Cache macaroon parse and validation

Turns out this can be quite expensive for requests, and is easily
cachable. We don't cache the lookup to the DB so invalidation still
works.

synapse/api/auth.py

@@ -23,6 +23,8 @@ from synapse import event_auth
 from synapse.api.constants import EventTypes, Membership, JoinRules
 from synapse.api.errors import AuthError, Codes
 from synapse.types import UserID
+from synapse.util.caches import register_cache, CACHE_SIZE_FACTOR
+from synapse.util.caches.lrucache import LruCache
 from synapse.util.metrics import Measure
 
 logger = logging.getLogger(__name__)
@@ -38,6 +40,10 @@ AuthEventTypes = (
 GUEST_DEVICE_ID = "guest_device"
 
 
+class _InvalidMacaroonException(Exception):
+    pass
+
+
 class Auth(object):
     """
     FIXME: This class contains a mix of functions for authenticating users
@@ -50,6 +56,9 @@ class Auth(object):
         self.state = hs.get_state_handler()
         self.TOKEN_NOT_FOUND_HTTP_STATUS = 401
 
+        self.token_cache = LruCache(CACHE_SIZE_FACTOR * 10000)
+        register_cache("token_cache", self.token_cache)
+
     @defer.inlineCallbacks
     def check_from_context(self, event, context, do_sig_check=True):
         auth_events_ids = yield self.compute_auth_events(
@@ -266,8 +275,8 @@ class Auth(object):
             AuthError if no user by that token exists or the token is invalid.
         """
         try:
-            macaroon = pymacaroons.Macaroon.deserialize(token)
-        except Exception:  # deserialize can throw more-or-less anything
+            user_id, guest = self._parse_and_validate_macaroon(token, rights)
+        except _InvalidMacaroonException:
             # doesn't look like a macaroon: treat it as an opaque token which
             # must be in the database.
             # TODO: it would be nice to get rid of this, but apparently some
@@ -276,19 +285,8 @@ class Auth(object):
             defer.returnValue(r)
 
         try:
-            user_id = self.get_user_id_from_macaroon(macaroon)
             user = UserID.from_string(user_id)
 
-            self.validate_macaroon(
-                macaroon, rights, self.hs.config.expire_access_token,
-                user_id=user_id,
-            )
-
-            guest = False
-            for caveat in macaroon.caveats:
-                if caveat.caveat_id == "guest = true":
-                    guest = True
-
             if guest:
                 # Guest access tokens are not stored in the database (there can
                 # only be one access token per guest, anyway).
@@ -360,6 +358,55 @@ class Auth(object):
                 errcode=Codes.UNKNOWN_TOKEN
             )
 
+    def _parse_and_validate_macaroon(self, token, rights="access"):
+        """Takes a macaroon and tries to parse and validate it. This is cached
+        if and only if rights == access and there isn't an expiry.
+
+        On invalid macaroon raises _InvalidMacaroonException
+
+        Returns:
+            (user_id, is_guest)
+        """
+        if rights == "access":
+            cached = self.token_cache.get(token, None)
+            if cached:
+                return cached
+
+        try:
+            macaroon = pymacaroons.Macaroon.deserialize(token)
+        except Exception:  # deserialize can throw more-or-less anything
+            # doesn't look like a macaroon: treat it as an opaque token which
+            # must be in the database.
+            # TODO: it would be nice to get rid of this, but apparently some
+            # people use access tokens which aren't macaroons
+            raise _InvalidMacaroonException()
+
+        try:
+            user_id = self.get_user_id_from_macaroon(macaroon)
+
+            has_expiry = False
+            guest = False
+            for caveat in macaroon.caveats:
+                if caveat.caveat_id.startswith("time "):
+                    has_expiry = True
+                elif caveat.caveat_id == "guest = true":
+                    guest = True
+
+            self.validate_macaroon(
+                macaroon, rights, self.hs.config.expire_access_token,
+                user_id=user_id,
+            )
+        except (pymacaroons.exceptions.MacaroonException, TypeError, ValueError):
+            raise AuthError(
+                self.TOKEN_NOT_FOUND_HTTP_STATUS, "Invalid macaroon passed.",
+                errcode=Codes.UNKNOWN_TOKEN
+            )
+
+        if not has_expiry and rights == "access":
+            self.token_cache[token] = (user_id, guest)
+
+        return user_id, guest
+
     def get_user_id_from_macaroon(self, macaroon):
         """Retrieve the user_id given by the caveats on the macaroon.