1 files changed, 44 insertions, 0 deletions
diff --git a/synapse/util/caches/cache_in_self_decorator.py b/synapse/util/caches/cache_in_self_decorator.py
new file mode 100644
index 0000000000..fea41b9d08
--- /dev/null
+++ b/synapse/util/caches/cache_in_self_decorator.py
@@ -0,0 +1,44 @@
+from typing import TYPE_CHECKING, Callable, Dict, List, Optional, TypeVar, cast
+
+T = TypeVar("T")
+
+
+def cache_in_self(builder: Callable[["HomeServer"], T]) -> Callable[["HomeServer"], T]:
+    """Wraps a function called e.g. `get_foo`, checking if `self.foo` exists and
+    returning if so. If not, calls the given function and sets `self.foo` to it.
+
+    Also ensures that dependency cycles throw an exception correctly, rather
+    than overflowing the stack.
+    """
+
+    if not builder.__name__.startswith("get_"):
+        raise Exception(
+            "@cache_in_self can only be used on functions starting with `get_`"
+        )
+
+    # get_attr -> _attr
+    depname = builder.__name__[len("get") :]
+
+    building = [False]
+
+    @functools.wraps(builder)
+    def _get(self: "HomeServer") -> T:
+        try:
+            return getattr(self, depname)
+        except AttributeError:
+            pass
+
+        # Prevent cyclic dependencies from deadlocking
+        if building[0]:
+            raise ValueError("Cyclic dependency while building %s" % (depname,))
+
+        building[0] = True
+        try:
+            dep = builder(self)
+            setattr(self, depname, dep)
+        finally:
+            building[0] = False
+
+        return dep
+
+    return _get