Sometimes repairing a broken hive with a hive log does not always guarantee the hive
in question has fully recovered. In worst cases it could happen the LOG itself is even
corrupt too and that would certainly lead to a total unbootable system. This is most likely
if the victim hive is the SYSTEM hive.
This can be anyhow solved by the help of a mirror hive, or also called an "alternate hive".
Alternate hives serve the purpose as backup hives for primary hives of which there is still
a risk that is not worth taking. For now only the SYSTEM hive is granted the right to have
a backup alternate hive.
=== NOTE ===
Currently the SYSTEM hive can only base upon the alternate SYSTEM.ALT hive, which means the
corresponding LOG file never gets updated. When time comes the existing code must be adapted
to allow the possibility to use .ALT and .LOG hives simultaneously.
In addition to that, in some functions like CmFlushKey, CmSaveKey and CmSaveMergedKeys we must validate the underlying hives as a matter of precaution that everything is alright and we don't fuck all the shit up.
Currently the failure code path doesn't do any kind of cleanup against
the hive that was being linked to master. The cleanup is pretty
straightforward as you just simply close the hive file handles and free
the registry kernel structures.
CORE-5772
CORE-17263
CORE-13559
