Race condition for transient with no expiration and object cache active

[hugod]

When object cache is installed and is temporarily unavailable, transient values with no expiration can be obsolete.

Here is a scenario:

• No object cache active, do set_transient( 'foo', 'bar' );
• Now activate object cache
• Do delete_transient( 'foo' );, foo is deleted from the cache but not from wp_options table ([see]​https://github.com/WordPress/wordpress-develop/blob/24dde230694b33c43c3b5e9ce87320f19d08da96/src/wp-includes/option.php#L1394)
• For some reason, the transient value must change, do set_transient( 'foo', 'baz' );. The new value is correct in cache, but not in wp_options table ([see]​https://github.com/WordPress/wordpress-develop/blob/24dde230694b33c43c3b5e9ce87320f19d08da96/src/wp-includes/option.php#L1542).
• Now let's say the cache is temporarily unavailable (update, malfunction, you name it). When doing get_transient( 'foo' ); we get 'bar' instead of 'baz' (we expect the last value set).

If some major feature depends on this transient (e.g. rewrite rules), it can lead to weird behaviors that are hard to debug.
For the record, we encountered such a bug in Polylang ([see]​https://github.com/polylang/polylang/pull/1653).

As per the title, force deletion or creation of the transient in wp_options table when object cache is active.
Preventing race condition between object cache and wp_options table for a transient without expiration.

Trac ticket: https://core.trac.wordpress.org/ticket/63450

[audrasjb]

Thanks for the report and patch, moving this for 6.9 consideration.