Fix ExpiringCache.__len__ to be accurate

It used to try and produce an estimate, which was sometimes negative.
This caused metrics to be sad, so lets always just calculate it from
scratch.

(This appears to have been a longstanding bug, but one which has been made more
of a problem by #3932 and #3933).

(This was originally done by Erik as part of #3933. I'm cherry-picking it
because really it's a fix in its own right)

synapse/util/caches/expiringcache.py

@@ -16,6 +16,8 @@
 import logging
 from collections import OrderedDict
 
+from six import itervalues
+
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.util.caches import register_cache
 
@@ -54,8 +56,6 @@ class ExpiringCache(object):
 
         self.iterable = iterable
 
-        self._size_estimate = 0
-
         self.metrics = register_cache("expiring", cache_name, self)
 
         if not self._expiry_ms:
@@ -74,16 +74,11 @@ class ExpiringCache(object):
         now = self._clock.time_msec()
         self._cache[key] = _CacheEntry(now, value)
 
-        if self.iterable:
-            self._size_estimate += len(value)
-
         # Evict if there are now too many items
         while self._max_len and len(self) > self._max_len:
             _key, value = self._cache.popitem(last=False)
             if self.iterable:
-                removed_len = len(value.value)
+                self.metrics.inc_evictions(len(value.value))
-                self.metrics.inc_evictions(removed_len)
-                self._size_estimate -= removed_len
             else:
                 self.metrics.inc_evictions()
 
@@ -134,7 +129,9 @@ class ExpiringCache(object):
         for k in keys_to_delete:
             value = self._cache.pop(k)
             if self.iterable:
-                self._size_estimate -= len(value.value)
+                self.metrics.inc_evictions(len(value.value))
+            else:
+                self.metrics.inc_evictions()
 
         logger.debug(
             "[%s] _prune_cache before: %d, after len: %d",
@@ -143,7 +140,7 @@ class ExpiringCache(object):
 
     def __len__(self):
         if self.iterable:
-            return self._size_estimate
+            return sum(len(entry.value) for entry in itervalues(self._cache))
         else:
             return len(self._cache)