gh-139184: set O_CLOEXEC for master_fd when call os.forkpty

Lib/test/test_pty.py

@@ -12,6 +12,7 @@
     raise unittest.SkipTest("pty is not available on this platform")
 
 import errno
+import fcntl
 import os
 import pty
 import tty
@@ -230,6 +231,10 @@ def test_fork(self):
                 os._exit(2)
             os._exit(4)
         else:
+            flags = fcntl.fcntl(master_fd, fcntl.F_GETFD)
+            cloexec_set = bool(flags & fcntl.FD_CLOEXEC)
+            flags = fcntl.fcntl(master_fd, fcntl.F_GETFD)
+            self.assertTrue(flags & fcntl.FD_CLOEXEC)
             debug("Waiting for child (%d) to finish." % pid)
             # In verbose mode, we have to consume the debug output from the
             # child or the child will block, causing this test to hang in the

Misc/NEWS.d/next/Library/2025-09-29-14-15-20.gh-issue-139184.dNl9O4.rst

@@ -0,0 +1 @@
+Set O_CLOEXEC for master_fd when call os.forkpty

Modules/posixmodule.c

@@ -9059,14 +9059,22 @@ os_forkpty_impl(PyObject *module)
     } else {
         /* parent: release the import lock. */
         PyOS_AfterFork_Parent();
+        /* set O_CLOEXEC on master_fd */
+        if (_Py_set_inheritable(master_fd, 0, NULL) < 0)
+            goto error;
         // After PyOS_AfterFork_Parent() starts the world to avoid deadlock.
         if (warn_about_fork_with_threads("forkpty") < 0)
             return NULL;
     }
     if (pid == -1) {
         return posix_error();
     }
+
     return Py_BuildValue("(Ni)", PyLong_FromPid(pid), master_fd);
+error:
+    if (master_fd != -1)
+        close(master_fd);
+    return NULL;
 }
 #endif /* HAVE_FORKPTY */