Redis cache store: avoid blocking the server in `#delete_matched`

Fixes #32610. Closes #32614.

Lua scripts in redis are *blocking*, meaning that no other client can
execute any commands while the script is running. See
https://redis.io/commands/eval#atomicity-of-scripts.

This results in the following exceptions once the number of keys is
sufficiently large:

    BUSY Redis is busy running a script.
    You can only call SCRIPT KILL or SHUTDOWN NOSAVE.

This commit replaces the lua-based implementation with one that uses
`SCAN` and `DEL` in batches. This doesn't block the server.

The primary limitation of `SCAN`, i.e. potential duplicate keys, is of
no consequence here, because `DEL` ignores keys that do not exist.

2 files changed, 18 insertions, 6 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 82c985fae2..483eb12ce1 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Redis cache store: `delete_matched` no longer blocks the Redis server.
+    (Switches from evaled Lua to a batched SCAN + DEL loop.)
+
+    *Gleb Mazovetskiy*
+
 *   Fix bug where `ActiveSupport::Cache` will massively inflate the storage
     size when compression is enabled (which is true by default). This patch
     does not attempt to repair existing data: please manually flush the cache
diff --git a/activesupport/lib/active_support/cache/redis_cache_store.rb b/activesupport/lib/active_support/cache/redis_cache_store.rb
index 74f935e02e..11c574258f 100644
--- a/activesupport/lib/active_support/cache/redis_cache_store.rb
+++ b/activesupport/lib/active_support/cache/redis_cache_store.rb
@@ -62,8 +62,9 @@ module ActiveSupport
         end
       end
 
-      DELETE_GLOB_LUA = "for i, name in ipairs(redis.call('KEYS', ARGV[1])) do redis.call('DEL', name); end"
-      private_constant :DELETE_GLOB_LUA
+      # The maximum number of entries to receive per SCAN call.
+      SCAN_BATCH_SIZE = 1000
+      private_constant :SCAN_BATCH_SIZE
 
       # Support raw values in the local cache strategy.
       module LocalCacheWithRaw # :nodoc:
@@ -231,12 +232,18 @@ module ActiveSupport
       # Failsafe: Raises errors.
       def delete_matched(matcher, options = nil)
         instrument :delete_matched, matcher do
-          case matcher
-          when String
-            redis.with { |c| c.eval DELETE_GLOB_LUA, [], [namespace_key(matcher, options)] }
-          else
+          unless String === matcher
             raise ArgumentError, "Only Redis glob strings are supported: #{matcher.inspect}"
           end
+          redis.with do |c|
+            pattern = namespace_key(matcher, options)
+            cursor = "0"
+            # Fetch keys in batches using SCAN to avoid blocking the Redis server.
+            begin
+              cursor, keys = c.scan(cursor, match: pattern, count: SCAN_BATCH_SIZE)
+              c.del(*keys) unless keys.empty?
+            end until cursor == "0"
+          end
         end
       end