From 041b6cba612f5640fe490859a54f0ef140e29d33 Mon Sep 17 00:00:00 2001
From: Mark Haines <mark.haines@matrix.org>
Date: Tue, 5 May 2015 16:32:44 +0100
Subject: [PATCH] SYN-369: Add comments to the sequence number logic in the
cache
---
synapse/storage/_base.py | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 840a4994bb..579ed56377 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -98,6 +98,8 @@ class Cache(object):
def update(self, sequence, *args):
self.check_thread()
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)
def prefill(self, *args): # because I can't *keyargs, value
@@ -117,6 +119,8 @@ class Cache(object):
self.check_thread()
if len(keyargs) != 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.cache.pop(keyargs, None)
@@ -159,6 +163,9 @@ def cached(max_entries=1000, num_args=1, lru=False):
raise ValueError("Stale cache entry")
defer.returnValue(cached_result)
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
ret = yield orig(self, *keyargs)
--
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