bpo-42815: Fix issue when thread doesn't copy context of parent thread

Lib/test/test_decimal.py

@@ -32,6 +32,7 @@
 import unittest
 import numbers
 import locale
+import contextvars
 from test.support import (run_unittest, run_doctest, is_resource_enabled,
                           requires_IEEE_754, requires_docstrings,
                           requires_legacy_unicode_capi)
@@ -1616,8 +1617,8 @@ def test_threading(self):
         self.finish1 = threading.Event()
         self.finish2 = threading.Event()
 
-        th1 = threading.Thread(target=thfunc1, args=(self,))
-        th2 = threading.Thread(target=thfunc2, args=(self,))
+        th1 = threading.Thread(target=thfunc1, args=(self,), context=contextvars.Context())
+        th2 = threading.Thread(target=thfunc2, args=(self,), context=contextvars.Context())
 
         th1.start()
         th2.start()

Lib/test/test_threading.py

@@ -8,6 +8,7 @@
 from test.support.import_helper import import_module
 from test.support.script_helper import assert_python_ok, assert_python_failure
 
+import contextvars
 import random
 import sys
 import _thread
@@ -928,6 +929,31 @@ def test_debug_deprecation(self):
                b'is deprecated and will be removed in Python 3.12')
         self.assertIn(msg, err)
 
+    def test_contextvars(self):
+        context_var = contextvars.ContextVar("context_var")
+        context_var.set("default")
+
+        result = None
+
+        def target():
+            nonlocal result
+            result = context_var.get()
+
+        t = threading.Thread(target=target)
+        t.start()
+        t.join()
+
+        self.assertEqual(result, "default")
+
+        custom_ctx = contextvars.Context()
+        custom_ctx.run(lambda: context_var.set("custom"))
+
+        t = threading.Thread(target=target, context=custom_ctx)
+        t.start()
+        t.join()
+
+        self.assertEqual(result, "custom")
+
 
 class ThreadJoinOnShutdown(BaseTestCase):
 

Lib/threading.py

@@ -8,6 +8,7 @@
 from time import monotonic as _time
 from _weakrefset import WeakSet
 from itertools import islice as _islice, count as _count
+from contextvars import copy_context as _copy_context
 try:
     from _collections import deque as _deque
 except ImportError:
@@ -818,7 +819,7 @@ class Thread:
     _initialized = False
 
     def __init__(self, group=None, target=None, name=None,
-                 args=(), kwargs=None, *, daemon=None):
+                 args=(), kwargs=None, *, daemon=None, context=None):
         """This constructor should always be called with keyword arguments. Arguments are:
 
         *group* should be None; reserved for future extension when a ThreadGroup
@@ -863,6 +864,10 @@ class is implemented.
         else:
             self._daemonic = current_thread().daemon
         self._ident = None
+        if context is not None:
+            self._context = context
+        else:
+            self._context = _copy_context()
         if _HAVE_THREAD_NATIVE_ID:
             self._native_id = None
         self._tstate_lock = None
@@ -943,11 +948,11 @@ def run(self):
         """
         try:
             if self._target is not None:
-                self._target(*self._args, **self._kwargs)
+                self._context.run(self._target, *self._args, **self._kwargs)
         finally:
             # Avoid a refcycle if the thread is running a function with
             # an argument that has a member that points to the thread.
-            del self._target, self._args, self._kwargs
+            del self._context, self._target, self._args, self._kwargs
 
     def _bootstrap(self):
         # Wrapper around the real bootstrap code that ignores

Misc/NEWS.d/next/Library/2021-01-03-13-44-44.bpo-42815.IJqHPr.rst

@@ -0,0 +1,2 @@
+Fix issue when new thread doesn't copy context of a parent thread. Patch
+provided by Yurii Karabas.

Modules/Setup

@@ -169,7 +169,7 @@ _symtable symtablemodule.c
 #array -DPy_BUILD_CORE_MODULE arraymodule.c	# array objects
 #cmath cmathmodule.c _math.c -DPy_BUILD_CORE_MODULE # -lm # complex math library functions
 #math mathmodule.c _math.c -DPy_BUILD_CORE_MODULE # -lm # math library functions, e.g. sin()
-#_contextvars _contextvarsmodule.c  # Context Variables
+_contextvars _contextvarsmodule.c  # Context Variables
 #_struct -DPy_BUILD_CORE_MODULE _struct.c	# binary structure packing/unpacking
 #_weakref _weakref.c	# basic weak reference support
 #_testcapi _testcapimodule.c    # Python C API test module