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, 4 insertions, 0 deletions

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 2a28fab2f51..3e0165acd6f 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -3335,6 +3335,8 @@ static PyStructSequence_Field flags_fields[] = {
     {"safe_path", "-P"},
     {"int_max_str_digits",      "-X int_max_str_digits"},
     {"gil",                     "-X gil"},
+    {"thread_inherit_context",  "-X thread_inherit_context"},
+    {"context_aware_warnings",    "-X context_aware_warnings"},
     {0}
 };
 
@@ -3435,6 +3437,8 @@ set_flags_from_config(PyInterpreterState *interp, PyObject *flags)
 #else
     SetFlagObj(PyLong_FromLong(1));
 #endif
+    SetFlag(config->thread_inherit_context);
+    SetFlag(config->context_aware_warnings);
 #undef SetFlagObj
 #undef SetFlag
     return 0;