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Extend `ResponseCache` to pass a context object into the callback (#10157) 

This is the first of two PRs which seek to address #8518. This first PR lays the groundwork by extending ResponseCache; a second PR (#10158) will update the SyncHandler to actually use it, and fix the bug.

The idea here is that we allow the callback given to ResponseCache.wrap to decide whether its result should be cached or not. We do that by (optionally) passing a ResponseCacheContext into it, which it can modify.
This commit is contained in:
Richard van der Hoff 2021-06-14 10:26:09 +01:00 committed by GitHub
parent 13577aa55e
commit d7808a2dde
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GPG Key ID: 4AEE18F83AFDEB23
5 changed files with 146 additions and 37 deletions
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1
changelog.d/10157.misc Normal file
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@ -0,0 +1 @@
Extend `ResponseCache` to pass a context object into the callback.

6
synapse/replication/http/_base.py
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@ -285,7 +285,7 @@ class ReplicationEndpoint(metaclass=abc.ABCMeta):
self.__class__.__name__,
)
def _check_auth_and_handle(self, request, **kwargs):
async def _check_auth_and_handle(self, request, **kwargs):
"""Called on new incoming requests when caching is enabled. Checks
if there is a cached response for the request and returns that,
otherwise calls `_handle_request` and caches its response.
@ -300,8 +300,8 @@ class ReplicationEndpoint(metaclass=abc.ABCMeta):
if self.CACHE:
txn_id = kwargs.pop("txn_id")
return self.response_cache.wrap(
return await self.response_cache.wrap(
txn_id, self._handle_request, request, **kwargs
)
return self._handle_request(request, **kwargs)
return await self._handle_request(request, **kwargs)

2
synapse/replication/http/membership.py
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@ -345,7 +345,7 @@ class ReplicationUserJoinedLeftRoomRestServlet(ReplicationEndpoint):
return {}
def _handle_request( # type: ignore
async def _handle_request( # type: ignore
self, request: Request, room_id: str, user_id: str, change: str
) -> Tuple[int, JsonDict]:
logger.info("user membership change: %s in %s", user_id, room_id)

99
synapse/util/caches/response_cache.py
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@ -12,7 +12,9 @@
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
from typing import Any, Callable, Dict, Generic, Optional, TypeVar
from typing import Any, Awaitable, Callable, Dict, Generic, Optional, TypeVar
import attr
from twisted.internet import defer
@ -23,10 +25,36 @@ from synapse.util.caches import register_cache
logger = logging.getLogger(__name__)
T = TypeVar("T")
# the type of the key in the cache
KV = TypeVar("KV")
# the type of the result from the operation
RV = TypeVar("RV")
class ResponseCache(Generic[T]):
@attr.s(auto_attribs=True)
class ResponseCacheContext(Generic[KV]):
"""Information about a missed ResponseCache hit
This object can be passed into the callback for additional feedback
"""
cache_key: KV
"""The cache key that caused the cache miss
This should be considered read-only.
TODO: in attrs 20.1, make it frozen with an on_setattr.
"""
should_cache: bool = True
"""Whether the result should be cached once the request completes.
This can be modified by the callback if it decides its result should not be cached.
"""
class ResponseCache(Generic[KV]):
"""
This caches a deferred response. Until the deferred completes it will be
returned from the cache. This means that if the client retries the request
@ -35,8 +63,10 @@ class ResponseCache(Generic[T]):
"""
def __init__(self, clock: Clock, name: str, timeout_ms: float = 0):
# Requests that haven't finished yet.
self.pending_result_cache = {} # type: Dict[T, ObservableDeferred]
# This is poorly-named: it includes both complete and incomplete results.
# We keep complete results rather than switching to absolute values because
# that makes it easier to cache Failure results.
self.pending_result_cache = {} # type: Dict[KV, ObservableDeferred]
self.clock = clock
self.timeout_sec = timeout_ms / 1000.0
@ -50,16 +80,13 @@ class ResponseCache(Generic[T]):
def __len__(self) -> int:
return self.size()
def get(self, key: T) -> Optional[defer.Deferred]:
def get(self, key: KV) -> Optional[defer.Deferred]:
"""Look up the given key.
Can return either a new Deferred (which also doesn't follow the synapse
logcontext rules), or, if the request has completed, the actual
result. You will probably want to make_deferred_yieldable the result.
Returns a new Deferred (which also doesn't follow the synapse
logcontext rules). You will probably want to make_deferred_yieldable the result.
If there is no entry for the key, returns None. It is worth noting that
this means there is no way to distinguish a completed result of None
from an absent cache entry.
If there is no entry for the key, returns None.
Args:
key: key to get/set in the cache
@ -76,42 +103,56 @@ class ResponseCache(Generic[T]):
self._metrics.inc_misses()
return None
def set(self, key: T, deferred: defer.Deferred) -> defer.Deferred:
def _set(
self, context: ResponseCacheContext[KV], deferred: defer.Deferred
) -> defer.Deferred:
"""Set the entry for the given key to the given deferred.
*deferred* should run its callbacks in the sentinel logcontext (ie,
you should wrap normal synapse deferreds with
synapse.logging.context.run_in_background).
Can return either a new Deferred (which also doesn't follow the synapse
logcontext rules), or, if *deferred* was already complete, the actual
result. You will probably want to make_deferred_yieldable the result.
Returns a new Deferred (which also doesn't follow the synapse logcontext rules).
You will probably want to make_deferred_yieldable the result.
Args:
key: key to get/set in the cache
context: Information about the cache miss
deferred: The deferred which resolves to the result.
Returns:
A new deferred which resolves to the actual result.
"""
result = ObservableDeferred(deferred, consumeErrors=True)
key = context.cache_key
self.pending_result_cache[key] = result
def remove(r):
if self.timeout_sec:
def on_complete(r):
# if this cache has a non-zero timeout, and the callback has not cleared
# the should_cache bit, we leave it in the cache for now and schedule
# its removal later.
if self.timeout_sec and context.should_cache:
self.clock.call_later(
self.timeout_sec, self.pending_result_cache.pop, key, None
)
else:
# otherwise, remove the result immediately.
self.pending_result_cache.pop(key, None)
return r
result.addBoth(remove)
# make sure we do this *after* adding the entry to pending_result_cache,
# in case the result is already complete (in which case flipping the order would
# leave us with a stuck entry in the cache).
result.addBoth(on_complete)
return result.observe()
def wrap(
self, key: T, callback: Callable[..., Any], *args: Any, **kwargs: Any
) -> defer.Deferred:
async def wrap(
self,
key: KV,
callback: Callable[..., Awaitable[RV]],
*args: Any,
cache_context: bool = False,
**kwargs: Any,
) -> RV:
"""Wrap together a *get* and *set* call, taking care of logcontexts
First looks up the key in the cache, and if it is present makes it
@ -140,22 +181,28 @@ class ResponseCache(Generic[T]):
*args: positional parameters to pass to the callback, if it is used
cache_context: if set, the callback will be given a `cache_context` kw arg,
which will be a ResponseCacheContext object.
**kwargs: named parameters to pass to the callback, if it is used
Returns:
Deferred which resolves to the result
The result of the callback (from the cache, or otherwise)
"""
result = self.get(key)
if not result:
logger.debug(
"[%s]: no cached result for [%s], calculating new one", self._name, key
)
context = ResponseCacheContext(cache_key=key)
if cache_context:
kwargs["cache_context"] = context
d = run_in_background(callback, *args, **kwargs)
result = self.set(key, d)
result = self._set(context, d)
elif not isinstance(result, defer.Deferred) or result.called:
logger.info("[%s]: using completed cached result for [%s]", self._name, key)
else:
logger.info(
"[%s]: using incomplete cached result for [%s]", self._name, key
)
return make_deferred_yieldable(result)
return await make_deferred_yieldable(result)

75
tests/util/caches/test_responsecache.py → tests/util/caches/test_response_cache.py
View File

@ -11,14 +11,17 @@
# 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 parameterized import parameterized
from synapse.util.caches.response_cache import ResponseCache
from twisted.internet import defer
from synapse.util.caches.response_cache import ResponseCache, ResponseCacheContext
from tests.server import get_clock
from tests.unittest import TestCase
class DeferredCacheTestCase(TestCase):
class ResponseCacheTestCase(TestCase):
"""
A TestCase class for ResponseCache.
@ -48,7 +51,9 @@ class DeferredCacheTestCase(TestCase):
expected_result = "howdy"
wrap_d = cache.wrap(0, self.instant_return, expected_result)
wrap_d = defer.ensureDeferred(
cache.wrap(0, self.instant_return, expected_result)
)
self.assertEqual(
expected_result,
@ -66,7 +71,9 @@ class DeferredCacheTestCase(TestCase):
expected_result = "howdy"
wrap_d = cache.wrap(0, self.instant_return, expected_result)
wrap_d = defer.ensureDeferred(
cache.wrap(0, self.instant_return, expected_result)
)
self.assertEqual(
expected_result,
@ -80,7 +87,9 @@ class DeferredCacheTestCase(TestCase):
expected_result = "howdy"
wrap_d = cache.wrap(0, self.instant_return, expected_result)
wrap_d = defer.ensureDeferred(
cache.wrap(0, self.instant_return, expected_result)
)
self.assertEqual(expected_result, self.successResultOf(wrap_d))
self.assertEqual(
@ -99,7 +108,10 @@ class DeferredCacheTestCase(TestCase):
expected_result = "howdy"
wrap_d = cache.wrap(0, self.delayed_return, expected_result)
wrap_d = defer.ensureDeferred(
cache.wrap(0, self.delayed_return, expected_result)
)
self.assertNoResult(wrap_d)
# function wakes up, returns result
@ -112,7 +124,9 @@ class DeferredCacheTestCase(TestCase):
expected_result = "howdy"
wrap_d = cache.wrap(0, self.delayed_return, expected_result)
wrap_d = defer.ensureDeferred(
cache.wrap(0, self.delayed_return, expected_result)
)
self.assertNoResult(wrap_d)
# stop at 1 second to callback cache eviction callLater at that time, then another to set time at 2
@ -129,3 +143,50 @@ class DeferredCacheTestCase(TestCase):
self.reactor.pump((2,))
self.assertIsNone(cache.get(0), "cache should not have the result now")
@parameterized.expand([(True,), (False,)])
def test_cache_context_nocache(self, should_cache: bool):
"""If the callback clears the should_cache bit, the result should not be cached"""
cache = self.with_cache("medium_cache", ms=3000)
expected_result = "howdy"
call_count = 0
async def non_caching(o: str, cache_context: ResponseCacheContext[int]):
nonlocal call_count
call_count += 1
await self.clock.sleep(1)
cache_context.should_cache = should_cache
return o
wrap_d = defer.ensureDeferred(
cache.wrap(0, non_caching, expected_result, cache_context=True)
)
# there should be no result to start with
self.assertNoResult(wrap_d)
# a second call should also return a pending deferred
wrap2_d = defer.ensureDeferred(
cache.wrap(0, non_caching, expected_result, cache_context=True)
)
self.assertNoResult(wrap2_d)
# and there should have been exactly one call
self.assertEqual(call_count, 1)
# let the call complete
self.reactor.advance(1)
# both results should have completed
self.assertEqual(expected_result, self.successResultOf(wrap_d))
self.assertEqual(expected_result, self.successResultOf(wrap2_d))
if should_cache:
self.assertEqual(
expected_result,
self.successResultOf(cache.get(0)),
"cache should still have the result",
)
else:
self.assertIsNone(cache.get(0), "cache should not have the result")