[SharedCache] Split view-specific state into a separate struct

The existing view-specific state was stored in several global unordered
maps. Many of these were accessed without locking, including
`viewSpecificMutexes`, which is racy in the face of multiple threads.

View-specific state is stored in a new heap-allocated
`ViewSpecificState` struct that is reference counted via
`std::shared_ptr`. A static map holds a `std::weak_ptr` to each
view-specific state, keyed by session id. `SharedCache` retrieves its
view-specific state during its constructor.

Since `ViewSpecificState` is reference counted it will naturally be
deallocated when the last `SharedCache` instance that references it goes
away. Its corresponding entry will remain in the static map, though
since it only holds a `std::weak_ptr` rather than any state it will not
use much memory. The next time view-specific state is retrieved any
expired entries will be removed from the map.

0 files changed, 0 insertions, 0 deletions