[vbox-dev] Fix to #13801

[a.urakov at drweb.com]

Hello!

We would like to contribute to VirtualBox under MIT license. We send our 
changes to fix bug described at https://www.virtualbox.org/ticket/13801 .

The idea is that there is no need to hold write lock on parent medium to 
change its locking type. In current version we unlock all session locked 
list first and only then we update locking type. But in this case only 
element of /pMediumLockList/ is changed, not /Medium/ object. May be 
parent write-locking code left after old version, where parent lock type 
updating looked like this (version /3.1.8/):

ComObjPtr<Medium> parent = pMedium->getParent();
/* make the relock atomic */
AutoWriteLock parentLock (parent);
rc = parent->UnlockWrite(NULL);
AssertComRC(rc);
rc = parent->LockRead(NULL);
AssertComRC(rc);

Regards,
Alexander

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