Fix typing for `@cached` wrapped functions (#8240)

This requires adding a mypy plugin to fiddle with the type signatures a bit.

1 files changed, 28 insertions, 14 deletions

diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 49d9fddcf0..825810eb16 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -18,11 +18,10 @@ import functools
 import inspect
 import logging
 import threading
-from typing import Any, Tuple, Union, cast
+from typing import Any, Callable, Generic, Optional, Tuple, TypeVar, Union, cast
 from weakref import WeakValueDictionary
 
 from prometheus_client import Gauge
-from typing_extensions import Protocol
 
 from twisted.internet import defer
 
@@ -38,8 +37,10 @@ logger = logging.getLogger(__name__)
 
 CacheKey = Union[Tuple, Any]
 
+F = TypeVar("F", bound=Callable[..., Any])
 
-class _CachedFunction(Protocol):
+
+class _CachedFunction(Generic[F]):
     invalidate = None  # type: Any
     invalidate_all = None  # type: Any
     invalidate_many = None  # type: Any
@@ -47,8 +48,11 @@ class _CachedFunction(Protocol):
     cache = None  # type: Any
     num_args = None  # type: Any
 
-    def __name__(self):
-        ...
+    __name__ = None  # type: str
+
+    # Note: This function signature is actually fiddled with by the synapse mypy
+    # plugin to a) make it a bound method, and b) remove any `cache_context` arg.
+    __call__ = None  # type: F
 
 
 cache_pending_metric = Gauge(
@@ -123,7 +127,7 @@ class Cache(object):
 
         self.name = name
         self.keylen = keylen
-        self.thread = None
+        self.thread = None  # type: Optional[threading.Thread]
         self.metrics = register_cache(
             "cache",
             name,
@@ -662,9 +666,13 @@ class _CacheContext:
 
 
 def cached(
-    max_entries=1000, num_args=None, tree=False, cache_context=False, iterable=False
-):
-    return lambda orig: CacheDescriptor(
+    max_entries: int = 1000,
+    num_args: Optional[int] = None,
+    tree: bool = False,
+    cache_context: bool = False,
+    iterable: bool = False,
+) -> Callable[[F], _CachedFunction[F]]:
+    func = lambda orig: CacheDescriptor(
         orig,
         max_entries=max_entries,
         num_args=num_args,
@@ -673,8 +681,12 @@ def cached(
         iterable=iterable,
     )
 
+    return cast(Callable[[F], _CachedFunction[F]], func)
 
-def cachedList(cached_method_name, list_name, num_args=None):
+
+def cachedList(
+    cached_method_name: str, list_name: str, num_args: Optional[int] = None
+) -> Callable[[F], _CachedFunction[F]]:
     """Creates a descriptor that wraps a function in a `CacheListDescriptor`.
 
     Used to do batch lookups for an already created cache. A single argument
@@ -684,11 +696,11 @@ def cachedList(cached_method_name, list_name, num_args=None):
     cache.
 
     Args:
-        cached_method_name (str): The name of the single-item lookup method.
+        cached_method_name: The name of the single-item lookup method.
             This is only used to find the cache to use.
-        list_name (str): The name of the argument that is the list to use to
+        list_name: The name of the argument that is the list to use to
             do batch lookups in the cache.
-        num_args (int): Number of arguments to use as the key in the cache
+        num_args: Number of arguments to use as the key in the cache
             (including list_name). Defaults to all named parameters.
 
     Example:
@@ -702,9 +714,11 @@ def cachedList(cached_method_name, list_name, num_args=None):
             def batch_do_something(self, first_arg, second_args):
                 ...
     """
-    return lambda orig: CacheListDescriptor(
+    func = lambda orig: CacheListDescriptor(
         orig,
         cached_method_name=cached_method_name,
         list_name=list_name,
         num_args=num_args,
     )
+
+    return cast(Callable[[F], _CachedFunction[F]], func)