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ResponseCache: fix handling of completed results

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Turns out that ObservableDeferred.observe doesn't return a deferred if the
result is already completed. Fix handling and improve documentation.
This commit is contained in:
Richard van der Hoff 2018-04-13 07:32:29 +01:00
parent b78395b7fe
commit 60f6014bb7
1 changed files with 19 additions and 13 deletions
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32
synapse/util/caches/response_cache.py
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@ -14,6 +14,8 @@
# limitations under the License.
import logging
from twisted.internet import defer
from synapse.util.async import ObservableDeferred
from synapse.util.caches import metrics as cache_metrics
from synapse.util.logcontext import make_deferred_yieldable, run_in_background
@ -48,15 +50,21 @@ class ResponseCache(object):
def get(self, key):
"""Look up the given key.
Returns a deferred which doesn't follow the synapse logcontext rules,
so you'll probably want to make_deferred_yieldable it.
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.
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.
Args:
key (hashable):
Returns:
twisted.internet.defer.Deferred|None: None if there is no entry
for this key; otherwise a deferred result.
twisted.internet.defer.Deferred|None|E: None if there is no entry
for this key; otherwise either a deferred result or the result
itself.
"""
result = self.pending_result_cache.get(key)
if result is not None:
@ -73,19 +81,17 @@ class ResponseCache(object):
you should wrap normal synapse deferreds with
logcontext.run_in_background).
Returns a new Deferred which also doesn't follow the synapse logcontext
rules, so you will want to make_deferred_yieldable it
(TODO: before using this more widely, it might make sense to refactor
it and get() so that they do the necessary wrapping rather than having
to do it everywhere ResponseCache is used.)
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.
Args:
key (hashable):
deferred (twisted.internet.defer.Deferred):
deferred (twisted.internet.defer.Deferred[T):
Returns:
twisted.internet.defer.Deferred
twisted.internet.defer.Deferred[T]|T: a new deferred, or the actual
result.
"""
result = ObservableDeferred(deferred, consumeErrors=True)
self.pending_result_cache[key] = result
@ -144,7 +150,7 @@ class ResponseCache(object):
self._name, key)
d = run_in_background(callback, *args, **kwargs)
result = self.set(key, d)
elif result.called:
elif not isinstance(result, defer.Deferred) or result.called:
logger.info("[%s]: using completed cached result for [%s]",
self._name, key)
else: