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, 8 insertions, 0 deletions

diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index 9ffd5cd581..f80eb22e18 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -280,6 +280,8 @@ since it is impossible to detect the termination of alien threads.
    base class constructor (``Thread.__init__()``) before doing anything else to
    the thread.
 
+   Daemon threads must not be used in subinterpreters.
+
    .. versionchanged:: 3.3
       Added the *daemon* argument.
 
@@ -294,6 +296,12 @@ since it is impossible to detect the termination of alien threads.
       This method will raise a :exc:`RuntimeError` if called more than once
       on the same thread object.
 
+      Raise a :exc:`RuntimeError` if the thread is a daemon thread and the
+      method is called from a subinterpreter.
+
+      .. versionchanged:: 3.9
+         In a subinterpreter, spawning a daemon thread now raises an exception.
+
    .. method:: run()
 
       Method representing the thread's activity.