Add DeferredCache.get_immediate method (#8568)

* Add `DeferredCache.get_immediate` method

A bunch of things that are currently calling `DeferredCache.get` are only
really interested in the result if it's completed. We can optimise and simplify
this case.

* Remove unused 'default' parameter to DeferredCache.get()

* another get_immediate instance

synapse/util/caches/deferred_cache.py

@@ -17,7 +17,16 @@
 
 import enum
 import threading
-from typing import Callable, Generic, Iterable, MutableMapping, Optional, TypeVar, cast
+from typing import (
+    Callable,
+    Generic,
+    Iterable,
+    MutableMapping,
+    Optional,
+    TypeVar,
+    Union,
+    cast,
+)
 
 from prometheus_client import Gauge
 
@@ -33,7 +42,7 @@ cache_pending_metric = Gauge(
     ["name"],
 )
 
-
+T = TypeVar("T")
 KT = TypeVar("KT")
 VT = TypeVar("VT")
 
@@ -119,21 +128,21 @@ class DeferredCache(Generic[KT, VT]):
     def get(
         self,
         key: KT,
-        default=_Sentinel.sentinel,
         callback: Optional[Callable[[], None]] = None,
         update_metrics: bool = True,
-    ):
+    ) -> Union[ObservableDeferred, VT]:
         """Looks the key up in the caches.
 
         Args:
             key(tuple)
-            default: What is returned if key is not in the caches. If not
-                specified then function throws KeyError instead
             callback(fn): Gets called when the entry in the cache is invalidated
             update_metrics (bool): whether to update the cache hit rate metrics
 
         Returns:
             Either an ObservableDeferred or the result itself
+
+        Raises:
+            KeyError if the key is not found in the cache
         """
         callbacks = [callback] if callback else []
         val = self._pending_deferred_cache.get(key, _Sentinel.sentinel)
@@ -145,13 +154,19 @@ class DeferredCache(Generic[KT, VT]):
                 m.inc_hits()
             return val.deferred
 
-        val = self.cache.get(
-            key, default, callbacks=callbacks, update_metrics=update_metrics
+        val2 = self.cache.get(
+            key, _Sentinel.sentinel, callbacks=callbacks, update_metrics=update_metrics
         )
-        if val is _Sentinel.sentinel:
+        if val2 is _Sentinel.sentinel:
             raise KeyError()
         else:
-            return val
+            return val2
+
+    def get_immediate(
+        self, key: KT, default: T, update_metrics: bool = True
+    ) -> Union[VT, T]:
+        """If we have a *completed* cached value, return it."""
+        return self.cache.get(key, default, update_metrics=update_metrics)
 
     def set(
         self,