bpo-40089: Fix threading._after_fork() (GH-19191)

If fork was not called by a thread spawned by threading.Thread,
threading._after_fork() now creates a _MainThread instance for
_main_thread, instead of a _DummyThread instance.

Lib/threading.py

@@ -1423,7 +1423,15 @@ def _after_fork():
 
     # fork() only copied the current thread; clear references to others.
     new_active = {}
-    current = current_thread()
+
+    try:
+        current = _active[get_ident()]
+    except KeyError:
+        # fork() was called in a thread which was not spawned
+        # by threading.Thread. For example, a thread spawned
+        # by thread.start_new_thread().
+        current = _MainThread()
+
     _main_thread = current
 
     # reset _shutdown() locks: threads re-register their _tstate_lock below

Misc/NEWS.d/next/Library/2020-03-27-17-22-34.bpo-40089.-lFsD0.rst

@@ -0,0 +1,3 @@
+Fix threading._after_fork(): if fork was not called by a thread spawned by
+threading.Thread, threading._after_fork() now creates a _MainThread instance
+for _main_thread, instead of a _DummyThread instance.