Remove unused `@lru_cache` decorator (#13595)

* Remove unused `@lru_cache` decorator

Spotted this working on something else.

Co-authored-by: David Robertson <davidr@element.io>

3 files changed, 5 insertions, 140 deletions

diff --git a/changelog.d/13595.misc b/changelog.d/13595.misc
new file mode 100644
index 0000000000..71959a6ee7
--- /dev/null
+++ b/changelog.d/13595.misc
@@ -0,0 +1 @@
+Remove unused `@lru_cache` decorator.
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index b3c748ef44..75428d19ba 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -12,7 +12,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import enum
 import functools
 import inspect
 import logging
@@ -146,109 +145,6 @@ class _CacheDescriptorBase:
         )
 
 
-class _LruCachedFunction(Generic[F]):
-    cache: LruCache[CacheKey, Any]
-    __call__: F
-
-
-def lru_cache(
-    *, max_entries: int = 1000, cache_context: bool = False
-) -> Callable[[F], _LruCachedFunction[F]]:
-    """A method decorator that applies a memoizing cache around the function.
-
-    This is more-or-less a drop-in equivalent to functools.lru_cache, although note
-    that the signature is slightly different.
-
-    The main differences with functools.lru_cache are:
-        (a) the size of the cache can be controlled via the cache_factor mechanism
-        (b) the wrapped function can request a "cache_context" which provides a
-            callback mechanism to indicate that the result is no longer valid
-        (c) prometheus metrics are exposed automatically.
-
-    The function should take zero or more arguments, which are used as the key for the
-    cache. Single-argument functions use that argument as the cache key; otherwise the
-    arguments are built into a tuple.
-
-    Cached functions can be "chained" (i.e. a cached function can call other cached
-    functions and get appropriately invalidated when they called caches are
-    invalidated) by adding a special "cache_context" argument to the function
-    and passing that as a kwarg to all caches called. For example:
-
-        @lru_cache(cache_context=True)
-        def foo(self, key, cache_context):
-            r1 = self.bar1(key, on_invalidate=cache_context.invalidate)
-            r2 = self.bar2(key, on_invalidate=cache_context.invalidate)
-            return r1 + r2
-
-    The wrapped function also has a 'cache' property which offers direct access to the
-    underlying LruCache.
-    """
-
-    def func(orig: F) -> _LruCachedFunction[F]:
-        desc = LruCacheDescriptor(
-            orig,
-            max_entries=max_entries,
-            cache_context=cache_context,
-        )
-        return cast(_LruCachedFunction[F], desc)
-
-    return func
-
-
-class LruCacheDescriptor(_CacheDescriptorBase):
-    """Helper for @lru_cache"""
-
-    class _Sentinel(enum.Enum):
-        sentinel = object()
-
-    def __init__(
-        self,
-        orig: Callable[..., Any],
-        max_entries: int = 1000,
-        cache_context: bool = False,
-    ):
-        super().__init__(
-            orig, num_args=None, uncached_args=None, cache_context=cache_context
-        )
-        self.max_entries = max_entries
-
-    def __get__(self, obj: Optional[Any], owner: Optional[Type]) -> Callable[..., Any]:
-        cache: LruCache[CacheKey, Any] = LruCache(
-            cache_name=self.name,
-            max_size=self.max_entries,
-        )
-
-        get_cache_key = self.cache_key_builder
-        sentinel = LruCacheDescriptor._Sentinel.sentinel
-
-        @functools.wraps(self.orig)
-        def _wrapped(*args: Any, **kwargs: Any) -> Any:
-            invalidate_callback = kwargs.pop("on_invalidate", None)
-            callbacks = (invalidate_callback,) if invalidate_callback else ()
-
-            cache_key = get_cache_key(args, kwargs)
-
-            ret = cache.get(cache_key, default=sentinel, callbacks=callbacks)
-            if ret != sentinel:
-                return ret
-
-            # Add our own `cache_context` to argument list if the wrapped function
-            # has asked for one
-            if self.add_cache_context:
-                kwargs["cache_context"] = _CacheContext.get_instance(cache, cache_key)
-
-            ret2 = self.orig(obj, *args, **kwargs)
-            cache.set(cache_key, ret2, callbacks=callbacks)
-
-            return ret2
-
-        wrapped = cast(CachedFunction, _wrapped)
-        wrapped.cache = cache
-        obj.__dict__[self.name] = wrapped
-
-        return wrapped
-
-
 class DeferredCacheDescriptor(_CacheDescriptorBase):
     """A method decorator that applies a memoizing cache around the function.
 
diff --git a/tests/util/caches/test_descriptors.py b/tests/util/caches/test_descriptors.py
index 78fd7b6961..43475a307f 100644
--- a/tests/util/caches/test_descriptors.py
+++ b/tests/util/caches/test_descriptors.py
@@ -28,7 +28,7 @@ from synapse.logging.context import (
     make_deferred_yieldable,
 )
 from synapse.util.caches import descriptors
-from synapse.util.caches.descriptors import cached, cachedList, lru_cache
+from synapse.util.caches.descriptors import cached, cachedList
 
 from tests import unittest
 from tests.test_utils import get_awaitable_result
@@ -36,38 +36,6 @@ from tests.test_utils import get_awaitable_result
 logger = logging.getLogger(__name__)
 
 
-class LruCacheDecoratorTestCase(unittest.TestCase):
-    def test_base(self):
-        class Cls:
-            def __init__(self):
-                self.mock = mock.Mock()
-
-            @lru_cache()
-            def fn(self, arg1, arg2):
-                return self.mock(arg1, arg2)
-
-        obj = Cls()
-        obj.mock.return_value = "fish"
-        r = obj.fn(1, 2)
-        self.assertEqual(r, "fish")
-        obj.mock.assert_called_once_with(1, 2)
-        obj.mock.reset_mock()
-
-        # a call with different params should call the mock again
-        obj.mock.return_value = "chips"
-        r = obj.fn(1, 3)
-        self.assertEqual(r, "chips")
-        obj.mock.assert_called_once_with(1, 3)
-        obj.mock.reset_mock()
-
-        # the two values should now be cached
-        r = obj.fn(1, 2)
-        self.assertEqual(r, "fish")
-        r = obj.fn(1, 3)
-        self.assertEqual(r, "chips")
-        obj.mock.assert_not_called()
-
-
 def run_on_reactor():
     d = defer.Deferred()
     reactor.callLater(0, d.callback, 0)
@@ -478,10 +446,10 @@ class DescriptorTestCase(unittest.TestCase):
 
             @cached(cache_context=True)
             async def func2(self, key, cache_context):
-                return self.func3(key, on_invalidate=cache_context.invalidate)
+                return await self.func3(key, on_invalidate=cache_context.invalidate)
 
-            @lru_cache(cache_context=True)
-            def func3(self, key, cache_context):
+            @cached(cache_context=True)
+            async def func3(self, key, cache_context):
                 self.invalidate = cache_context.invalidate
                 return 42