cache: Do not fail the request on cache-set failures.

Failure to update the cache should log an exception but continue.

zerver/lib/cache.py

@@ -12,6 +12,7 @@ from functools import _lru_cache_wrapper, lru_cache, wraps
 from itertools import islice, product
 from typing import TYPE_CHECKING, Any, Generic, TypeVar
 
+from bmemcached.exceptions import MemcachedException
 from django.conf import settings
 from django.core.cache import caches
 from django.core.cache.backends.base import BaseCache
@@ -226,7 +227,10 @@ def cache_set(
 
     remote_cache_stats_start()
     cache_backend = get_cache_backend(cache_name)
-    cache_backend.set(final_key, (val,), timeout=timeout)
+    try:
+        cache_backend.set(final_key, (val,), timeout=timeout)
+    except MemcachedException as e:
+        logger.exception(e)
     remote_cache_stats_finish()
 
 
@@ -278,7 +282,10 @@ def cache_set_many(
         new_items[new_key] = item
     items = new_items
     remote_cache_stats_start()
-    get_cache_backend(cache_name).set_many(items, timeout=timeout)
+    try:
+        get_cache_backend(cache_name).set_many(items, timeout=timeout)
+    except MemcachedException as e:
+        logger.exception(e)
     remote_cache_stats_finish()
 
 

zerver/tests/test_cache.py

@@ -1,5 +1,6 @@
 from unittest.mock import Mock, patch
 
+from bmemcached.exceptions import MemcachedException
 from django.conf import settings
 
 from zerver.apps import flush_cache
@@ -158,6 +159,36 @@ class CacheWithKeyDecoratorTest(ZulipTestCase):
         self.assertEqual(result_two, None)
 
 
+class SetCacheExceptionTest(ZulipTestCase):
+    def test_set_cache_exception(self) -> None:
+        with (
+            patch("zerver.lib.cache.get_cache_backend") as mock_backend,
+            self.assertLogs("", level="INFO") as logs,
+        ):
+            mock_backend.return_value.set.side_effect = MemcachedException(
+                b"Out of memory during read", 130
+            )
+            cache_set("test-key", 1)
+            mock_backend.assert_called_once()
+            mock_backend.return_value.set.assert_called_once()
+            self.assert_length(logs.output, 1)
+            self.assertIn("Out of memory during read", logs.output[0])
+
+    def test_set_many_cache_exception(self) -> None:
+        with (
+            patch("zerver.lib.cache.get_cache_backend") as mock_backend,
+            self.assertLogs("", level="INFO") as logs,
+        ):
+            mock_backend.return_value.set_many.side_effect = MemcachedException(
+                b"Out of memory during read", 130
+            )
+            cache_set_many({"test-key": 1, "other-key": 2})
+            mock_backend.assert_called_once()
+            mock_backend.return_value.set_many.assert_called_once()
+            self.assert_length(logs.output, 1)
+            self.assertIn("Out of memory during read", logs.output[0])
+
+
 class SafeCacheFunctionsTest(ZulipTestCase):
     def test_safe_cache_functions_with_all_good_keys(self) -> None:
         items = {