bpo-40140: test_builtin.PtyTests registers SIGHUP handler (GH-19314)

test_builtin.PtyTests now registers an handler for SIGHUP signal.
Closing the PTY file descriptor can emit a SIGHUP signal: just ignore
it.

run_child() now also closes the PTY file descriptor before waiting
for the process completition, otherwise the test hangs on AIX.

Lib/test/test_builtin.py

@@ -1837,7 +1837,21 @@ class PtyTests(unittest.TestCase):
     """Tests that use a pseudo terminal to guarantee stdin and stdout are
     terminals in the test environment"""
 
+    @staticmethod
+    def handle_sighup(signum, frame):
+        # bpo-40140: if the process is the session leader, os.close(fd)
+        # of "pid, fd = pty.fork()" can raise SIGHUP signal:
+        # just ignore the signal.
+        pass
+
     def run_child(self, child, terminal_input):
+        old_sighup = signal.signal(signal.SIGHUP, self.handle_sighup)
+        try:
+            return self._run_child(child, terminal_input)
+        finally:
+            signal.signal(signal.SIGHUP, old_sighup)
+
+    def _run_child(self, child, terminal_input):
         r, w = os.pipe()  # Pipe test results from child back to parent
         try:
             pid, fd = pty.fork()
@@ -1893,13 +1907,12 @@ class PtyTests(unittest.TestCase):
             self.fail("got %d lines in pipe but expected 2, child output was:\n%s"
                       % (len(lines), child_output))
 
-        # Wait until the child process completes before closing the PTY to
-        # prevent sending SIGHUP to the child process.
-        support.wait_process(pid, exitcode=0)
-
-        # Close the PTY
+        # bpo-40155: Close the PTY before waiting for the child process
+        # completion, otherwise the child process hangs on AIX.
         os.close(fd)
 
+        support.wait_process(pid, exitcode=0)
+
         return lines
 
     def check_input_tty(self, prompt, terminal_input, stdio_encoding=None):

Lib/test/test_pty.py

@@ -70,7 +70,7 @@ class PtyTest(unittest.TestCase):
         self.addCleanup(signal.signal, signal.SIGALRM, old_alarm)
 
         old_sighup = signal.signal(signal.SIGHUP, self.handle_sighup)
-        self.addCleanup(signal.signal, signal.SIGHUP, old_alarm)
+        self.addCleanup(signal.signal, signal.SIGHUP, old_sighup)
 
         # isatty() and close() can hang on some platforms. Set an alarm
         # before running the test to make sure we don't hang forever.
@@ -81,8 +81,8 @@ class PtyTest(unittest.TestCase):
         self.fail("isatty hung")
 
     @staticmethod
-    def handle_sighup(sig, frame):
-        # if the process is the session leader, os.close(master_fd)
+    def handle_sighup(signum, frame):
+        # bpo-38547: if the process is the session leader, os.close(master_fd)
         # of "master_fd, slave_name = pty.master_open()" raises SIGHUP
         # signal: just ignore the signal.
         pass