bpo-37076: _thread.start_new_thread() calls _PyErr_WriteUnraisableMsg()

Lib/test/test_thread.py

@@ -154,6 +154,24 @@ def mywrite(self, *args):
                 started.acquire()
         self.assertIn("Traceback", stderr.getvalue())
 
+    def test_unraisable_exception(self):
+        def task():
+            started.release()
+            raise ValueError("task failed")
+
+        started = thread.allocate_lock()
+        with support.catch_unraisable_exception() as cm:
+            with support.wait_threads_exit():
+                started.acquire()
+                thread.start_new_thread(task, ())
+                started.acquire()
+
+            self.assertEqual(str(cm.unraisable.exc_value), "task failed")
+            self.assertIs(cm.unraisable.object, task)
+            self.assertEqual(cm.unraisable.err_msg,
+                             "Exception ignored in thread started by")
+            self.assertIsNotNone(cm.unraisable.exc_traceback)
+
 
 class Barrier:
     def __init__(self, num_threads):

Misc/NEWS.d/next/Library/2019-05-28-12-17-10.bpo-37076.Bk2xOs.rst

@@ -0,0 +1,3 @@
+:func:`_thread.start_new_thread` now logs uncaught exception raised by the
+function using :func:`sys.unraisablehook`, rather than :func:`sys.excepthook`,
+so the hook gets access to the function which raised the exception.

Modules/_threadmodule.c

@@ -1004,23 +1004,12 @@ t_bootstrap(void *boot_raw)
         if (PyErr_ExceptionMatches(PyExc_SystemExit))
             PyErr_Clear();
         else {
-            PyObject *file;
-            PyObject *exc, *value, *tb;
-            PySys_WriteStderr(
-                "Unhandled exception in thread started by ");
-            PyErr_Fetch(&exc, &value, &tb);
-            file = _PySys_GetObjectId(&PyId_stderr);
-            if (file != NULL && file != Py_None)
-                PyFile_WriteObject(boot->func, file, 0);
-            else
-                PyObject_Print(boot->func, stderr, 0);
-            PySys_WriteStderr("\n");
-            PyErr_Restore(exc, value, tb);
-            PyErr_PrintEx(0);
+            _PyErr_WriteUnraisableMsg("in thread started by", boot->func);
         }
     }
-    else
+    else {
         Py_DECREF(res);
+    }
     Py_DECREF(boot->func);
     Py_DECREF(boot->args);
     Py_XDECREF(boot->keyw);