Issue #23309: Avoid a deadlock at shutdown if a daemon thread is aborted

while it is holding a lock to a buffered I/O object, and the main thread
tries to use the same I/O object (typically stdout or stderr).  A fatal
error is emitted instead.

3 files changed, 67 insertions, 13 deletions

diff --git a/Lib/test/script_helper.py b/Lib/test/script_helper.py
index 8743dba..7ac19bf 100644
--- a/Lib/test/script_helper.py
+++ b/Lib/test/script_helper.py
@@ -1,6 +1,7 @@
 # Common utility functions used by various script execution tests
 #  e.g. test_cmd_line, test_cmd_line_script and test_runpy
 
+import collections
 import importlib
 import sys
 import os
@@ -50,8 +51,12 @@ def interpreter_requires_environment():
     return __cached_interp_requires_environment
 
 
+_PythonRunResult = collections.namedtuple("_PythonRunResult",
+                                          ("rc", "out", "err"))
+
+
 # Executing the interpreter in a subprocess
-def _assert_python(expected_success, *args, **env_vars):
+def run_python_until_end(*args, **env_vars):
     env_required = interpreter_requires_environment()
     if '__isolated' in env_vars:
         isolated = env_vars.pop('__isolated')
@@ -85,9 +90,14 @@ def _assert_python(expected_success, *args, **env_vars):
         p.stderr.close()
     rc = p.returncode
     err = strip_python_stderr(err)
-    if (rc and expected_success) or (not rc and not expected_success):
+    return _PythonRunResult(rc, out, err), cmd_line
+
+def _assert_python(expected_success, *args, **env_vars):
+    res, cmd_line = run_python_until_end(*args, **env_vars)
+    if (res.rc and expected_success) or (not res.rc and not expected_success):
         # Limit to 80 lines to ASCII characters
         maxlen = 80 * 100
+        out, err = res.out, res.err
         if len(out) > maxlen:
             out = b'(... truncated stdout ...)' + out[-maxlen:]
         if len(err) > maxlen:
@@ -106,10 +116,10 @@ def _assert_python(expected_success, *args, **env_vars):
                              "---\n"
                              "%s\n"
                              "---"
-                             % (rc, cmd_line,
+                             % (res.rc, cmd_line,
                                 out,
                                 err))
-    return rc, out, err
+    return res
 
 def assert_python_ok(*args, **env_vars):
     """
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 9fb85f2..2d02a31 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -35,7 +35,7 @@ import weakref
 from collections import deque, UserList
 from itertools import cycle, count
 from test import support
-from test.script_helper import assert_python_ok
+from test.script_helper import assert_python_ok, run_python_until_end
 
 import codecs
 import io  # C implementation of io
@@ -3437,6 +3437,49 @@ class CMiscIOTest(MiscIOTest):
         b = bytearray(2)
         self.assertRaises(ValueError, bufio.readinto, b)
 
+    @unittest.skipUnless(threading, 'Threading required for this test.')
+    def check_daemon_threads_shutdown_deadlock(self, stream_name):
+        # Issue #23309: deadlocks at shutdown should be avoided when a
+        # daemon thread and the main thread both write to a file.
+        code = """if 1:
+            import sys
+            import time
+            import threading
+
+            file = sys.{stream_name}
+
+            def run():
+                while True:
+                    file.write('.')
+                    file.flush()
+
+            thread = threading.Thread(target=run)
+            thread.daemon = True
+            thread.start()
+
+            time.sleep(0.5)
+            file.write('!')
+            file.flush()
+            """.format_map(locals())
+        res, _ = run_python_until_end("-c", code)
+        err = res.err.decode()
+        if res.rc != 0:
+            # Failure: should be a fatal error
+            self.assertIn("Fatal Python error: could not acquire lock "
+                          "for <_io.BufferedWriter name='<{stream_name}>'> "
+                          "at interpreter shutdown, possibly due to "
+                          "daemon threads".format_map(locals()),
+                          err)
+        else:
+            self.assertFalse(err.strip('.!'))
+
+    def test_daemon_threads_shutdown_stdout_deadlock(self):
+        self.check_daemon_threads_shutdown_deadlock('stdout')
+
+    def test_daemon_threads_shutdown_stderr_deadlock(self):
+        self.check_daemon_threads_shutdown_deadlock('stderr')
+
+
 class PyMiscIOTest(MiscIOTest):
     io = pyio
 
diff --git a/Lib/test/test_script_helper.py b/Lib/test/test_script_helper.py
index 7f5d654..8694530 100644
--- a/Lib/test/test_script_helper.py
+++ b/Lib/test/test_script_helper.py
@@ -8,26 +8,27 @@ from unittest import mock
 
 
 class TestScriptHelper(unittest.TestCase):
-    def test_assert_python_expect_success(self):
-        t = script_helper._assert_python(True, '-c', 'import sys; sys.exit(0)')
+
+    def test_assert_python_ok(self):
+        t = script_helper.assert_python_ok('-c', 'import sys; sys.exit(0)')
         self.assertEqual(0, t[0], 'return code was not 0')
 
-    def test_assert_python_expect_failure(self):
+    def test_assert_python_failure(self):
         # I didn't import the sys module so this child will fail.
-        rc, out, err = script_helper._assert_python(False, '-c', 'sys.exit(0)')
+        rc, out, err = script_helper.assert_python_failure('-c', 'sys.exit(0)')
         self.assertNotEqual(0, rc, 'return code should not be 0')
 
-    def test_assert_python_raises_expect_success(self):
+    def test_assert_python_ok_raises(self):
         # I didn't import the sys module so this child will fail.
         with self.assertRaises(AssertionError) as error_context:
-            script_helper._assert_python(True, '-c', 'sys.exit(0)')
+            script_helper.assert_python_ok('-c', 'sys.exit(0)')
         error_msg = str(error_context.exception)
         self.assertIn('command line:', error_msg)
         self.assertIn('sys.exit(0)', error_msg, msg='unexpected command line')
 
-    def test_assert_python_raises_expect_failure(self):
+    def test_assert_python_failure_raises(self):
         with self.assertRaises(AssertionError) as error_context:
-            script_helper._assert_python(False, '-c', 'import sys; sys.exit(0)')
+            script_helper.assert_python_failure('-c', 'import sys; sys.exit(0)')
         error_msg = str(error_context.exception)
         self.assertIn('Process return code is 0\n', error_msg)
         self.assertIn('import sys; sys.exit(0)', error_msg,