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SYN-369: Add comments to the sequence number logic in the cache

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Mark Haines 2015-05-05 16:32:44 +01:00
parent 63075118a5
commit 041b6cba61
1 changed files with 7 additions and 0 deletions
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7
synapse/storage/_base.py
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@ -98,6 +98,8 @@ class Cache(object):
def update(self, sequence, *args): def update(self, sequence, *args):
self.check_thread() self.check_thread()
if self.sequence == sequence: if self.sequence == sequence:
# Only update the cache if the caches sequence number matches the
# number that the cache had before the SELECT was started (SYN-369)
self.prefill(*args) self.prefill(*args)
def prefill(self, *args): # because I can't *keyargs, value def prefill(self, *args): # because I can't *keyargs, value
@ -117,6 +119,8 @@ class Cache(object):
self.check_thread() self.check_thread()
if len(keyargs) != self.keylen: if len(keyargs) != self.keylen:
raise ValueError("Expected a key to have %d items", self.keylen) raise ValueError("Expected a key to have %d items", self.keylen)
# Increment the sequence number so that any SELECT statements that
# raced with the INSERT don't update the cache (SYN-369)
self.sequence += 1 self.sequence += 1
self.cache.pop(keyargs, None) self.cache.pop(keyargs, None)
@ -159,6 +163,9 @@ def cached(max_entries=1000, num_args=1, lru=False):
raise ValueError("Stale cache entry") raise ValueError("Stale cache entry")
defer.returnValue(cached_result) defer.returnValue(cached_result)
except KeyError: except KeyError:
# Get the sequence number of the cache before reading from the
# database so that we can tell if the cache is invalidated
# while the SELECT is executing (SYN-369)
sequence = cache.sequence sequence = cache.sequence
ret = yield orig(self, *keyargs) ret = yield orig(self, *keyargs)