bpo-31234: Enhance test_thread.test_forkinthread() (#3516)

* test_thread.test_forkinthread() now waits until the thread completes.
* Check the status in the test method, not in the thread function
* Don't ignore RuntimeError anymore: since the commit
  346cbd35 (bpo-16500,
  os.register_at_fork(), os.fork() cannot fail anymore with
  RuntimeError.
* Replace 0.01 literal with a new POLL_SLEEP constant
* test_forkinthread(): test if os.fork() exists rather than testing
  the platform.

Lib/test/test_thread.py

@@ -11,6 +11,7 @@ from test import lock_tests
 
 NUMTASKS = 10
 NUMTRIPS = 3
+POLL_SLEEP = 0.010 # seconds = 10 ms
 
 _print_mutex = thread.allocate_lock()
 
@@ -114,7 +115,7 @@ class ThreadRunningTests(BasicThreadTest):
             mut.release()
         thread.start_new_thread(task, ())
         while not started:
-            time.sleep(0.01)
+            time.sleep(POLL_SLEEP)
         self.assertEqual(thread._count(), orig + 1)
         # Allow the task to finish.
         mut.release()
@@ -125,7 +126,7 @@ class ThreadRunningTests(BasicThreadTest):
         wr = weakref.ref(task, lambda _: done.append(None))
         del task
         while not done:
-            time.sleep(0.01)
+            time.sleep(POLL_SLEEP)
         self.assertEqual(thread._count(), orig)
 
     def test_save_exception_state_on_error(self):
@@ -148,7 +149,7 @@ class ThreadRunningTests(BasicThreadTest):
             thread.start_new_thread(task, ())
             started.acquire()
             while thread._count() > c:
-                time.sleep(0.01)
+                time.sleep(POLL_SLEEP)
         self.assertIn("Traceback", stderr.getvalue())
 
 
@@ -221,30 +222,36 @@ class TestForkInThread(unittest.TestCase):
     def setUp(self):
         self.read_fd, self.write_fd = os.pipe()
 
-    @unittest.skipIf(sys.platform.startswith('win'),
-                     "This test is only appropriate for POSIX-like systems.")
+    @unittest.skipUnless(hasattr(os, 'fork'), 'need os.fork')
     @support.reap_threads
     def test_forkinthread(self):
+        running = True
+        status = "not set"
+
         def thread1():
-            try:
-                pid = os.fork() # fork in a thread
-            except RuntimeError:
-                os._exit(1) # exit the child
+            nonlocal running, status
 
-            if pid == 0: # child
+            # fork in a thread
+            pid = os.fork()
+            if pid == 0:
+                # child
                 try:
                     os.close(self.read_fd)
                     os.write(self.write_fd, b"OK")
                 finally:
                     os._exit(0)
-            else: # parent
+            else:
+                # parent
                 os.close(self.write_fd)
                 pid, status = os.waitpid(pid, 0)
-                self.assertEqual(status, 0)
+                running = False
 
         thread.start_new_thread(thread1, ())
         self.assertEqual(os.read(self.read_fd, 2), b"OK",
                          "Unable to fork() in thread")
+        while running:
+            time.sleep(POLL_SLEEP)
+        self.assertEqual(status, 0)
 
     def tearDown(self):
         try: