Use a per-deployment memcached key prefix

Some cache keys used by Django (like sessions) will not have the key
prefixes, but those values shouldn't change across most restarts.

(imported from commit 2fe61028111fe9d5700432214a611b3341412654)

zephyr/lib/cache.py

@@ -4,9 +4,16 @@ from functools import wraps
 
 from django.core.cache import cache as djcache
 from django.core.cache import get_cache
+from django.conf import settings
 
 from zephyr.lib.utils import statsd, statsd_key, make_safe_digest
 import time
+import base64
+import random
+import sys
+import os
+import os.path
+import hashlib
 
 memcached_time_start = 0
 memcached_total_time = 0
@@ -29,6 +36,33 @@ def memcached_stats_finish():
     memcached_total_requests += 1
     memcached_total_time += (time.time() - memcached_time_start)
 
+def get_or_create_key_prefix():
+    filename = os.path.join(settings.SITE_ROOT, "..", "memcached_prefix")
+    try:
+        fd = os.open(filename, os.O_CREAT | os.O_EXCL | os.O_RDWR, 0444)
+        prefix = base64.b16encode(hashlib.sha256(str(random.getrandbits(256))).digest())[:32].lower() + ':'
+        # This does close the underlying file
+        with os.fdopen(fd, 'w') as f:
+            f.write(prefix + "\n")
+    except OSError:
+        # The file already exists
+        tries = 1
+        while tries < 10:
+            with file(filename, 'r') as f:
+                prefix = f.readline()[:-1]
+            if len(prefix) == 33:
+                break
+            tries += 1
+            prefix = ''
+            time.sleep(0.5)
+
+    if not prefix:
+        sys.exit("Could not read memcache key prefix file")
+
+    return prefix
+
+KEY_PREFIX = get_or_create_key_prefix()
+
 def cache_with_key(keyfunc, cache_name=None, timeout=None, with_statsd_key=None):
     """Decorator which applies Django caching to a function.
 
@@ -40,7 +74,7 @@ def cache_with_key(keyfunc, cache_name=None, timeout=None, with_statsd_key=None)
     def decorator(func):
         @wraps(func)
         def func_with_caching(*args, **kwargs):
-            key = keyfunc(*args, **kwargs)
+            key = KEY_PREFIX + keyfunc(*args, **kwargs)
 
             memcached_stats_start()
             if cache_name is None:
@@ -91,6 +125,7 @@ def cache_set(key, val, cache_name=None, timeout=None):
     return ret
 
 def cache_get_many(keys, cache_name=None):
+    keys = [KEY_PREFIX + key for key in keys]
     memcached_stats_start()
     if cache_name is None:
         cache_backend = djcache
@@ -101,6 +136,10 @@ def cache_get_many(keys, cache_name=None):
     return ret
 
 def cache_set_many(items, cache_name=None, timeout=None):
+    new_items = {}
+    for key in items:
+        new_items[KEY_PREFIX + key] = items[key]
+    items = new_items
     memcached_stats_start()
     if cache_name is None:
         cache_backend = djcache
@@ -152,4 +191,4 @@ def status_dict_cache_key(user_profile):
 
 def update_user_presence_cache(sender, **kwargs):
     user_profile = kwargs['instance'].user_profile
-    djcache.delete(status_dict_cache_key(user_profile))
+    djcache.delete(KEY_PREFIX + status_dict_cache_key(user_profile))