Disconnect the Redis socket on fatal shutdown

[kovshenin]

A fatal error during socket read/write (such as a memory exhausted error) can result in a corrupted persistent socket for future PHP requests. This change explicitly disconnects the Redis socket on shutdown that happened with a fatal error.

The problem can be reproduced by enabling persistent connections, writing a large value to Redis:

$buffer = '';
for ( $i = 0; $i <= 1024 * 1024 * 32; $i++ ) {
	$buffer .= chr( rand( 65, 90 ) );
}
wp_cache_set( 'foo', $buffer );

Then attempting to read that value with a low enough memory limit:

ini_set( 'memory_limit', '4M' );
wp_cache_get_multiple( [ 'foo', 'bar', 'baz' ] );

This should cause a memory exhausted fatal error in predis/src/Connection/StreamConnection.php exiting the script but leaving the persistent socket in a connected state, which in subsequent requests leads to poisoning/corruption.

[mikelittle]

Looks good to me

[owaincuvelier]

letsagoooo