bpo-40094: Add test.support.wait_process() (GH-19254)

Moreover, the following tests now check the child process exit code:

* test_os.PtyTests
* test_mailbox.test_lock_conflict()
* test_tempfile.test_process_awareness()
* test_uuid.testIssue8621()
* multiprocessing resource tracker tests

19 files changed, 125 insertions, 103 deletions

diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 54ad620d..c33465d 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -825,6 +825,21 @@ The :mod:`test.support` module defines the following functions:
    target of the "as" clause, if there is one.
 
 
+.. function:: wait_process(pid, *, exitcode, timeout=None)
+
+   Wait until process *pid* completes and check that the process exit code is
+   *exitcode*.
+
+   Raise an :exc:`AssertionError` if the process exit code is not equal to
+   *exitcode*.
+
+   If the process runs longer than *timeout* seconds (:data:`SHORT_TIMEOUT` by
+   default), kill the process and raise an :exc:`AssertionError`. The timeout
+   feature is not available on Windows.
+
+   .. versionadded:: 3.9
+
+
 .. function:: wait_threads_exit(timeout=60.0)
 
    Context manager to wait until all threads created in the ``with`` statement
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 4a87b17..d00e928 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -5124,7 +5124,7 @@ class TestResourceTracker(unittest.TestCase):
         pid = _resource_tracker._pid
         if pid is not None:
             os.kill(pid, signal.SIGKILL)
-            os.waitpid(pid, 0)
+            support.wait_process(pid, exitcode=-signal.SIGKILL)
         with warnings.catch_warnings():
             warnings.simplefilter("ignore")
             _resource_tracker.ensure_running()
diff --git a/Lib/test/fork_wait.py b/Lib/test/fork_wait.py
index f6bbffe..8c17755 100644
--- a/Lib/test/fork_wait.py
+++ b/Lib/test/fork_wait.py
@@ -44,16 +44,7 @@ class ForkWait(unittest.TestCase):
                 pass
 
     def wait_impl(self, cpid):
-        for i in range(10):
-            # waitpid() shouldn't hang, but some of the buildbots seem to hang
-            # in the forking tests.  This is an attempt to fix the problem.
-            spid, status = os.waitpid(cpid, os.WNOHANG)
-            if spid == cpid:
-                break
-            time.sleep(2 * SHORTSLEEP)
-
-        self.assertEqual(spid, cpid)
-        self.assertEqual(status, 0, "cause = %d, exit = %d" % (status&0xff, status>>8))
+        support.wait_process(cpid, exitcode=0)
 
     def test_wait(self):
         for i in range(NUM_THREADS):
diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 259c706..5b9aebb 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -3400,3 +3400,62 @@ class catch_threading_exception:
         del self.exc_value
         del self.exc_traceback
         del self.thread
+
+
+def wait_process(pid, *, exitcode, timeout=None):
+    """
+    Wait until process pid completes and check that the process exit code is
+    exitcode.
+
+    Raise an AssertionError if the process exit code is not equal to exitcode.
+
+    If the process runs longer than timeout seconds (SHORT_TIMEOUT by default),
+    kill the process (if signal.SIGKILL is available) and raise an
+    AssertionError. The timeout feature is not available on Windows.
+    """
+    if os.name != "nt":
+        if timeout is None:
+            timeout = SHORT_TIMEOUT
+        t0 = time.monotonic()
+        deadline = t0 + timeout
+        sleep = 0.001
+        max_sleep = 0.1
+        while True:
+            pid2, status = os.waitpid(pid, os.WNOHANG)
+            if pid2 != 0:
+                break
+            # process is still running
+
+            dt = time.monotonic() - t0
+            if dt > SHORT_TIMEOUT:
+                try:
+                    os.kill(pid, signal.SIGKILL)
+                    os.waitpid(pid, 0)
+                except OSError:
+                    # Ignore errors like ChildProcessError or PermissionError
+                    pass
+
+                raise AssertionError(f"process {pid} is still running "
+                                     f"after {dt:.1f} seconds")
+
+            sleep = min(sleep * 2, max_sleep)
+            time.sleep(sleep)
+
+        if os.WIFEXITED(status):
+            exitcode2 = os.WEXITSTATUS(status)
+        elif os.WIFSIGNALED(status):
+            exitcode2 = -os.WTERMSIG(status)
+        else:
+            raise ValueError(f"invalid wait status: {status!r}")
+    else:
+        # Windows implementation
+        pid2, status = os.waitpid(pid, 0)
+        exitcode2 = (status >> 8)
+
+    if exitcode2 != exitcode:
+        raise AssertionError(f"process {pid} exited with code {exitcode2}, "
+                             f"but exit code {exitcode} is expected")
+
+    # sanity check: it should not fail in practice
+    if pid2 != pid:
+        raise AssertionError(f"pid {pid2} != pid {pid}")
diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
index 1e9012e..86e5f1d 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -25,6 +25,7 @@ from itertools import product
 from textwrap import dedent
 from types import AsyncGeneratorType, FunctionType
 from operator import neg
+from test import support
 from test.support import (
     EnvironmentVarGuard, TESTFN, check_warnings, swap_attr, unlink,
     maybe_get_event_loop_policy)
@@ -1890,7 +1891,7 @@ class PtyTests(unittest.TestCase):
         os.close(fd)
 
         # Wait until the child process completes
-        os.waitpid(pid, 0)
+        support.wait_process(pid, exitcode=0)
 
         return lines
 
diff --git a/Lib/test/test_logging.py b/Lib/test/test_logging.py
index e223522..2ad3c5c 100644
--- a/Lib/test/test_logging.py
+++ b/Lib/test/test_logging.py
@@ -727,30 +727,19 @@ class HandlerTest(BaseTest):
 
         locks_held__ready_to_fork.wait()
         pid = os.fork()
-        if pid == 0:  # Child.
+        if pid == 0:
+            # Child process
             try:
                 test_logger.info(r'Child process did not deadlock. \o/')
             finally:
                 os._exit(0)
-        else:  # Parent.
+        else:
+            # Parent process
             test_logger.info(r'Parent process returned from fork. \o/')
             fork_happened__release_locks_and_end_thread.set()
             lock_holder_thread.join()
-            start_time = time.monotonic()
-            while True:
-                test_logger.debug('Waiting for child process.')
-                waited_pid, status = os.waitpid(pid, os.WNOHANG)
-                if waited_pid == pid:
-                    break  # child process exited.
-                if time.monotonic() - start_time > 7:
-                    break  # so long? implies child deadlock.
-                time.sleep(0.05)
-            test_logger.debug('Done waiting.')
-            if waited_pid != pid:
-                os.kill(pid, signal.SIGKILL)
-                waited_pid, status = os.waitpid(pid, 0)
-                self.fail("child process deadlocked.")
-            self.assertEqual(status, 0, msg="child process error")
+
+            support.wait_process(pid, exitcode=0)
 
 
 class BadStream(object):
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 36a2653..fdda1d1 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -1092,7 +1092,7 @@ class _TestMboxMMDF(_TestSingleFile):
             # Signal the child it can now release the lock and exit.
             p.send(b'p')
             # Wait for child to exit.  Locking should now succeed.
-            exited_pid, status = os.waitpid(pid, 0)
+            support.wait_process(pid, exitcode=0)
 
         self._box.lock()
         self._box.unlock()
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 9c96544..be85616 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -2792,8 +2792,7 @@ class PidTests(unittest.TestCase):
         args = [sys.executable, '-c', 'pass']
         # Add an implicit test for PyUnicode_FSConverter().
         pid = os.spawnv(os.P_NOWAIT, FakePath(args[0]), args)
-        status = os.waitpid(pid, 0)
-        self.assertEqual(status, (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
 
 class SpawnTests(unittest.TestCase):
@@ -2877,14 +2876,7 @@ class SpawnTests(unittest.TestCase):
     def test_nowait(self):
         args = self.create_args()
         pid = os.spawnv(os.P_NOWAIT, args[0], args)
-        result = os.waitpid(pid, 0)
-        self.assertEqual(result[0], pid)
-        status = result[1]
-        if hasattr(os, 'WIFEXITED'):
-            self.assertTrue(os.WIFEXITED(status))
-            self.assertEqual(os.WEXITSTATUS(status), self.exitcode)
-        else:
-            self.assertEqual(status, self.exitcode << 8)
+        support.wait_process(pid, exitcode=self.exitcode)
 
     @requires_os_func('spawnve')
     def test_spawnve_bytes(self):
diff --git a/Lib/test/test_platform.py b/Lib/test/test_platform.py
index 3084663..f167fb1 100644
--- a/Lib/test/test_platform.py
+++ b/Lib/test/test_platform.py
@@ -236,9 +236,7 @@ class PlatformTest(unittest.TestCase):
 
         else:
             # parent
-            cpid, sts = os.waitpid(pid, 0)
-            self.assertEqual(cpid, pid)
-            self.assertEqual(sts, 0)
+            support.wait_process(pid, exitcode=0)
 
     def test_libc_ver(self):
         # check that libc_ver(executable) doesn't raise an exception
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index fad26d8..be121ae 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -37,6 +37,7 @@ def _supports_sched():
 
 requires_sched = unittest.skipUnless(_supports_sched(), 'requires POSIX scheduler API')
 
+
 class PosixTester(unittest.TestCase):
 
     def setUp(self):
@@ -180,7 +181,6 @@ class PosixTester(unittest.TestCase):
 
     @unittest.skipUnless(getattr(os, 'execve', None) in os.supports_fd, "test needs execve() to support the fd parameter")
     @unittest.skipUnless(hasattr(os, 'fork'), "test needs os.fork()")
-    @unittest.skipUnless(hasattr(os, 'waitpid'), "test needs os.waitpid()")
     def test_fexecve(self):
         fp = os.open(sys.executable, os.O_RDONLY)
         try:
@@ -189,7 +189,7 @@ class PosixTester(unittest.TestCase):
                 os.chdir(os.path.split(sys.executable)[0])
                 posix.execve(fp, [sys.executable, '-c', 'pass'], os.environ)
             else:
-                self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+                support.wait_process(pid, exitcode=0)
         finally:
             os.close(fp)
 
@@ -1539,7 +1539,7 @@ class _PosixSpawnMixin:
             """
         args = self.python_args('-c', script)
         pid = self.spawn_func(args[0], args, os.environ)
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
         with open(pidfile) as f:
             self.assertEqual(f.read(), str(pid))
 
@@ -1569,7 +1569,7 @@ class _PosixSpawnMixin:
         args = self.python_args('-c', script)
         pid = self.spawn_func(args[0], args,
                               {**os.environ, 'foo': 'bar'})
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
         with open(envfile) as f:
             self.assertEqual(f.read(), 'bar')
 
@@ -1580,7 +1580,7 @@ class _PosixSpawnMixin:
             os.environ,
             file_actions=None
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_empty_file_actions(self):
         pid = self.spawn_func(
@@ -1589,7 +1589,7 @@ class _PosixSpawnMixin:
             os.environ,
             file_actions=[]
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_resetids_explicit_default(self):
         pid = self.spawn_func(
@@ -1598,7 +1598,7 @@ class _PosixSpawnMixin:
             os.environ,
             resetids=False
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_resetids(self):
         pid = self.spawn_func(
@@ -1607,7 +1607,7 @@ class _PosixSpawnMixin:
             os.environ,
             resetids=True
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_resetids_wrong_type(self):
         with self.assertRaises(TypeError):
@@ -1622,7 +1622,7 @@ class _PosixSpawnMixin:
             os.environ,
             setpgroup=os.getpgrp()
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_setpgroup_wrong_type(self):
         with self.assertRaises(TypeError):
@@ -1643,7 +1643,7 @@ class _PosixSpawnMixin:
             os.environ,
             setsigmask=[signal.SIGUSR1]
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_setsigmask_wrong_type(self):
         with self.assertRaises(TypeError):
@@ -1684,7 +1684,8 @@ class _PosixSpawnMixin:
         finally:
             os.close(wfd)
 
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
+
         output = os.read(rfd, 100)
         child_sid = int(output)
         parent_sid = os.getsid(os.getpid())
@@ -1707,10 +1708,7 @@ class _PosixSpawnMixin:
         finally:
             signal.signal(signal.SIGUSR1, original_handler)
 
-        pid2, status = os.waitpid(pid, 0)
-        self.assertEqual(pid2, pid)
-        self.assertTrue(os.WIFSIGNALED(status), status)
-        self.assertEqual(os.WTERMSIG(status), signal.SIGUSR1)
+        support.wait_process(pid, exitcode=-signal.SIGUSR1)
 
     def test_setsigdef_wrong_type(self):
         with self.assertRaises(TypeError):
@@ -1744,7 +1742,7 @@ class _PosixSpawnMixin:
             os.environ,
             scheduler=(None, os.sched_param(priority))
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     @requires_sched
     @unittest.skipIf(sys.platform.startswith(('freebsd', 'netbsd')),
@@ -1764,7 +1762,7 @@ class _PosixSpawnMixin:
             os.environ,
             scheduler=(policy, os.sched_param(priority))
         )
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_multiple_file_actions(self):
         file_actions = [
@@ -1776,7 +1774,7 @@ class _PosixSpawnMixin:
                               self.NOOP_PROGRAM,
                               os.environ,
                               file_actions=file_actions)
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+        support.wait_process(pid, exitcode=0)
 
     def test_bad_file_actions(self):
         args = self.NOOP_PROGRAM
@@ -1822,7 +1820,8 @@ class _PosixSpawnMixin:
         args = self.python_args('-c', script)
         pid = self.spawn_func(args[0], args, os.environ,
                               file_actions=file_actions)
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+
+        support.wait_process(pid, exitcode=0)
         with open(outfile) as f:
             self.assertEqual(f.read(), 'hello')
 
@@ -1840,7 +1839,8 @@ class _PosixSpawnMixin:
         args = self.python_args('-c', script)
         pid = self.spawn_func(args[0], args, os.environ,
                               file_actions=[(os.POSIX_SPAWN_CLOSE, 0)])
-        self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+
+        support.wait_process(pid, exitcode=0)
         with open(closefile) as f:
             self.assertEqual(f.read(), 'is closed %d' % errno.EBADF)
 
@@ -1858,7 +1858,7 @@ class _PosixSpawnMixin:
             args = self.python_args('-c', script)
             pid = self.spawn_func(args[0], args, os.environ,
                                   file_actions=file_actions)
-            self.assertEqual(os.waitpid(pid, 0), (pid, 0))
+            support.wait_process(pid, exitcode=0)
         with open(dupfile) as f:
             self.assertEqual(f.read(), 'hello')
 
@@ -1890,13 +1890,12 @@ class TestPosixSpawnP(unittest.TestCase, _PosixSpawnMixin):
         spawn_args = (program, '-I', '-S', '-c', 'pass')
         code = textwrap.dedent("""
             import os
+            from test import support
+
             args = %a
             pid = os.posix_spawnp(args[0], args, os.environ)
-            pid2, status = os.waitpid(pid, 0)
-            if pid2 != pid:
-                raise Exception(f"pid {pid2} != {pid}")
-            if status != 0:
-                raise Exception(f"status {status} != 0")
+
+            support.wait_process(pid, exitcode=0)
         """ % (spawn_args,))
 
         # Use a subprocess to test os.posix_spawnp() with a modified PATH
diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py
index c147105..548af70 100644
--- a/Lib/test/test_random.py
+++ b/Lib/test/test_random.py
@@ -1103,8 +1103,7 @@ class TestModule(unittest.TestCase):
                 child_val = eval(f.read())
             self.assertNotEqual(val, child_val)
 
-            pid, status = os.waitpid(pid, 0)
-            self.assertEqual(status, 0)
+            support.wait_process(pid, exitcode=0)
 
 
 if __name__ == "__main__":
diff --git a/Lib/test/test_socketserver.py b/Lib/test/test_socketserver.py
index 85382a0..f818df0 100644
--- a/Lib/test/test_socketserver.py
+++ b/Lib/test/test_socketserver.py
@@ -65,9 +65,7 @@ def simple_subprocess(testcase):
     except:
         raise
     finally:
-        pid2, status = os.waitpid(pid, 0)
-        testcase.assertEqual(pid2, pid)
-        testcase.assertEqual(72 << 8, status)
+        test.support.wait_process(pid, exitcode=72)
 
 
 class SocketServerTest(unittest.TestCase):
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index 0093a49..4184665 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -408,8 +408,7 @@ class BasicSocketTests(unittest.TestCase):
         else:
             os.close(wfd)
             self.addCleanup(os.close, rfd)
-            _, status = os.waitpid(pid, 0)
-            self.assertEqual(status, 0)
+            support.wait_process(pid, exitcode=0)
 
             child_random = os.read(rfd, 16)
             self.assertEqual(len(child_random), 16)
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 868f279..7cf31e1 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -3114,12 +3114,10 @@ class POSIXProcessTestCase(BaseTestCase):
         proc = subprocess.Popen(args)
 
         # Wait until the real process completes to avoid zombie process
-        pid = proc.pid
-        pid, status = os.waitpid(pid, 0)
-        self.assertEqual(status, 0)
+        support.wait_process(proc.pid, exitcode=0)
 
         status = _testcapi.W_STOPCODE(3)
-        with mock.patch('subprocess.os.waitpid', return_value=(pid, status)):
+        with mock.patch('subprocess.os.waitpid', return_value=(proc.pid, status)):
             returncode = proc.wait()
 
         self.assertEqual(returncode, -3)
@@ -3130,10 +3128,7 @@ class POSIXProcessTestCase(BaseTestCase):
         proc = subprocess.Popen(ZERO_RETURN_CMD)
 
         # wait until the process completes without using the Popen APIs.
-        pid, status = os.waitpid(proc.pid, 0)
-        self.assertEqual(pid, proc.pid)
-        self.assertTrue(os.WIFEXITED(status), status)
-        self.assertEqual(os.WEXITSTATUS(status), 0)
+        support.wait_process(proc.pid, exitcode=0)
 
         # returncode is still None but the process completed.
         self.assertIsNone(proc.returncode)
diff --git a/Lib/test/test_support.py b/Lib/test/test_support.py
index 175f7c8..99a4cad 100644
--- a/Lib/test/test_support.py
+++ b/Lib/test/test_support.py
@@ -176,13 +176,10 @@ class TestSupport(unittest.TestCase):
             with support.temp_cwd() as temp_path:
                 pid = os.fork()
                 if pid != 0:
-                    # parent process (child has pid == 0)
+                    # parent process
 
                     # wait for the child to terminate
-                    (pid, status) = os.waitpid(pid, 0)
-                    if status != 0:
-                        raise AssertionError(f"Child process failed with exit "
-                                             f"status indication 0x{status:x}.")
+                    support.wait_process(pid, exitcode=0)
 
                     # Make sure that temp_path is still present. When the child
                     # process leaves the 'temp_cwd'-context, the __exit__()-
diff --git a/Lib/test/test_tempfile.py b/Lib/test/test_tempfile.py
index 5fe9506..524ab7c 100644
--- a/Lib/test/test_tempfile.py
+++ b/Lib/test/test_tempfile.py
@@ -200,15 +200,7 @@ class TestRandomNameSequence(BaseTestCase):
             child_value = os.read(read_fd, len(parent_value)).decode("ascii")
         finally:
             if pid:
-                # best effort to ensure the process can't bleed out
-                # via any bugs above
-                try:
-                    os.kill(pid, signal.SIGKILL)
-                except OSError:
-                    pass
-
-                # Read the process exit status to avoid zombie process
-                os.waitpid(pid, 0)
+                support.wait_process(pid, exitcode=0)
 
             os.close(read_fd)
             os.close(write_fd)
diff --git a/Lib/test/test_tracemalloc.py b/Lib/test/test_tracemalloc.py
index 7283d9c..635a9d3 100644
--- a/Lib/test/test_tracemalloc.py
+++ b/Lib/test/test_tracemalloc.py
@@ -314,10 +314,7 @@ class TestTracemallocEnabled(unittest.TestCase):
             finally:
                 os._exit(exitcode)
         else:
-            pid2, status = os.waitpid(pid, 0)
-            self.assertTrue(os.WIFEXITED(status))
-            exitcode = os.WEXITSTATUS(status)
-            self.assertEqual(exitcode, 0)
+            support.wait_process(pid, exitcode=0)
 
 
 class TestSnapshot(unittest.TestCase):
diff --git a/Lib/test/test_uuid.py b/Lib/test/test_uuid.py
index 27fc56d..0b267f4 100644
--- a/Lib/test/test_uuid.py
+++ b/Lib/test/test_uuid.py
@@ -657,7 +657,7 @@ class BaseTestUUID:
             os.close(fds[1])
             self.addCleanup(os.close, fds[0])
             parent_value = self.uuid.uuid4().hex
-            os.waitpid(pid, 0)
+            support.wait_process(pid, exitcode=0)
             child_value = os.read(fds[0], 100).decode('latin-1')
 
             self.assertNotEqual(parent_value, child_value)
diff --git a/Misc/NEWS.d/next/Tests/2020-03-31-18-57-52.bpo-40094.m3fTJe.rst b/Misc/NEWS.d/next/Tests/2020-03-31-18-57-52.bpo-40094.m3fTJe.rst
new file mode 100644
index 0000000..cae001b
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2020-03-31-18-57-52.bpo-40094.m3fTJe.rst
@@ -0,0 +1 @@
+Add :func:`test.support.wait_process` function.