api: Avoid programming errors due to nested Annotated types.

We want to reject ambiguous type annotations that set ApiParamConfig
inside a Union. If a parameter is Optional and has a default of None, we
prefer Annotated[Optional[T], ...] over Optional[Annotated[T, ...]].

This implements a check that detects Optional[Annotated[T, ...]] and
raise an assertion error if ApiParamConfig is in the annotation. It also
checks if the type annotation contains any ApiParamConfig objects that
are ignored, which can happen if the Annotated type is nested inside
another type like List, Union, etc.

Note that because
param: Annotated[Optional[T], ...] = None
and
param: Optional[Annotated[Optional[T], ...]] = None
are equivalent in runtime prior to Python 3.11, there is no way for us
to distinguish the two. So we cannot detect that in runtime.
See also: python/cpython#90353

tools/semgrep.yml

@@ -169,3 +169,26 @@ rules:
     message: 'A batched migration should not be atomic. Add "atomic = False" to the Migration class'
     languages: [python]
     severity: ERROR
+
+  - id: dont-nest-annotated-types-with-param-config
+    patterns:
+      - pattern-not: |
+          def $F(..., invalid_param: typing.Optional[<...  zerver.lib.typed_endpoint.ApiParamConfig(...) ...>], ...) -> ...:
+            ...
+      - pattern-not: |
+          def $F(..., $A: typing_extensions.Annotated[<...  zerver.lib.typed_endpoint.ApiParamConfig(...) ...>], ...) -> ...:
+            ...
+      - pattern-not: |
+          def $F(..., $A: typing_extensions.Annotated[<...  zerver.lib.typed_endpoint.ApiParamConfig(...) ...>] = ..., ...) -> ...:
+            ...
+      - pattern-either:
+          - pattern: |
+              def $F(..., $A: $B[<...  zerver.lib.typed_endpoint.ApiParamConfig(...) ...>], ...) -> ...:
+                ...
+          - pattern: |
+              def $F(..., $A: $B[<...  zerver.lib.typed_endpoint.ApiParamConfig(...) ...>] = ..., ...) -> ...:
+                ...
+    message: |
+      Annotated types containing zerver.lib.typed_endpoint.ApiParamConfig should not be nested inside Optional. Use Annotated[Optional[...], zerver.lib.typed_endpoint.ApiParamConfig(...)] instead.
+    languages: [python]
+    severity: ERROR

zerver/lib/typed_endpoint.py

@@ -1,5 +1,6 @@
 import inspect
 import json
+import sys
 from dataclasses import dataclass
 from enum import Enum, auto
 from functools import wraps
@@ -194,6 +195,41 @@ def is_annotated(type_annotation: Type[object]) -> bool:
     return origin is Annotated
 
 
+def is_optional(type_annotation: Type[object]) -> bool:
+    origin = get_origin(type_annotation)
+    type_args = get_args(type_annotation)
+    return origin is Union and type(None) in type_args and len(type_args) == 2
+
+
+API_PARAM_CONFIG_USAGE_HINT = f"""
+    Detected incorrect usage of Annotated types for parameter {{param_name}}!
+    Check the placement of the {ApiParamConfig.__name__} object in the type annotation:
+
+    {{param_name}}: {{param_type}}
+
+    The Annotated[T, ...] type annotation containing the
+    {ApiParamConfig.__name__} object should not be nested inside another type.
+
+    Correct examples:
+
+    # Using Optional inside Annotated
+    param: Annotated[Optional[int], ApiParamConfig(...)]
+    param: Annotated[Optional[int], ApiParamConfig(...)]] = None
+
+    # Not using Optional when the default is not None
+    param: Annotated[int, ApiParamConfig(...)]
+
+    Incorrect examples:
+
+    # Nesting Annotated inside Optional
+    param: Optional[Annotated[int, ApiParamConfig(...)]]
+    param: Optional[Annotated[int, ApiParamConfig(...)]] = None
+
+    # Nesting the Annotated type carrying ApiParamConfig inside other types like Union
+    param: Union[str, Annotated[int, ApiParamConfig(...)]]
+"""
+
+
 def parse_single_parameter(
     param_name: str, param_type: Type[T], parameter: inspect.Parameter
 ) -> FuncParam[T]:
@@ -209,12 +245,21 @@ def parse_single_parameter(
     # lost. This is fixed in Python 3.11:
     # https://github.com/python/cpython/issues/90353
     if param_default is None:
-        origin = get_origin(param_type)
         type_args = get_args(param_type)
-        if origin is Union and type(None) in type_args and len(type_args) == 2:
-            inner_type = type_args[0] if type_args[1] is type(None) else type_args[1]
-            if is_annotated(inner_type):
-                param_type = inner_type
+        assert is_optional(param_type)
+        inner_type = type_args[0] if type_args[1] is type(None) else type_args[1]
+        if is_annotated(inner_type):
+            annotated_type, *annotations = get_args(inner_type)
+            has_api_param_config = any(
+                isinstance(annotation, ApiParamConfig) for annotation in annotations
+            )
+            # This prohibits the use of `Optional[Annotated[T, ApiParamConfig(...)]] = None`
+            # and encourage `Annotated[Optional[T], ApiParamConfig(...)] = None`
+            # to avoid confusion when the parameter metadata is unintentionally nested.
+            assert not has_api_param_config or is_optional(
+                annotated_type
+            ), API_PARAM_CONFIG_USAGE_HINT.format(param_name=param_name, param_type=param_type)
+            param_type = inner_type
 
     param_config: Optional[ApiParamConfig] = None
     json_wrapper = False
@@ -223,14 +268,22 @@ def parse_single_parameter(
         # metadata attached to Annotated. Note that we do not transform
         # param_type to its underlying type because the Annotated metadata might
         # still be needed by other parties like Pydantic.
-        _, *annotations = get_args(param_type)
+        ignored_type, *annotations = get_args(param_type)
         for annotation in annotations:
             if type(annotation) is Json:
                 json_wrapper = True
             if not isinstance(annotation, ApiParamConfig):
                 continue
             assert param_config is None, "ApiParamConfig can only be defined once per parameter"
             param_config = annotation
+    else:
+        # When no parameter configuration is found, assert that there is none
+        # nested somewhere in a Union type to avoid silently ignoring it. If it
+        # does present in the stringified parameter type, it is very likely a
+        # programming error.
+        assert ApiParamConfig.__name__ not in str(param_type), API_PARAM_CONFIG_USAGE_HINT.format(
+            param_name=param_name, param_type=param_type
+        )
     # Set param_config to a default early to avoid additional None-checks.
     if param_config is None:
         param_config = ApiParamConfig()

zerver/tests/test_typed_endpoint.py

@@ -3,7 +3,7 @@
 import orjson
 from django.core.exceptions import ValidationError as DjangoValidationError
 from django.http import HttpRequest, HttpResponse
-from pydantic import BaseModel, ConfigDict, Json
+from pydantic import BaseModel, ConfigDict, Json, StringConstraints
 from pydantic.dataclasses import dataclass
 from typing_extensions import Annotated
 
@@ -399,6 +399,45 @@ def view3(
         )
         self.assertFalse(result)
 
+        # Not nesting the Annotated type with the ApiParamConfig inside Optional is fine
+        @typed_endpoint
+        def no_nesting(
+            request: HttpRequest,
+            bar: Annotated[
+                Optional[str],
+                StringConstraints(strip_whitespace=True, max_length=3),
+                ApiParamConfig("test"),
+            ] = None,
+        ) -> None:
+            raise AssertionError
+
+        with self.assertRaisesMessage(ApiParamValidationError, "test is too long"):
+            call_endpoint(no_nesting, HostRequestMock({"test": "long"}))
+
+        # Nesting Annotated with ApiParamConfig inside Optional is not fine
+        def nesting_with_config(
+            request: HttpRequest,
+            invalid_param: Optional[Annotated[str, ApiParamConfig("test")]] = None,
+        ) -> None:
+            raise AssertionError
+
+        with self.assertRaisesRegex(
+            AssertionError,
+            "Detected incorrect usage of Annotated types for parameter invalid_param!",
+        ):
+            typed_endpoint(nesting_with_config)
+
+        # Nesting Annotated inside Optional, when ApiParamConfig is not also nested is fine
+        @typed_endpoint
+        def nesting_without_config(
+            request: HttpRequest,
+            bar: Optional[Annotated[str, StringConstraints(max_length=3)]] = None,
+        ) -> None:
+            raise AssertionError
+
+        with self.assertRaisesMessage(ApiParamValidationError, "bar is too long"):
+            call_endpoint(nesting_without_config, HostRequestMock({"bar": "long"}))
+
     def test_aliases(self) -> None:
         @typed_endpoint
         def foo(