Improve Non-Persistent Group Handling in WP_Object_Cache (Sentinel Value)

[mreishus]

Improve Non-Persistent Group Handling in WP_Object_Cache

This PR addresses a critical bug in the handling of non-persistent cache groups, particularly affecting the 'themes' group. The issue prevented the add() method from working correctly after a failed get() operation on a non-existent key.

Key Changes:

1. Introduced a NOT_SET_SENTINEL constant to distinguish between truly non-existent keys and keys that have been checked but not found.
2. Modified add(), replace(), get(), and delete() methods to properly handle the NOT_SET_SENTINEL for non-persistent groups.
3. Test coverage added to ensure correct behavior in various scenarios.

Bug Fix:

Previously, when attempting to get() a non-existent key in a non-persistent group (like 'themes'), the cache would mark this key as existing but empty. This prevented subsequent add() operations from succeeding, as the key was incorrectly considered to already exist.

The new implementation allows add() to succeed after a failed get(), which is the expected behavior for a cache system.

Benefits:

• Improves reliability of theme data caching and retrieval.
• Enhances overall performance by reducing unnecessary operations on non-existent keys.
• Provides a more consistent and predictable behavior for non-persistent cache groups.

Testing:

Tests have been added to cover individual operations, multiple operations, edge cases, and the specific scenario that triggered the original bug.

This change is crucial for ensuring correct behavior in WordPress installations using non-persistent cache groups, particularly for theme-related operations.

To test, run bin/test.sh.

[mreishus]

Alternate implementation w/o a sentinel value is here: #9. But it has the disadvantage of wp_cache_delete('key_in_nonpersistent_group') acting differently than than unset( $this->cache['key_in_nonpersistent_group'] );.