bpo-35424: Fix test_multiprocessing_main_handling (GH-11223)

Fix test_multiprocessing_main_handling: use multiprocessing.Pool with
a context manager and then explicitly join the pool.

Lib/test/test_multiprocessing_main_handling.py

@@ -54,9 +54,9 @@ if "check_sibling" in __file__:
 if __name__ == '__main__':
     start_method = sys.argv[1]
     set_start_method(start_method)
-    p = Pool(5)
     results = []
-    p.map_async(f, [1, 2, 3], callback=results.extend)
+    with Pool(5) as pool:
+        pool.map_async(f, [1, 2, 3], callback=results.extend)
         start_time = time.monotonic()
         while not results:
             time.sleep(0.05)
@@ -64,8 +64,11 @@ if __name__ == '__main__':
             dt = time.monotonic() - start_time
             if dt > 60.0:
                 raise RuntimeError("Timed out waiting for results (%.1f sec)" % dt)
+
     results.sort()
     print(start_method, "->", results)
+
+    pool.join()
 """
 
 test_source_main_skipped_in_children = """\
@@ -84,18 +87,21 @@ from multiprocessing import Pool, set_start_method
 
 start_method = sys.argv[1]
 set_start_method(start_method)
-p = Pool(5)
 results = []
-p.map_async(int, [1, 4, 9], callback=results.extend)
-start_time = time.monotonic()
-while not results:
+with Pool(5) as pool:
+    pool.map_async(int, [1, 4, 9], callback=results.extend)
+    start_time = time.monotonic()
+    while not results:
         time.sleep(0.05)
         # up to 1 min to report the results
         dt = time.monotonic() - start_time
         if dt > 60.0:
             raise RuntimeError("Timed out waiting for results (%.1f sec)" % dt)
+
 results.sort()
 print(start_method, "->", results)
+
+pool.join()
 """
 
 # These helpers were copied from test_cmd_line_script & tweaked a bit...

Misc/NEWS.d/next/Tests/2018-12-18-22-36-53.bpo-35424.1Pz4IS.rst

@@ -0,0 +1,2 @@
+Fix test_multiprocessing_main_handling: use :class:`multiprocessing.Pool` with
+a context manager and then explicitly join the pool.