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Push some deferred wrangling down into DeferredCache

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This commit is contained in:
Richard van der Hoff 2020-10-16 12:34:55 +01:00
parent 7b71695388
commit 1f4269700c
5 changed files with 67 additions and 46 deletions
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1
changelog.d/8572.misc Normal file
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@ -0,0 +1 @@
Modify `DeferredCache.get()` to return `Deferred`s instead of `ObservableDeferred`s.

57
synapse/util/caches/deferred_cache.py
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@ -57,7 +57,7 @@ class DeferredCache(Generic[KT, VT]):
"""Wraps an LruCache, adding support for Deferred results.
It expects that each entry added with set() will be a Deferred; likewise get()
may return an ObservableDeferred.
will return a Deferred.
"""
__slots__ = (
@ -130,16 +130,22 @@ class DeferredCache(Generic[KT, VT]):
key: KT,
callback: Optional[Callable[[], None]] = None,
update_metrics: bool = True,
) -> Union[ObservableDeferred, VT]:
) -> defer.Deferred:
"""Looks the key up in the caches.
For symmetry with set(), this method does *not* follow the synapse logcontext
rules: the logcontext will not be cleared on return, and the Deferred will run
its callbacks in the sentinel context. In other words: wrap the result with
make_deferred_yieldable() before `await`ing it.
Args:
key(tuple)
callback(fn): Gets called when the entry in the cache is invalidated
key:
callback: Gets called when the entry in the cache is invalidated
update_metrics (bool): whether to update the cache hit rate metrics
Returns:
Either an ObservableDeferred or the result itself
A Deferred which completes with the result. Note that this may later fail
if there is an ongoing set() operation which later completes with a failure.
Raises:
KeyError if the key is not found in the cache
@ -152,7 +158,7 @@ class DeferredCache(Generic[KT, VT]):
m = self.cache.metrics
assert m # we always have a name, so should always have metrics
m.inc_hits()
return val.deferred
return val.deferred.observe()
val2 = self.cache.get(
key, _Sentinel.sentinel, callbacks=callbacks, update_metrics=update_metrics
@ -160,7 +166,7 @@ class DeferredCache(Generic[KT, VT]):
if val2 is _Sentinel.sentinel:
raise KeyError()
else:
return val2
return defer.succeed(val2)
def get_immediate(
self, key: KT, default: T, update_metrics: bool = True
@ -173,7 +179,36 @@ class DeferredCache(Generic[KT, VT]):
key: KT,
value: defer.Deferred,
callback: Optional[Callable[[], None]] = None,
) -> ObservableDeferred:
) -> defer.Deferred:
"""Adds a new entry to the cache (or updates an existing one).
The given `value` *must* be a Deferred.
First any existing entry for the same key is invalidated. Then a new entry
is added to the cache for the given key.
Until the `value` completes, calls to `get()` for the key will also result in an
incomplete Deferred, which will ultimately complete with the same result as
`value`.
If `value` completes successfully, subsequent calls to `get()` will then return
a completed deferred with the same result. If it *fails*, the cache is
invalidated and subequent calls to `get()` will raise a KeyError.
If another call to `set()` happens before `value` completes, then (a) any
invalidation callbacks registered in the interim will be called, (b) any
`get()`s in the interim will continue to complete with the result from the
*original* `value`, (c) any future calls to `get()` will complete with the
result from the *new* `value`.
It is expected that `value` does *not* follow the synapse logcontext rules - ie,
if it is incomplete, it runs its callbacks in the sentinel context.
Args:
key: Key to be set
value: a deferred which will complete with a result to add to the cache
callback: An optional callback to be called when the entry is invalidated
"""
if not isinstance(value, defer.Deferred):
raise TypeError("not a Deferred")
@ -187,6 +222,8 @@ class DeferredCache(Generic[KT, VT]):
if existing_entry:
existing_entry.invalidate()
# XXX: why don't we invalidate the entry in `self.cache` yet?
self._pending_deferred_cache[key] = entry
def compare_and_pop():
@ -230,7 +267,9 @@ class DeferredCache(Generic[KT, VT]):
# _pending_deferred_cache to the real cache.
#
observer.addCallbacks(cb, eb)
return observable
# we return a new Deferred which will be called before any subsequent observers.
return observable.observe()
def prefill(self, key: KT, value: VT, callback: Callable[[], None] = None):
callbacks = [callback] if callback else []

32
synapse/util/caches/descriptors.py
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@ -23,7 +23,6 @@ from twisted.internet import defer
from synapse.logging.context import make_deferred_yieldable, preserve_fn
from synapse.util import unwrapFirstError
from synapse.util.async_helpers import ObservableDeferred
from synapse.util.caches.deferred_cache import DeferredCache
logger = logging.getLogger(__name__)
@ -156,7 +155,7 @@ class CacheDescriptor(_CacheDescriptorBase):
keylen=self.num_args,
tree=self.tree,
iterable=self.iterable,
) # type: DeferredCache[Tuple, Any]
) # type: DeferredCache[CacheKey, Any]
def get_cache_key_gen(args, kwargs):
"""Given some args/kwargs return a generator that resolves into
@ -208,26 +207,12 @@ class CacheDescriptor(_CacheDescriptorBase):
kwargs["cache_context"] = _CacheContext.get_instance(cache, cache_key)
try:
cached_result_d = cache.get(cache_key, callback=invalidate_callback)
if isinstance(cached_result_d, ObservableDeferred):
observer = cached_result_d.observe()
else:
observer = defer.succeed(cached_result_d)
ret = cache.get(cache_key, callback=invalidate_callback)
except KeyError:
ret = defer.maybeDeferred(preserve_fn(self.orig), obj, *args, **kwargs)
ret = cache.set(cache_key, ret, callback=invalidate_callback)
def onErr(f):
cache.invalidate(cache_key)
return f
ret.addErrback(onErr)
result_d = cache.set(cache_key, ret, callback=invalidate_callback)
observer = result_d.observe()
return make_deferred_yieldable(observer)
return make_deferred_yieldable(ret)
wrapped = cast(_CachedFunction, _wrapped)
@ -286,7 +271,7 @@ class CacheListDescriptor(_CacheDescriptorBase):
def __get__(self, obj, objtype=None):
cached_method = getattr(obj, self.cached_method_name)
cache = cached_method.cache
cache = cached_method.cache # type: DeferredCache[CacheKey, Any]
num_args = cached_method.num_args
@functools.wraps(self.orig)
@ -326,14 +311,11 @@ class CacheListDescriptor(_CacheDescriptorBase):
for arg in list_args:
try:
res = cache.get(arg_to_cache_key(arg), callback=invalidate_callback)
if not isinstance(res, ObservableDeferred):
results[arg] = res
elif not res.has_succeeded():
res = res.observe()
if not res.called:
res.addCallback(update_results_dict, arg)
cached_defers.append(res)
else:
results[arg] = res.get_result()
results[arg] = res.result
except KeyError:
missing.add(arg)

18
tests/util/caches/test_deferred_cache.py
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@ -13,15 +13,16 @@
# See the License for the specific language governing permissions and
# limitations under the License.
import unittest
from functools import partial
from twisted.internet import defer
from synapse.util.caches.deferred_cache import DeferredCache
from tests.unittest import TestCase
class DeferredCacheTestCase(unittest.TestCase):
class DeferredCacheTestCase(TestCase):
def test_empty(self):
cache = DeferredCache("test")
failed = False
@ -36,7 +37,7 @@ class DeferredCacheTestCase(unittest.TestCase):
cache = DeferredCache("test")
cache.prefill("foo", 123)
self.assertEquals(cache.get("foo"), 123)
self.assertEquals(self.successResultOf(cache.get("foo")), 123)
def test_get_immediate(self):
cache = DeferredCache("test")
@ -82,16 +83,15 @@ class DeferredCacheTestCase(unittest.TestCase):
d2 = defer.Deferred()
cache.set("key2", d2, partial(record_callback, 1))
# lookup should return observable deferreds
self.assertFalse(cache.get("key1").has_called())
self.assertFalse(cache.get("key2").has_called())
# lookup should return pending deferreds
self.assertFalse(cache.get("key1").called)
self.assertFalse(cache.get("key2").called)
# let one of the lookups complete
d2.callback("result2")
# for now at least, the cache will return real results rather than an
# observabledeferred
self.assertEqual(cache.get("key2"), "result2")
# now the cache will return a completed deferred
self.assertEqual(self.successResultOf(cache.get("key2")), "result2")
# now do the invalidation
cache.invalidate_all()

5
tests/util/caches/test_descriptors.py
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@ -27,7 +27,6 @@ from synapse.logging.context import (
current_context,
make_deferred_yieldable,
)
from synapse.util.async_helpers import ObservableDeferred
from synapse.util.caches import descriptors
from synapse.util.caches.descriptors import cached
@ -419,9 +418,9 @@ class CacheDecoratorTestCase(unittest.HomeserverTestCase):
a = A()
a.func.prefill(("foo",), ObservableDeferred(d))
a.func.prefill(("foo",), 456)
self.assertEquals(a.func("foo").result, d.result)
self.assertEquals(a.func("foo").result, 456)
self.assertEquals(callcount[0], 0)
@defer.inlineCallbacks