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, 4 insertions, 0 deletions

diff --git a/Lib/_dummy_thread.py b/Lib/_dummy_thread.py
index a2cae54b05..2407f9bf5d 100644
--- a/Lib/_dummy_thread.py
+++ b/Lib/_dummy_thread.py
@@ -161,3 +161,7 @@ def interrupt_main():
     else:
         global _interrupt
         _interrupt = True
+
+
+def _is_main_interpreter():
+    return True