pythongh-110367: Enhance regrtest -jN --verbose3

When using worker processes (-jN) with --verbose3 option, regrtest
can now display the worker output even if a worker process does
crash.  Previously, sys.stdout and sys.stderr were replaced and so
the worker output was lost on a crash.

Lib/test/libregrtest/run_workers.py

@@ -262,6 +262,9 @@ def create_worker_runtests(self, test_name: TestName, json_file: JsonFile) -> Ru
         kwargs = {}
         if match_tests:
             kwargs['match_tests'] = match_tests
+        if self.runtests.output_on_failure:
+            kwargs['verbose'] = True
+            kwargs['output_on_failure'] = False
         return self.runtests.copy(
             tests=tests,
             json_file=json_file,
@@ -562,8 +565,16 @@ def _process_result(self, item: QueueOutput) -> TestResult:
         self.results.accumulate_result(result, self.runtests)
         self.display_result(mp_result)
 
-        if mp_result.worker_stdout:
-            print(mp_result.worker_stdout, flush=True)
+        # Display worker stdout
+        if not self.runtests.output_on_failure:
+            show_stdout = True
+        else:
+            # --verbose3 ignores stdout on success
+            show_stdout = (result.state != State.PASSED)
+        if show_stdout:
+            stdout = mp_result.worker_stdout
+            if stdout:
+                print(stdout, flush=True)
 
         return result
 

Lib/test/test_regrtest.py

@@ -2036,6 +2036,26 @@ def test_add_python_opts(self):
             with self.subTest(opt=opt):
                 self.check_add_python_opts(opt)
 
+    def test_worker_output_on_failure(self):
+        code = textwrap.dedent(r"""
+            import ctypes
+            import unittest
+
+            class CrashTests(unittest.TestCase):
+                def test_crash(self):
+                    print("just before crash!", flush=True)
+
+                    # Reading at NULL is likely to crash
+                    ctypes.string_at(0)
+        """)
+        testname = self.create_test(code=code)
+
+        output = self.run_tests("-j1", testname, exitcode=EXITCODE_BAD_TEST)
+        self.check_executed_tests(output, testname,
+                                  failed=[testname],
+                                  stats=0, parallel=True)
+        self.check_line(output, "just before crash!", full=True, regex=False)
+
 
 class TestUtils(unittest.TestCase):
     def test_format_duration(self):

Misc/NEWS.d/next/Tests/2023-10-04-18-27-47.gh-issue-110367.Nnq1I7.rst

@@ -0,0 +1,4 @@
+regrtest: When using worker processes (-jN) with --verbose3 option, regrtest
+can now display the worker output even if a worker process does crash.
+Previously, sys.stdout and sys.stderr were replaced and so the worker output
+was lost on a crash. Patch by Victor Stinner.