Thanks for working on the PR, Zackery. Would you be interested in working on improvements to mmap for 3.10? With support in mmap, the Windows-specific initialization of SharedMemory could be as simple as the following:

    # Windows Named Shared Memory

    while True:
        tagname = _make_filename() if name is None else name
        try:
            self._mmap = mmap.mmap(-1, size if create else 0,
                tagname, create=create)
            break
        except FileExistsError:
            if name is not None:
                raise

    self._name = tagname
    self._size = len(self._mmap)

The new mmap `create` parameter would default to None, which uses the current behavior that allows either opening or creating a file mapping with no sanity checking (e.g. a valid fd gets passed in, but it opens an unrelated file mapping via tagname). If `create` is true, and there's an existing file mapping named `tagname`, then raise FileExistsError. If `create` is false, call OpenFileMappingW instead of CreateFileMappingW. In this case, `fileno` must be -1, `length` is allowed to be 0, and `tagname` must be a non-empty string. If `length` is 0, map the entire file mapping and get the size via VirtualQuery: RegionSize.