prevent writing to stderr from messing up the exception state (closes #14474)

Lib/test/test_thread.py

@@ -130,6 +130,30 @@ class ThreadRunningTests(BasicThreadTest):
             time.sleep(0.01)
         self.assertEqual(thread._count(), orig)
 
+    def test_save_exception_state_on_error(self):
+        # See issue #14474
+        def task():
+            started.release()
+            sys.stderr = stderr
+            raise SyntaxError
+        def mywrite(self, *args):
+            try:
+                raise ValueError
+            except ValueError:
+                pass
+            real_write(self, *args)
+        c = thread._count()
+        started = thread.allocate_lock()
+        with test_support.captured_output("stderr") as stderr:
+            real_write = stderr.write
+            stderr.write = mywrite
+            started.acquire()
+            thread.start_new_thread(task, ())
+            started.acquire()
+            while thread._count() > c:
+                pass
+        self.assertIn("Traceback", stderr.getvalue())
+
 
 class Barrier:
     def __init__(self, num_threads):

Misc/NEWS

@@ -9,6 +9,9 @@ What's New in Python 2.7.4
 Core and Builtins
 -----------------
 
+- Issue #14474: Save and restore exception state in thread.start_new_thread()
+  while writing error message if the thread leaves a unhandled exception.
+
 - Issue #13019: Fix potential reference leaks in bytearray.extend().  Patch
   by Suman Saha.
 

Modules/threadmodule.c

@@ -618,6 +618,8 @@ t_bootstrap(void *boot_raw)
             PyErr_Clear();
         else {
             PyObject *file;
+            PyObject *exc, *value, *tb;
+            PyErr_Fetch(&exc, &value, &tb);
             PySys_WriteStderr(
                 "Unhandled exception in thread started by ");
             file = PySys_GetObject("stderr");
@@ -625,6 +627,7 @@ t_bootstrap(void *boot_raw)
                 PyFile_WriteObject(boot->func, file, 0);
             else
                 PyObject_Print(boot->func, stderr, 0);
+            PyErr_Restore(exc, value, tb);
             PySys_WriteStderr("\n");
             PyErr_PrintEx(0);
         }