Clean up caching/locking of OIDC metadata load (#9362)

Ensure that we lock correctly to prevent multiple concurrent metadata load
requests, and generally clean up the way we construct the metadata cache.

2 files changed, 182 insertions, 36 deletions

diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
index 3adc75fa4a..5f3e8a77f5 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -41,6 +41,7 @@ from synapse.http.site import SynapseRequest
 from synapse.logging.context import make_deferred_yieldable
 from synapse.types import JsonDict, UserID, map_username_to_mxid_localpart
 from synapse.util import json_decoder
+from synapse.util.caches.cached_call import RetryOnExceptionCachedCall
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -245,6 +246,7 @@ class OidcProvider:
 
         self._token_generator = token_generator
 
+        self._config = provider
         self._callback_url = hs.config.oidc_callback_url  # type: str
 
         self._scopes = provider.scopes
@@ -253,14 +255,16 @@ class OidcProvider:
             provider.client_id, provider.client_secret, provider.client_auth_method,
         )  # type: ClientAuth
         self._client_auth_method = provider.client_auth_method
-        self._provider_metadata = OpenIDProviderMetadata(
-            issuer=provider.issuer,
-            authorization_endpoint=provider.authorization_endpoint,
-            token_endpoint=provider.token_endpoint,
-            userinfo_endpoint=provider.userinfo_endpoint,
-            jwks_uri=provider.jwks_uri,
-        )  # type: OpenIDProviderMetadata
-        self._provider_needs_discovery = provider.discover
+
+        # cache of metadata for the identity provider (endpoint uris, mostly). This is
+        # loaded on-demand from the discovery endpoint (if discovery is enabled), with
+        # possible overrides from the config.  Access via `load_metadata`.
+        self._provider_metadata = RetryOnExceptionCachedCall(self._load_metadata)
+
+        # cache of JWKs used by the identity provider to sign tokens. Loaded on demand
+        # from the IdP's jwks_uri, if required.
+        self._jwks = RetryOnExceptionCachedCall(self._load_jwks)
+
         self._user_mapping_provider = provider.user_mapping_provider_class(
             provider.user_mapping_provider_config
         )
@@ -286,7 +290,7 @@ class OidcProvider:
 
         self._sso_handler.register_identity_provider(self)
 
-    def _validate_metadata(self):
+    def _validate_metadata(self, m: OpenIDProviderMetadata) -> None:
         """Verifies the provider metadata.
 
         This checks the validity of the currently loaded provider. Not
@@ -305,7 +309,6 @@ class OidcProvider:
         if self._skip_verification is True:
             return
 
-        m = self._provider_metadata
         m.validate_issuer()
         m.validate_authorization_endpoint()
         m.validate_token_endpoint()
@@ -340,11 +343,7 @@ class OidcProvider:
                 )
         else:
             # If we're not using userinfo, we need a valid jwks to validate the ID token
-            if m.get("jwks") is None:
-                if m.get("jwks_uri") is not None:
-                    m.validate_jwks_uri()
-                else:
-                    raise ValueError('"jwks_uri" must be set')
+            m.validate_jwks_uri()
 
     @property
     def _uses_userinfo(self) -> bool:
@@ -361,11 +360,15 @@ class OidcProvider:
             or self._user_profile_method == "userinfo_endpoint"
         )
 
-    async def load_metadata(self) -> OpenIDProviderMetadata:
-        """Load and validate the provider metadata.
+    async def load_metadata(self, force: bool = False) -> OpenIDProviderMetadata:
+        """Return the provider metadata.
+
+        If this is the first call, the metadata is built from the config and from the
+        metadata discovery endpoint (if enabled), and then validated. If the metadata
+        is successfully validated, it is then cached for future use.
 
-        The values metadatas are discovered if ``oidc_config.discovery`` is
-        ``True`` and then cached.
+        Args:
+            force: If true, any cached metadata is discarded to force a reload.
 
         Raises:
             ValueError: if something in the provider is not valid
@@ -373,18 +376,32 @@ class OidcProvider:
         Returns:
             The provider's metadata.
         """
-        # If we are using the OpenID Discovery documents, it needs to be loaded once
-        # FIXME: should there be a lock here?
-        if self._provider_needs_discovery:
-            url = get_well_known_url(self._provider_metadata["issuer"], external=True)
+        if force:
+            # reset the cached call to ensure we get a new result
+            self._provider_metadata = RetryOnExceptionCachedCall(self._load_metadata)
+
+        return await self._provider_metadata.get()
+
+    async def _load_metadata(self) -> OpenIDProviderMetadata:
+        # init the metadata from our config
+        metadata = OpenIDProviderMetadata(
+            issuer=self._config.issuer,
+            authorization_endpoint=self._config.authorization_endpoint,
+            token_endpoint=self._config.token_endpoint,
+            userinfo_endpoint=self._config.userinfo_endpoint,
+            jwks_uri=self._config.jwks_uri,
+        )
+
+        # load any data from the discovery endpoint, if enabled
+        if self._config.discover:
+            url = get_well_known_url(self._config.issuer, external=True)
             metadata_response = await self._http_client.get_json(url)
             # TODO: maybe update the other way around to let user override some values?
-            self._provider_metadata.update(metadata_response)
-            self._provider_needs_discovery = False
+            metadata.update(metadata_response)
 
-        self._validate_metadata()
+        self._validate_metadata(metadata)
 
-        return self._provider_metadata
+        return metadata
 
     async def load_jwks(self, force: bool = False) -> JWKS:
         """Load the JSON Web Key Set used to sign ID tokens.
@@ -414,27 +431,27 @@ class OidcProvider:
                     ]
                 }
         """
+        if force:
+            # reset the cached call to ensure we get a new result
+            self._jwks = RetryOnExceptionCachedCall(self._load_jwks)
+        return await self._jwks.get()
+
+    async def _load_jwks(self) -> JWKS:
         if self._uses_userinfo:
             # We're not using jwt signing, return an empty jwk set
             return {"keys": []}
 
-        # First check if the JWKS are loaded in the provider metadata.
-        # It can happen either if the provider gives its JWKS in the discovery
-        # document directly or if it was already loaded once.
         metadata = await self.load_metadata()
-        jwk_set = metadata.get("jwks")
-        if jwk_set is not None and not force:
-            return jwk_set
 
-        # Loading the JWKS using the `jwks_uri` metadata
+        # Load the JWKS using the `jwks_uri` metadata.
         uri = metadata.get("jwks_uri")
         if not uri:
+            # this should be unreachable: load_metadata validates that
+            # there is a jwks_uri in the metadata if _uses_userinfo is unset
             raise RuntimeError('Missing "jwks_uri" in metadata')
 
         jwk_set = await self._http_client.get_json(uri)
 
-        # Caching the JWKS in the provider's metadata
-        self._provider_metadata["jwks"] = jwk_set
         return jwk_set
 
     async def _exchange_code(self, code: str) -> Token:
diff --git a/synapse/util/caches/cached_call.py b/synapse/util/caches/cached_call.py
new file mode 100644
index 0000000000..3ee0f2317a
--- /dev/null
+++ b/synapse/util/caches/cached_call.py
@@ -0,0 +1,129 @@
+# -*- coding: utf-8 -*-
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Awaitable, Callable, Generic, Optional, TypeVar, Union
+
+from twisted.internet.defer import Deferred
+from twisted.python.failure import Failure
+
+from synapse.logging.context import make_deferred_yieldable, run_in_background
+
+TV = TypeVar("TV")
+
+
+class CachedCall(Generic[TV]):
+    """A wrapper for asynchronous calls whose results should be shared
+
+    This is useful for wrapping asynchronous functions, where there might be multiple
+    callers, but we only want to call the underlying function once (and have the result
+    returned to all callers).
+
+    Similar results can be achieved via a lock of some form, but that typically requires
+    more boilerplate (and ends up being less efficient).
+
+    Correctly handles Synapse logcontexts (logs and resource usage for the underlying
+    function are logged against the logcontext which is active when get() is first
+    called).
+
+    Example usage:
+
+        _cached_val = CachedCall(_load_prop)
+
+        async def handle_request() -> X:
+            # We can call this multiple times, but it will result in a single call to
+            # _load_prop().
+            return await _cached_val.get()
+
+        async def _load_prop() -> X:
+            await difficult_operation()
+
+
+    The implementation is deliberately single-shot (ie, once the call is initiated,
+    there is no way to ask for it to be run). This keeps the implementation and
+    semantics simple. If you want to make a new call, simply replace the whole
+    CachedCall object.
+    """
+
+    __slots__ = ["_callable", "_deferred", "_result"]
+
+    def __init__(self, f: Callable[[], Awaitable[TV]]):
+        """
+        Args:
+            f: The underlying function. Only one call to this function will be alive
+                at once (per instance of CachedCall)
+        """
+        self._callable = f  # type: Optional[Callable[[], Awaitable[TV]]]
+        self._deferred = None  # type: Optional[Deferred]
+        self._result = None  # type: Union[None, Failure, TV]
+
+    async def get(self) -> TV:
+        """Kick off the call if necessary, and return the result"""
+
+        # Fire off the callable now if this is our first time
+        if not self._deferred:
+            self._deferred = run_in_background(self._callable)
+
+            # we will never need the callable again, so make sure it can be GCed
+            self._callable = None
+
+            # once the deferred completes, store the result. We cannot simply leave the
+            # result in the deferred, since if it's a Failure, GCing the deferred
+            # would then log a critical error about unhandled Failures.
+            def got_result(r):
+                self._result = r
+
+            self._deferred.addBoth(got_result)
+
+        # TODO: consider cancellation semantics. Currently, if the call to get()
+        #    is cancelled, the underlying call will continue (and any future calls
+        #    will get the result/exception), which I think is *probably* ok, modulo
+        #    the fact the underlying call may be logged to a cancelled logcontext,
+        #    and any eventual exception may not be reported.
+
+        # we can now await the deferred, and once it completes, return the result.
+        await make_deferred_yieldable(self._deferred)
+
+        # I *think* this is the easiest way to correctly raise a Failure without having
+        # to gut-wrench into the implementation of Deferred.
+        d = Deferred()
+        d.callback(self._result)
+        return await d
+
+
+class RetryOnExceptionCachedCall(Generic[TV]):
+    """A wrapper around CachedCall which will retry the call if an exception is thrown
+
+    This is used in much the same way as CachedCall, but adds some extra functionality
+    so that if the underlying function throws an exception, then the next call to get()
+    will initiate another call to the underlying function. (Any calls to get() which
+    are already pending will raise the exception.)
+    """
+
+    slots = ["_cachedcall"]
+
+    def __init__(self, f: Callable[[], Awaitable[TV]]):
+        async def _wrapper() -> TV:
+            try:
+                return await f()
+            except Exception:
+                # the call raised an exception: replace the underlying CachedCall to
+                # trigger another call next time get() is called
+                self._cachedcall = CachedCall(_wrapper)
+                raise
+
+        self._cachedcall = CachedCall(_wrapper)
+
+    async def get(self) -> TV:
+        return await self._cachedcall.get()