Clear multiple prefixes at once

Patch isn't attached ;)

I think this is a pretty good idea - we've just hit on a situation where we need the same sort of functionality.

Here's our patch, targeted at d7.

It's probably nit-picky, but I'd suggest something that doesn't use the empty() checks ... maybe something like the following would ultimately be more performant?

if ($prefix == '') {
  $prefix = variable_get('memcache_key_prefix', '');
}
if ($prefix != '') {
  $prefix .= '-';
}

After a bit of back and forth locally, here's an updated patch:

And apparently I failed to hit save after making my changes...

And apparently I am an idiot today.

I like this approach because it explicitly sets the prefix passed in to NULL so that it can't be mistaken for a legitimate cache prefix and should perform marginally better.

The patch is for d7 so bumping to there to solve confusion.

if ($prefix == NULL) { should be if ($prefix === NULL) {

Updated the patch to reflect cyberswat's not in comment 12:

Sorry for the slow review.

I've rewritten the logic to use the existing prefix variable for maximum flexibility.

The feature is currently documented as follows:

It is also possible to specify multiple prefixes per bin. Only the first prefix will be used when setting/getting cache items, but all prefixes will be cleared when deleting cache items. This provides support for more complicated configurations such as a live instance and an administrative instance each with their own prefixes and therefore their own unique caches. Any time a cache item is deleted on either instance, it gets flushed on both -- thus, should an admin do something that flushes the page cache, it will appropriately get flushed on both instances. (For more discussion see the issue where support was added, https://www.drupal.org/node/1084448.) This feature is enabled when you configure prefixes as arrays within arrays. For example:

  // Live instance.
  $conf['memcache_key_prefix'] = array(
    'default' => array(
      'live_unique', // live cache prefix
      'admin_unique', // admin cache prefix
    ),
  );

The above would be the configuration of your live instance. Then, on your administrative instance you would flip the keys:

  // Administrative instance.
  $conf['memcache_key_prefix'] = array(
    'default' => array(
      'admin_unique', // admin cache prefix
      'live_unique', // live cache prefix
    ),
  );