Fix #8518 (sync requests being cached wrongly on timeout) (#9358)

This fixes #8518 by adding a conditional check on `SyncResult` in a function when `prev_stream_token == current_stream_token`, as a sanity check. In `CachedResponse.set.<remove>()`, the result is immediately popped from the cache if the conditional function returns "false".

This prevents the caching of a timed-out `SyncResult` (that has `next_key` as the stream key that produced that `SyncResult`). The cache is prevented from returning a `SyncResult` that makes the client request the same stream key over and over again, effectively making it stuck in a loop of requesting and getting a response immediately for as long as the cache keeps those values.

Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>
This commit is contained in:
Jonathan de Jong 2021-02-24 14:57:00 +01:00 committed by GitHub
parent 2927921942
commit f5c93fc993
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3 changed files with 35 additions and 3 deletions

1
changelog.d/9358.misc Normal file
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@ -0,0 +1 @@
Added a fix that invalidates cache for empty timed-out sync responses.

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@ -277,8 +277,9 @@ class SyncHandler:
user_id = sync_config.user.to_string()
await self.auth.check_auth_blocking(requester=requester)
res = await self.response_cache.wrap(
res = await self.response_cache.wrap_conditional(
sync_config.request_key,
lambda result: since_token != result.next_batch,
self._wait_for_sync_for_user,
sync_config,
since_token,

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@ -13,7 +13,7 @@
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
from typing import TYPE_CHECKING, Any, Callable, Dict, Generic, Optional, TypeVar
from typing import TYPE_CHECKING, Any, Callable, Dict, Generic, Optional, Set, TypeVar
from twisted.internet import defer
@ -40,6 +40,7 @@ class ResponseCache(Generic[T]):
def __init__(self, hs: "HomeServer", name: str, timeout_ms: float = 0):
# Requests that haven't finished yet.
self.pending_result_cache = {} # type: Dict[T, ObservableDeferred]
self.pending_conditionals = {} # type: Dict[T, Set[Callable[[Any], bool]]]
self.clock = hs.get_clock()
self.timeout_sec = timeout_ms / 1000.0
@ -101,7 +102,11 @@ class ResponseCache(Generic[T]):
self.pending_result_cache[key] = result
def remove(r):
if self.timeout_sec:
should_cache = all(
func(r) for func in self.pending_conditionals.pop(key, [])
)
if self.timeout_sec and should_cache:
self.clock.call_later(
self.timeout_sec, self.pending_result_cache.pop, key, None
)
@ -112,6 +117,31 @@ class ResponseCache(Generic[T]):
result.addBoth(remove)
return result.observe()
def add_conditional(self, key: T, conditional: Callable[[Any], bool]):
self.pending_conditionals.setdefault(key, set()).add(conditional)
def wrap_conditional(
self,
key: T,
should_cache: Callable[[Any], bool],
callback: "Callable[..., Any]",
*args: Any,
**kwargs: Any
) -> defer.Deferred:
"""The same as wrap(), but adds a conditional to the final execution.
When the final execution completes, *all* conditionals need to return True for it to properly cache,
else it'll not be cached in a timed fashion.
"""
# See if there's already a result on this key that hasn't yet completed. Due to the single-threaded nature of
# python, adding a key immediately in the same execution thread will not cause a race condition.
result = self.get(key)
if not result or isinstance(result, defer.Deferred) and not result.called:
self.add_conditional(key, should_cache)
return self.wrap(key, callback, *args, **kwargs)
def wrap(
self, key: T, callback: "Callable[..., Any]", *args: Any, **kwargs: Any
) -> defer.Deferred: