gh-109162: libregrtest: add worker.py (#109229)

Add new worker.py file:

* Move create_worker_process() and worker_process() to this file.
* Add main() function to worker.py. create_worker_process() now
  runs the command: "python -m test.libregrtest.worker JSON".
* create_worker_process() now starts the worker process in the
  current working directory. Regrtest now gets the absolute path of
  the reflog.txt filename: -R command line option filename.
* Remove --worker-json command line option.
  Remove test_regrtest.test_worker_json().

Related changes:

* Add write_json() and from_json() methods to TestResult.
* Rename select_temp_dir() to get_temp_dir() and move it to utils.
* Rename make_temp_dir() to get_work_dir() and move it to utils.
  It no longer calls os.makedirs(): Regrtest.main() now calls it.
* Move fix_umask() to utils. The function is now called by
  setup_tests().
* Move StrPath to utils.
* Add exit_timeout() context manager to utils.
* RunTests: Replace junit_filename (StrPath) with use_junit (bool).

Lib/test/libregrtest/worker.py

@@ -0,0 +1,93 @@
+import subprocess
+import sys
+import os
+from typing import TextIO, NoReturn
+
+from test import support
+from test.support import os_helper
+
+from test.libregrtest.setup import setup_tests, setup_test_dir
+from test.libregrtest.runtest import (
+    run_single_test, StrJSON, FilterTuple, RunTests)
+from test.libregrtest.utils import get_work_dir, exit_timeout, StrPath
+
+
+USE_PROCESS_GROUP = (hasattr(os, "setsid") and hasattr(os, "killpg"))
+
+
+def create_worker_process(runtests: RunTests,
+                          output_file: TextIO,
+                          tmp_dir: StrPath | None = None) -> subprocess.Popen:
+    python_cmd = runtests.python_cmd
+    worker_json = runtests.as_json()
+
+    if python_cmd is not None:
+        executable = python_cmd
+    else:
+        executable = [sys.executable]
+    cmd = [*executable, *support.args_from_interpreter_flags(),
+           '-u',    # Unbuffered stdout and stderr
+           '-m', 'test.libregrtest.worker',
+           worker_json]
+
+    env = dict(os.environ)
+    if tmp_dir is not None:
+        env['TMPDIR'] = tmp_dir
+        env['TEMP'] = tmp_dir
+        env['TMP'] = tmp_dir
+
+    # Running the child from the same working directory as regrtest's original
+    # invocation ensures that TEMPDIR for the child is the same when
+    # sysconfig.is_python_build() is true. See issue 15300.
+    kw = dict(
+        env=env,
+        stdout=output_file,
+        # bpo-45410: Write stderr into stdout to keep messages order
+        stderr=output_file,
+        text=True,
+        close_fds=(os.name != 'nt'),
+    )
+    if USE_PROCESS_GROUP:
+        kw['start_new_session'] = True
+    return subprocess.Popen(cmd, **kw)
+
+
+def worker_process(worker_json: StrJSON) -> NoReturn:
+    runtests = RunTests.from_json(worker_json)
+    test_name = runtests.tests[0]
+    match_tests: FilterTuple | None = runtests.match_tests
+
+    setup_test_dir(runtests.test_dir)
+    setup_tests(runtests)
+
+    if runtests.rerun:
+        if match_tests:
+            matching = "matching: " + ", ".join(match_tests)
+            print(f"Re-running {test_name} in verbose mode ({matching})", flush=True)
+        else:
+            print(f"Re-running {test_name} in verbose mode", flush=True)
+
+    result = run_single_test(test_name, runtests)
+    print()   # Force a newline (just in case)
+
+    # Serialize TestResult as dict in JSON
+    result.write_json(sys.stdout)
+    sys.stdout.flush()
+    sys.exit(0)
+
+
+def main():
+    if len(sys.argv) != 2:
+        print("usage: python -m test.libregrtest.worker JSON")
+        sys.exit(1)
+    worker_json = sys.argv[1]
+
+    work_dir = get_work_dir(worker=True)
+
+    with exit_timeout():
+        with os_helper.temp_cwd(work_dir, quiet=True):
+            worker_process(worker_json)
+
+
+if __name__ == "__main__":
+    main()