virobject: Introduce virObjectRWLockable
Up until now we only had virObjectLockable which uses mutexes for mutually excluding each other in critical section. Well, this is not enough. Future work will require RW locks so we might as well have virObjectRWLockable which is introduced here. Moreover, polymorphism is introduced to our code for the first time. Yay! More specifically, virObjectLock will grab a write lock, virObjectLockRead will grab a read lock then (what a surprise right?). This has great advantage that an object can be made derived from virObjectRWLockable in a single line and still continue functioning properly (mutexes can be viewed as grabbing write locks only). Then just those critical sections that can grab a read lock need fixing. Therefore the resulting change is going to be way smaller. In order to avoid writer starvation, the object initializes RW lock that prefers writers. Signed-off-by: NMichal Privoznik <mprivozn@redhat.com> Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
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