Shared memory regions between Erlang and Python contexts.

A region is a fixed-size file mapped MAP_SHARED through iommap. Erlang reads it with no copy (binary/3) and writes with one copy (write/3); Python maps the same file and sees it as a buffer (erlang.SharedMemory), in every context mode. The handle is a plain term, {$py_shm', Id, Path, Size}', so it travels inside any call argument or result.

   {ok, Shm} = py_shm:new(64 * 1024 * 1024),
   ok = py_shm:write(Shm, 0, Data),
   {ok, Sum} = py_context:call(Ctx, myapp, sum_floats, [Shm]),
   Out = py_shm:binary(Shm, 0, 1024),
   ok = py_shm:close(Shm).

iommap is an optional dependency: add {iommap, "1.1.3"} to your deps. Without it new/1,2 returns {error, iommap_not_available}.

The module also backs shared py_buffers (py_buffer:new(#{shared => true})): a region used as a ring, with the write position and the closed flag in a header page and flow control through the _py_buffer_wait and _py_buffer_consumed callbacks the Python side calls.

Owns: the region table (ETS), the backing files, and the ring state of shared buffers. Talks to: iommap (through apply/3, optional dependency), py_buffer (shared variant), py_callback (registers _py_buffer_wait, _py_buffer_consumed, _py_buffer_state). Never: maps memory into Python itself; that is _erlang_impl/_shm.py in each interpreter.