field is not None and self._rules is not None:
             rule_value = _proto_message_get_field(self._rules, rule_field)
-            rule_cel = _field_to_cel(self._rules, rule_field)
+            rule_cel = field_to_cel(self._rules, rule_field)
         self._cel.append(
             CelRunner(
                 runner=prog,

protovalidate/internal/extra_func.py

@@ -22,6 +22,7 @@
 from celpy import celtypes
 
 from protovalidate.internal import string_format
+from protovalidate.internal.constraints import MessageType, field_to_cel
 
 
 def _validate_hostname(host):
@@ -112,6 +113,19 @@ def validate_ip(val: typing.Union[str, bytes], version: typing.Optional[int] = N
         return False
 
 
+def get_field(message: celtypes.Value, field_name: celtypes.Value) -> celpy.Result:
+    if not isinstance(message, MessageType):
+        msg = "invalid argument, expected message"
+        raise celpy.CELEvalError(msg)
+    if not isinstance(field_name, celtypes.StringType):
+        msg = "invalid argument, expected string"
+        raise celpy.CELEvalError(msg)
+    if field_name not in message.desc.fields_by_name:
+        msg = f"no such field: {field_name}"
+        raise celpy.CELEvalError(msg)
+    return field_to_cel(message.msg, message.desc.fields_by_name[field_name])
+
+
 def is_ip(val: celtypes.Value, version: typing.Optional[celtypes.Value] = None) -> celpy.Result:
     if not isinstance(val, (celtypes.BytesType, celtypes.StringType)):
         msg = "invalid argument, expected string or bytes"
@@ -245,6 +259,7 @@ def make_extra_funcs(locale: str) -> dict[str, celpy.CELFunction]:
         # Missing standard functions
         "format": string_fmt.format,
         # protovalidate specific functions
+        "getField": get_field,
         "isNan": is_nan,
         "isInf": is_inf,
         "isIp": is_ip,