Comment # 1 on bug 899755 from Michael Andres

It's actually not zyppers fault, but libzypps. The mutex ZyppJob locks and
unlocks is PK internal and not visible to zypper.

But the ZYpp instance created by ZyppJob::get_zypp 

> zypp = ZYppFactory::instance ().getZYpp ();

is the one that keeps the inter process lock, which prevents zypper from
starting. This instance however tends to stay in memory, even if all ZYpp::Ptr
references were dropped. This is what needs to be fixed otherwise only
terminating packagekitd (unloading libzypp) is the only way to release the
lock.