From 3854d0f94947ddd5a9ee98198af8d7ae839962c9 Mon Sep 17 00:00:00 2001
From: Brendan Abolivier <babolivier@matrix.org>
Date: Wed, 28 Dec 2022 14:48:21 +0100
Subject: [PATCH] Add a `cached` helper to the module API (#14663)

---
 changelog.d/14663.feature      |  1 +
 synapse/module_api/__init__.py | 40 +++++++++++++++++++++++++++++++++-
 2 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/14663.feature

diff --git a/changelog.d/14663.feature b/changelog.d/14663.feature
new file mode 100644
index 000000000000..b03f3ee54e33
--- /dev/null
+++ b/changelog.d/14663.feature
@@ -0,0 +1 @@
+Add a `cached` function to `synapse.module_api` that returns a decorator to cache return values of functions.
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index 0092a03c59a7..6f4a934b0509 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -18,6 +18,7 @@
     TYPE_CHECKING,
     Any,
     Callable,
+    Collection,
     Dict,
     Generator,
     Iterable,
@@ -126,7 +127,7 @@
 from synapse.types.state import StateFilter
 from synapse.util import Clock
 from synapse.util.async_helpers import maybe_awaitable
-from synapse.util.caches.descriptors import CachedFunction, cached
+from synapse.util.caches.descriptors import CachedFunction, cached as _cached
 from synapse.util.frozenutils import freeze
 
 if TYPE_CHECKING:
@@ -136,6 +137,7 @@
 
 T = TypeVar("T")
 P = ParamSpec("P")
+F = TypeVar("F", bound=Callable[..., Any])
 
 """
 This package defines the 'stable' API which can be used by extension modules which
@@ -185,6 +187,42 @@ class UserIpAndAgent:
     last_seen: int
 
 
+def cached(
+    *,
+    max_entries: int = 1000,
+    num_args: Optional[int] = None,
+    uncached_args: Optional[Collection[str]] = None,
+) -> Callable[[F], CachedFunction[F]]:
+    """Returns a decorator that applies a memoizing cache around the function. This
+    decorator behaves similarly to functools.lru_cache.
+
+    Example:
+
+        @cached()
+        def foo('a', 'b'):
+            ...
+
+    Added in Synapse v1.74.0.
+
+    Args:
+        max_entries: The maximum number of entries in the cache. If the cache is full
+            and a new entry is added, the least recently accessed entry will be evicted
+            from the cache.
+        num_args: The number of positional arguments (excluding `self`) to use as cache
+            keys. Defaults to all named args of the function.
+        uncached_args: A list of argument names to not use as the cache key. (`self` is
+            always ignored.) Cannot be used with num_args.
+
+    Returns:
+        A decorator that applies a memoizing cache around the function.
+    """
+    return _cached(
+        max_entries=max_entries,
+        num_args=num_args,
+        uncached_args=uncached_args,
+    )
+
+
 class ModuleApi:
     """A proxy object that gets passed to various plugin modules so they
     can register new users etc if necessary.