bpo-37266: Daemon threads are now denied in subinterpreters (GH-14049)

In a subinterpreter, spawning a daemon thread now raises an
exception. Daemon threads were never supported in subinterpreters.
Previously, the subinterpreter finalization crashed with a Pyton
fatal error if a daemon thread was still running.

* Add _thread._is_main_interpreter()
* threading.Thread.start() now raises RuntimeError if the thread is a
  daemon thread and the method is called from a subinterpreter.
* The _thread module now uses Argument Clinic for the new function.
* Use textwrap.dedent() in test_threading.SubinterpThreadingTests

1 files changed, 22 insertions, 0 deletions

diff --git a/Modules/clinic/_threadmodule.c.h b/Modules/clinic/_threadmodule.c.h
new file mode 100644
index 0000000..07ea08b
--- /dev/null
+++ b/Modules/clinic/_threadmodule.c.h
@@ -0,0 +1,22 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(_thread__is_main_interpreter__doc__,
+"_is_main_interpreter($module, /)\n"
+"--\n"
+"\n"
+"Return True if the current interpreter is the main Python interpreter.");
+
+#define _THREAD__IS_MAIN_INTERPRETER_METHODDEF    \
+    {"_is_main_interpreter", (PyCFunction)_thread__is_main_interpreter, METH_NOARGS, _thread__is_main_interpreter__doc__},
+
+static PyObject *
+_thread__is_main_interpreter_impl(PyObject *module);
+
+static PyObject *
+_thread__is_main_interpreter(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return _thread__is_main_interpreter_impl(module);
+}
+/*[clinic end generated code: output=505840d1b9101789 input=a9049054013a1b77]*/