bpo-33532: Fix test_multiprocessing_forkserver.test_ignore() (GH-7319)

Use also support.SOCK_MAX_SIZE, not only support.PIPE_MAX_SIZE, to
get the size for a blocking send into a multiprocessing pipe.
(cherry picked from commit 252f6ab)

Co-authored-by: Victor Stinner <vstinner@redhat.com>

Author: miss-islington

Lib/test/_test_multiprocessing.py

@@ -4295,6 +4295,9 @@ def test_closefd(self):
 
 class TestIgnoreEINTR(unittest.TestCase):
 
+    # Sending CONN_MAX_SIZE bytes into a multiprocessing pipe must block
+    CONN_MAX_SIZE = max(support.PIPE_MAX_SIZE, support.SOCK_MAX_SIZE)
+
     @classmethod
     def _test_ignore(cls, conn):
         def handler(signum, frame):
@@ -4303,7 +4306,7 @@ def handler(signum, frame):
         conn.send('ready')
         x = conn.recv()
         conn.send(x)
-        conn.send_bytes(b'x' * support.PIPE_MAX_SIZE)
+        conn.send_bytes(b'x' * cls.CONN_MAX_SIZE)
 
     @unittest.skipUnless(hasattr(signal, 'SIGUSR1'), 'requires SIGUSR1')
     def test_ignore(self):
@@ -4322,7 +4325,7 @@ def test_ignore(self):
             self.assertEqual(conn.recv(), 1234)
             time.sleep(0.1)
             os.kill(p.pid, signal.SIGUSR1)
-            self.assertEqual(conn.recv_bytes(), b'x' * support.PIPE_MAX_SIZE)
+            self.assertEqual(conn.recv_bytes(), b'x' * self.CONN_MAX_SIZE)
             time.sleep(0.1)
             p.join()
         finally: