From ab41c231b87c96ccb1aaf27817cb8543aa6d8399 Mon Sep 17 00:00:00 2001
From: Zackery Spytz <zspytz@gmail.com>
Date: Wed, 2 Dec 2020 16:57:45 -0700
Subject: [PATCH] bpo-38520: Add multiprocessing.main_process()

Add a multiprocessing.main_process() function to access the main
process.
---
 Doc/library/multiprocessing.rst                    |  6 ++++++
 Doc/whatsnew/3.10.rst                              |  7 +++++++
 Lib/multiprocessing/context.py                     |  1 +
 Lib/multiprocessing/dummy/__init__.py              |  2 ++
 Lib/multiprocessing/process.py                     | 11 +++++++++--
 Lib/test/_test_multiprocessing.py                  | 14 ++++++++++++++
 .../2020-12-02-16-57-20.bpo-38520.bgN3rB.rst       |  2 ++
 7 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-12-02-16-57-20.bpo-38520.bgN3rB.rst

diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 128aa43b8b7..5237dea38d0 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -970,6 +970,12 @@ Miscellaneous
 
    .. versionadded:: 3.8
 
+.. function:: main_process()
+
+   Return the main :class:`Process` object.
+
+   .. versionadded:: 3.10
+
 .. function:: freeze_support()
 
    Add support for when a program which uses :mod:`multiprocessing` has been
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index a8f1080a504..ecc850f4313 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -229,6 +229,13 @@ linecache
 When a module does not define ``__loader__``, fall back to ``__spec__.loader``.
 (Contributed by Brett Cannon in :issue:`42133`.)
 
+multiprocessing
+---------------
+
+Add a new :func:`multiprocessing.main_process` function to access the main
+process.
+(Contributed by Zackery Spytz in :issue:`38520`.)
+
 os
 --
 
diff --git a/Lib/multiprocessing/context.py b/Lib/multiprocessing/context.py
index 8d0525d5d62..47d19c1c1e7 100644
--- a/Lib/multiprocessing/context.py
+++ b/Lib/multiprocessing/context.py
@@ -37,6 +37,7 @@ class BaseContext(object):
     current_process = staticmethod(process.current_process)
     parent_process = staticmethod(process.parent_process)
     active_children = staticmethod(process.active_children)
+    main_process = staticmethod(process.main_process)
 
     def cpu_count(self):
         '''Returns the number of CPUs in the system'''
diff --git a/Lib/multiprocessing/dummy/__init__.py b/Lib/multiprocessing/dummy/__init__.py
index 6a1468609e3..b2cee5b6f16 100644
--- a/Lib/multiprocessing/dummy/__init__.py
+++ b/Lib/multiprocessing/dummy/__init__.py
@@ -64,6 +64,8 @@ class DummyProcess(threading.Thread):
 Process = DummyProcess
 current_process = threading.current_thread
 current_process()._children = weakref.WeakKeyDictionary()
+main_process = threading.main_thread
+
 
 def active_children():
     children = current_process()._children
diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
index 0b2e0b45b23..7bdc914ab30 100644
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -8,7 +8,7 @@
 #
 
 __all__ = ['BaseProcess', 'current_process', 'active_children',
-           'parent_process']
+           'parent_process', 'main_process']
 
 #
 # Imports
@@ -54,6 +54,13 @@ def parent_process():
     '''
     return _parent_process
 
+
+def main_process():
+    '''
+    Return process object representing the main process
+    '''
+    return _main_process
+
 #
 #
 #
@@ -413,7 +420,7 @@ class _MainProcess(BaseProcess):
 
 
 _parent_process = None
-_current_process = _MainProcess()
+_current_process = _main_process = _MainProcess()
 _process_counter = itertools.count(1)
 _children = set()
 del _MainProcess
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index fd3b4303f03..afabb8d4093 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -479,6 +479,18 @@ class _TestProcess(BaseTestCase):
         p.join()
         self.assertNotIn(p, self.active_children())
 
+    @classmethod
+    def _test_main_process(cls, conn):
+        conn.send(cls.main_process() is cls.current_process())
+
+    def test_main_process(self):
+        self.assertIs(self.main_process(), self.current_process())
+        reader, writer = self.Pipe(duplex=False)
+        p = self.Process(target=self._test_main_process, args=(writer,))
+        p.start()
+        p.join()
+        self.assertEqual(reader.recv(), False)
+
     @classmethod
     def _test_recursion(cls, wconn, id):
         wconn.send(id)
@@ -5634,6 +5646,7 @@ class ProcessesMixin(BaseMixin):
     current_process = staticmethod(multiprocessing.current_process)
     parent_process = staticmethod(multiprocessing.parent_process)
     active_children = staticmethod(multiprocessing.active_children)
+    main_process = staticmethod(multiprocessing.main_process)
     Pool = staticmethod(multiprocessing.Pool)
     Pipe = staticmethod(multiprocessing.Pipe)
     Queue = staticmethod(multiprocessing.Queue)
@@ -5718,6 +5731,7 @@ class ThreadsMixin(BaseMixin):
     connection = multiprocessing.dummy.connection
     current_process = staticmethod(multiprocessing.dummy.current_process)
     active_children = staticmethod(multiprocessing.dummy.active_children)
+    main_process = staticmethod(multiprocessing.dummy.main_process)
     Pool = staticmethod(multiprocessing.dummy.Pool)
     Pipe = staticmethod(multiprocessing.dummy.Pipe)
     Queue = staticmethod(multiprocessing.dummy.Queue)
diff --git a/Misc/NEWS.d/next/Library/2020-12-02-16-57-20.bpo-38520.bgN3rB.rst b/Misc/NEWS.d/next/Library/2020-12-02-16-57-20.bpo-38520.bgN3rB.rst
new file mode 100644
index 00000000000..7dc75f424d4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-12-02-16-57-20.bpo-38520.bgN3rB.rst
@@ -0,0 +1,2 @@
+Add a :func:`multiprocessing.main_process` function to access the main
+process.