Properly count subregion reference destruction towards root region unmapping

[manopapad]

Without this change we could miss the unmapping of an inline-mapped RegionField, even after we recycle it for a different Store. This would cause a conflict if we then try to inline map the second Store, e.g. in the following example:

for _ in range(8):
    arr = lg.ones((5,5,5))[1:2,1:2,1:2]
    arr._thunk.__numpy_array__()

which would produce the following logging output:

attaching Region(tid: 1, is: 13, fs: 1, fid: 1048577) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048577) :: got to root, attaching
reducing inline ref count for RegionField(tid: 1, is: 13, fs: 1, fid: 1048577), count = 0
parent: RegionField(tid: 1, is: 1, fs: 1, fid: 1048577), count = 1
self.physical_region is None => fast exit
attaching Region(tid: 1, is: 13, fs: 1, fid: 1048578) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048578) :: got to root, attaching
reducing inline ref count for RegionField(tid: 1, is: 13, fs: 1, fid: 1048578), count = 0
parent: RegionField(tid: 1, is: 1, fs: 1, fid: 1048578), count = 1
self.physical_region is None => fast exit
attaching Region(tid: 1, is: 13, fs: 1, fid: 1048579) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048579) :: got to root, attaching
reducing inline ref count for RegionField(tid: 1, is: 13, fs: 1, fid: 1048579), count = 0
parent: RegionField(tid: 1, is: 1, fs: 1, fid: 1048579), count = 1
self.physical_region is None => fast exit
attaching Region(tid: 1, is: 13, fs: 1, fid: 1048580) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048580) :: got to root, attaching
reducing inline ref count for RegionField(tid: 1, is: 13, fs: 1, fid: 1048580), count = 0
parent: RegionField(tid: 1, is: 1, fs: 1, fid: 1048580), count = 1
self.physical_region is None => fast exit
attaching Region(tid: 1, is: 13, fs: 1, fid: 1048581) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048581) :: got to root, attaching
reducing inline ref count for RegionField(tid: 1, is: 13, fs: 1, fid: 1048581), count = 0
parent: RegionField(tid: 1, is: 1, fs: 1, fid: 1048581), count = 1
self.physical_region is None => fast exit
attaching Region(tid: 1, is: 13, fs: 1, fid: 1048582) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048582) :: got to root, attaching
reducing inline ref count for RegionField(tid: 1, is: 13, fs: 1, fid: 1048582), count = 0
parent: RegionField(tid: 1, is: 1, fs: 1, fid: 1048582), count = 1
self.physical_region is None => fast exit
attaching Region(tid: 1, is: 13, fs: 1, fid: 1048583) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048583) :: got to root, attaching
reducing inline ref count for RegionField(tid: 1, is: 13, fs: 1, fid: 1048583), count = 0
parent: RegionField(tid: 1, is: 1, fs: 1, fid: 1048583), count = 1
self.physical_region is None => fast exit
attaching Region(tid: 1, is: 13, fs: 1, fid: 1048577) :: going to parent
attaching Region(tid: 1, is: 1, fs: 1, fid: 1048577) :: got to root, attaching
[0 - 7f5bac551700]    2.738872 {5}{runtime}: [error 395] LEGION ERROR: Attempted an inline mapping of region (1,1,1) that conflicts with previous inline mapping in task legion_python_main (ID 1) that would ultimately result in deadlock.  Instead you
receive this error message. (from file /gpfs/fs1/mpapadakis/legate.core/legion/runtime/legion/legion_context.cc:6568)