bpo-38707: Fix for multiprocessing.Process MainThread.native_id (GH-17088)

This PR implements a fix for `multiprocessing.Process` objects; the error occurs when Processes are created using either `fork` or `forkserver` as the `start_method`.

In these instances, the `MainThread` of the newly created `Process` object retains all attributes from its parent's `MainThread` object, including the `native_id` attribute. The resulting behavior is such that the new process' `MainThread` captures an incorrect/outdated `native_id` (the parent's instead of its own). 

This change forces the Process object to update its `native_id` attribute during the bootstrap process.

cc @vstinner





https://bugs.python.org/issue38707



Automerge-Triggered-By: @pitrou

Lib/multiprocessing/process.py

@@ -301,6 +301,8 @@ class BaseProcess(object):
             _current_process = self
             _parent_process = _ParentProcess(
                 self._parent_name, self._parent_pid, parent_sentinel)
+            if threading._HAVE_THREAD_NATIVE_ID:
+                threading.main_thread()._set_native_id()
             try:
                 util._finalizer_registry.clear()
                 util._run_after_forkers()