memcache: truncate keys at 250 chars

max memcache key len:
https://github.com/memcached/memcached/wiki/Commands#standard-protocol
https://pymemcache.readthedocs.io/en/latest/apidoc/pymemcache.client.base.html

fixes https://console.cloud.google.com/errors/detail/CNPzqYzWg4_TyQE;time=P30D?project=bridgy-federated

activitypub.py

@@ -1034,7 +1034,7 @@ def inbox(protocol=None, id=None):
             logger.info(f'Discarding, {id} is on an opted out domain')
             return '', 204
 
-        memcache_key = f'AP-id-{id}'
+        memcache_key = common.memcache_key(f'AP-id-{id}')
         if memcache.get(memcache_key):
             logger.info(f'Already seen this activity {id}')
             return '', 204

common.py

@@ -91,6 +91,9 @@ RUN_TASKS_INLINE = False  # overridden by unit tests
 # for Protocol.REQUIRES_OLD_ACCOUNT, how old is old enough
 OLD_ACCOUNT_AGE = timedelta(days=14)
 
+# https://github.com/memcached/memcached/wiki/Commands#standard-protocol
+MEMCACHE_KEY_MAX_LEN = 250
+
 if appengine_info.DEBUG or appengine_info.LOCAL_SERVER:
     logger.info('Using in memory mock memcache')
     memcache = MockMemcacheClient()
@@ -98,7 +101,8 @@ if appengine_info.DEBUG or appengine_info.LOCAL_SERVER:
 else:
     logger.info('Using production Memorystore memcache')
     memcache = pymemcache.client.base.PooledClient(
-        '10.126.144.3', timeout=10, connect_timeout=10)  # seconds
+        '10.126.144.3', timeout=10, connect_timeout=10,  # seconds
+        allow_unicode_keys=True)
     global_cache = MemcacheCache(memcache)
 
 
@@ -397,3 +401,15 @@ def global_cache_timeout_policy(key):
         return int(timedelta(hours=2).total_seconds())
 
     return int(timedelta(minutes=30).total_seconds())
+
+
+def memcache_key(key):
+    """Preprocesses a memcache key. Right now just truncates it to 250 chars.
+
+    https://pymemcache.readthedocs.io/en/latest/apidoc/pymemcache.client.base.html
+    https://github.com/memcached/memcached/wiki/Commands#standard-protocol
+
+    TODO: truncate to 250 *UTF-8* chars, to handle Unicode chars in URLs. Related:
+    pymemcache Client's allow_unicode_keys constructor kwarg.
+    """
+    return key[:MEMCACHE_KEY_MAX_LEN]

protocol.py

@@ -770,7 +770,7 @@ class Protocol:
             error(f'Activity {id} is blocklisted')
 
         # lease this object atomically
-        lease_memcache_key = f'receive-{id}'
+        lease_memcache_key = common.memcache_key(f'receive-{id}')
         if not common.memcache.add(lease_memcache_key, 'leased',
                                    noreply=False, expire=5 * 60):  # 5 min
             error('This object is already being received elsewhere', status=204)