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, 1 insertions, 0 deletions

diff --git a/Include/internal/pycore_interp_structs.h b/Include/internal/pycore_interp_structs.h
index 44a4231fbc5..573b56a57e1 100644
--- a/Include/internal/pycore_interp_structs.h
+++ b/Include/internal/pycore_interp_structs.h
@@ -582,6 +582,7 @@ struct _warnings_runtime_state {
     PyObject *default_action; /* String */
     _PyRecursiveMutex lock;
     long filters_version;
+    PyObject *context;
 };
 
 struct _Py_mem_interp_free_queue {