gh-128384: Use a context variable for warnings.catch_warnings (gh-130010)

Make `warnings.catch_warnings()` use a context variable for holding
the warning filtering state if the `sys.flags.context_aware_warnings`
flag is set to true.  This makes using the context manager thread-safe in
multi-threaded programs.

Add the `sys.flags.thread_inherit_context` flag.  If true, starting a new
thread with `threading.Thread` will use a copy of the context
from the caller of `Thread.start()`.

Both these flags are set to true by default for the free-threaded build
and false for the default build.

Move the Python implementation of warnings.py into _py_warnings.py.

Make _contextvars a builtin module.

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

1 files changed, 22 insertions, 2 deletions

diff --git a/Lib/threading.py b/Lib/threading.py
index 0dc1d324c98..c4cc4dd2b00 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -3,6 +3,7 @@
 import os as _os
 import sys as _sys
 import _thread
+import _contextvars
 
 from time import monotonic as _time
 from _weakrefset import WeakSet
@@ -876,7 +877,7 @@ 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
@@ -893,6 +894,14 @@ class Thread:
         *kwargs* is a dictionary of keyword arguments for the target
         invocation. Defaults to {}.
 
+        *context* is the contextvars.Context value to use for the thread.
+        The default value is None, which means to check
+        sys.flags.thread_inherit_context.  If that flag is true, use a copy
+        of the context of the caller.  If false, use an empty context.  To
+        explicitly start with an empty context, pass a new instance of
+        contextvars.Context().  To explicitly start with a copy of the current
+        context, pass the value from contextvars.copy_context().
+
         If a subclass overrides the constructor, it must make sure to invoke
         the base class constructor (Thread.__init__()) before doing anything
         else to the thread.
@@ -922,6 +931,7 @@ class Thread:
             self._daemonic = daemon
         else:
             self._daemonic = current_thread().daemon
+        self._context = context
         self._ident = None
         if _HAVE_THREAD_NATIVE_ID:
             self._native_id = None
@@ -977,6 +987,16 @@ class Thread:
 
         with _active_limbo_lock:
             _limbo[self] = self
+
+        if self._context is None:
+            # No context provided
+            if _sys.flags.thread_inherit_context:
+                # start with a copy of the context of the caller
+                self._context = _contextvars.copy_context()
+            else:
+                # start with an empty context
+                self._context = _contextvars.Context()
+
         try:
             # Start joinable thread
             _start_joinable_thread(self._bootstrap, handle=self._handle,
@@ -1056,7 +1076,7 @@ class Thread:
                 _sys.setprofile(_profile_hook)
 
             try:
-                self.run()
+                self._context.run(self.run)
             except:
                 self._invoke_excepthook(self)
         finally: