diff --git a/synapse/config/cache.py b/synapse/config/cache.py
index 0672538796..aff5b21ab2 100644
--- a/synapse/config/cache.py
+++ b/synapse/config/cache.py
@@ -15,6 +15,7 @@
import os
import re
+import threading
from typing import Callable, Dict
from ._base import Config, ConfigError
@@ -25,6 +26,9 @@ _CACHE_PREFIX = "SYNAPSE_CACHE_FACTOR"
# Map from canonicalised cache name to cache.
_CACHES = {}
+# a lock on the contents of _CACHES
+_CACHES_LOCK = threading.Lock()
+
_DEFAULT_FACTOR_SIZE = 0.5
_DEFAULT_EVENT_CACHE_SIZE = "10K"
@@ -66,7 +70,10 @@ def add_resizable_cache(cache_name: str, cache_resize_callback: Callable):
# Some caches have '*' in them which we strip out.
cache_name = _canonicalise_cache_name(cache_name)
- _CACHES[cache_name] = cache_resize_callback
+ # sometimes caches are initialised from background threads, so we need to make
+ # sure we don't conflict with another thread running a resize operation
+ with _CACHES_LOCK:
+ _CACHES[cache_name] = cache_resize_callback
# Ensure all loaded caches are sized appropriately
#
@@ -87,7 +94,8 @@ class CacheConfig(Config):
os.environ.get(_CACHE_PREFIX, _DEFAULT_FACTOR_SIZE)
)
properties.resize_all_caches_func = None
- _CACHES.clear()
+ with _CACHES_LOCK:
+ _CACHES.clear()
def generate_config_section(self, **kwargs):
return """\
@@ -193,6 +201,8 @@ class CacheConfig(Config):
For each cache, run the mapped callback function with either
a specific cache factor or the default, global one.
"""
- for cache_name, callback in _CACHES.items():
- new_factor = self.cache_factors.get(cache_name, self.global_factor)
- callback(new_factor)
+ # block other threads from modifying _CACHES while we iterate it.
+ with _CACHES_LOCK:
+ for cache_name, callback in _CACHES.items():
+ new_factor = self.cache_factors.get(cache_name, self.global_factor)
+ callback(new_factor)
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