Memcache fails its own tests.

That's a known issue, I believe it may not be possible to fix it without some kind of variation on #891600: Page and block caches are cleared the same way on cron as on content change in core. In Drupal 6 cache_clear_all() was replacable by cache backends, in D7 it's hard coded in cache.inc and the cache backends themselves can't do much about it.

Might be possible to do a debug_backtrace() from ->clear(), then detect it was called via cache_clear_all() with no arguments vs. the other cases. I didn't have any non-nasty ideas for fixing that in memcache itself.

The test failure means that when content is posted, content won't be flushed from the cache - this mainly affects the page cache.

The large D7 site I worked on (not currently) that uses memcache actively avoided cache_clear_all() from node and comment submits because it clears the cache too often on busy sites. Other sites may use varnish for the page cache, and would then not notice this either.

We could comment the tests out until the core bug is fixed, it'd be nicer to get expectedFail() into core to document this stuff better.

http://drupal.org/node/891600#comment-4502492 still describes what needs to happen here, I may work on a patch for that, however I've been hoping that #81461: Clean up the cache API (stop overloading function arguments, remove procedural wrappers) would go in soon (which contains a garbage collection method) then we could backport the relevant bits. Reviews of that patch would be great.

It won't be a straight backport. But we'd add a ->garbageCollection() method to memcache.inc, that just returns FALSE, then modify ->clear() to clear the cache when it gets NULL, $bin as arguments (currently that particular combination is ignored).

Yes and yes.

However if you were just to make that change now, without the core change, then every single cron run will flush the page cache, which is arguably a worse bug.

#81461 can't be backported wholesale from D8 to D7 because it's too large of an API change. We might be able to backport to http://drupal.org/project/cache_backport but that's a different thing.

I'd like to backport just the garbageCollection() method (and maybe one or two other bits) in core so that it's possible to differentiate between garbage collection and flushing of temporary items, but that depends on the patch getting into first before it can be applied to memcache.

With #1015946: Eliminate $user->cache and {session}.cache in favor of $_SESSION['cache_expiration'][$bin] would be happy just moving to $_SESSION['cache_something'] directly instead of $user->cache regardless of what core does there, but we should fix it for all bins as per #1079518: When one cache table is cleared, cached items from other tables invalidated.