fix: limit memcache keys to 250 bytes (#663)

Fixes #619

Author: Chris Rossi

packages/google-cloud-ndb/google/cloud/ndb/global_cache.py

@@ -17,6 +17,7 @@
 import abc
 import base64
 import collections
+import hashlib
 import os
 import pymemcache.exceptions
 import redis.exceptions
@@ -470,7 +471,10 @@ def _parse_host_string(host_string):
 
     @staticmethod
     def _key(key):
-        return base64.b64encode(key)
+        encoded = base64.b64encode(key)
+        if len(encoded) > 250:
+            encoded = hashlib.sha1(encoded).hexdigest()
+        return encoded
 
     @classmethod
     def from_environment(cls, max_pool_size=4, strict_read=False, strict_write=True):

packages/google-cloud-ndb/tests/unit/test_global_cache.py

@@ -354,6 +354,12 @@ def test_clear():
 
 
 class TestMemcacheCache:
+    @staticmethod
+    def test__key_long_key():
+        key = b"ou812" * 100
+        encoded = global_cache.MemcacheCache._key(key)
+        assert len(encoded) == 40  # sha1 hashes are 40 bytes
+
     @staticmethod
     @mock.patch("google.cloud.ndb.global_cache.pymemcache")
     def test_from_environment_not_configured(pymemcache):