diff --git a/Doc/library/os.rst b/Doc/library/os.rst
@@ -990,6 +990,17 @@ as internal buffering of data.
    .. versionadded:: 3.3
 
 
+.. function:: login_tty(fd)
+
+   Prepare the tty of which fd is a file descriptor for a new login session.
+   Make the calling process a session leader; make the tty the controlling tty,
+   the stdin, the stdout, and the stderr of the calling process; close fd.
+
+   .. availability:: Unix.
+
+   .. versionadded:: 3.11
+
+
 .. function:: lseek(fd, pos, how)
 
    Set the current position of file descriptor *fd* to position *pos*, modified

diff --git a/Misc/NEWS.d/next/Library/2020-12-11-04-56-12.bpo-41818.bVX3sO.rst b/Misc/NEWS.d/next/Library/2020-12-11-04-56-12.bpo-41818.bVX3sO.rst
@@ -0,0 +1 @@
+Soumendra Ganguly: New function os.login_tty() for Unix.
diff --git a/Modules/clinic/posixmodule.c.h b/Modules/clinic/posixmodule.c.h
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -7118,22 +7118,21 @@ os_sched_getaffinity_impl(PyObject *module, pid_t pid)
 #  define DEV_PTY_FILE "/dev/ptmx"
 #endif
 
-#if defined(HAVE_OPENPTY) || defined(HAVE_FORKPTY) || defined(HAVE_DEV_PTMX)
-#ifdef HAVE_PTY_H
+#if defined(HAVE_OPENPTY) || defined(HAVE_FORKPTY) || defined(HAVE_LOGIN_TTY) || defined(HAVE_DEV_PTMX)
+#if defined(HAVE_PTY_H)
 #include <pty.h>
-#else
-#ifdef HAVE_LIBUTIL_H
+#if defined(HAVE_UTMP_H)
+#include <utmp.h>
+#endif /* HAVE_UTMP_H */
+#elif defined(HAVE_LIBUTIL_H)
 #include <libutil.h>
-#else
-#ifdef HAVE_UTIL_H
+#elif defined(HAVE_UTIL_H)
 #include <util.h>
-#endif /* HAVE_UTIL_H */
-#endif /* HAVE_LIBUTIL_H */
 #endif /* HAVE_PTY_H */
 #ifdef HAVE_STROPTS_H
 #include <stropts.h>
 #endif
-#endif /* defined(HAVE_OPENPTY) || defined(HAVE_FORKPTY) || defined(HAVE_DEV_PTMX) */
+#endif /* if defined(HAVE_OPENPTY) || defined(HAVE_FORKPTY) || defined(HAVE_LOGIN_TTY) || defined(HAVE_DEV_PTMX) */
 
 
 #if defined(HAVE_OPENPTY) || defined(HAVE__GETPTY) || defined(HAVE_DEV_PTMX)
@@ -7235,6 +7234,84 @@ os_openpty_impl(PyObject *module)
 }
 #endif /* defined(HAVE_OPENPTY) || defined(HAVE__GETPTY) || defined(HAVE_DEV_PTMX) */
 
+#if defined(HAVE_SETSID)
+#if defined(TIOCSCTTY) || defined(HAVE_TTYNAME)
+#define HAVE_FALLBACK_LOGIN_TTY 1
+#endif /* defined(TIOCSCTTY) || defined(HAVE_TTYNAME) */
+#endif /* HAVE_SETSID */
+
+#if defined(HAVE_LOGIN_TTY) || defined(HAVE_FALLBACK_LOGIN_TTY)
+/*[clinic input]
+os.login_tty
+
+    fd: fildes
+    /
+
+Prepare the tty of which fd is a file descriptor for a new login session.
+
+Make the calling process a session leader; make the tty the
+controlling tty, the stdin, the stdout, and the stderr of the
+calling process; close fd.
+[clinic start generated code]*/
+
+static PyObject *
+os_login_tty_impl(PyObject *module, int fd)
+/*[clinic end generated code: output=495a79911b4cc1bc input=5f298565099903a2]*/
+{
+#if defined(HAVE_LOGIN_TTY)
+    if (login_tty(fd) == -1) {
+        return posix_error();
+    }
+#else /* defined(HAVE_FALLBACK_LOGIN_TTY) */
+    /* Establish a new session. */
+    if (setsid() == -1) {
+        return posix_error();
+    }
+
+    /* The tty becomes the controlling terminal. */
+#if defined(TIOCSCTTY)
+    if (ioctl(fd, TIOCSCTTY, (char *)NULL) == -1) {
+        return posix_error();
+    }
+#else /* defined(HAVE_TTYNAME) */
+    /* Fallback method (archaic); from Advanced Programming in the UNIX(R)
+     * Environment, Third edition, 2013, Section 9.6 - Controlling Terminal:
+     * "Systems derived from UNIX System V allocate the controlling
+     * terminal for a session when the session leader opens the first
+     * terminal device that is not already associated with a session, as
+     * long as the call to open does not specify the O_NOCTTY flag." */
+    char *tmppath = ttyname(fd);
+    if (tmppath == NULL) {
+        return posix_error();
+    }
+
+#define CLOSE_IF_NOT_FD(otherfd) \
+    if (fd != otherfd) { \
+        close(otherfd); \
+    } \
+
+    CLOSE_IF_NOT_FD(0);
+    CLOSE_IF_NOT_FD(1);
+    CLOSE_IF_NOT_FD(2);
+
+    int tmpfd = open(tmppath, O_RDWR);
+    if (tmpfd == -1) {
+        return posix_error();
+    }
+    close(tmpfd);
+#endif /* defined(TIOCSCTTY) */
+
+    /* The tty becomes stdin/stdout/stderr */
+    if (dup2(fd, 0) == -1 || dup2(fd, 1) == -1 || dup2(fd, 2) == -1) {
+        return posix_error();
+    }
+    if (fd > 2) {
+        close(fd);
+    }
+#endif /* defined(HAVE_LOGIN_TTY) */
+    Py_RETURN_NONE;
+}
+#endif /* defined(HAVE_LOGIN_TTY) || defined(HAVE_FALLBACK_LOGIN_TTY) */
 
 #ifdef HAVE_FORKPTY
 /*[clinic input]
@@ -7271,8 +7348,9 @@ os_forkpty_impl(PyObject *module)
         /* parent: release the import lock. */
         PyOS_AfterFork_Parent();
     }
-    if (pid == -1)
+    if (pid == -1) {
         return posix_error();
+    }
     return Py_BuildValue("(Ni)", PyLong_FromPid(pid), master_fd);
 }
 #endif /* HAVE_FORKPTY */
@@ -7613,7 +7691,7 @@ os_initgroups_impl(PyObject *module, PyObject *oname, gid_t gid)
     const char *username = PyBytes_AS_STRING(oname);
 
     if (initgroups(username, gid) == -1)
-        return PyErr_SetFromErrno(PyExc_OSError);
+        return posix_error();
 
     Py_RETURN_NONE;
 }
@@ -14652,6 +14730,7 @@ static PyMethodDef posix_methods[] = {
     OS_SCHED_GETAFFINITY_METHODDEF
     OS_OPENPTY_METHODDEF
     OS_FORKPTY_METHODDEF
+    OS_LOGIN_TTY_METHODDEF
     OS_GETEGID_METHODDEF
     OS_GETEUID_METHODDEF
     OS_GETGID_METHODDEF

@@ -787,6 +787,7 @@ infodir
 docdir
 oldincludedir
 includedir
+runstatedir
 localstatedir
 sharedstatedir
 sysconfdir
@@ -905,6 +906,7 @@ datadir='${datarootdir}'
 sysconfdir='${prefix}/etc'
 sharedstatedir='${prefix}/com'
 localstatedir='${prefix}/var'
+runstatedir='${localstatedir}/run'
 includedir='${prefix}/include'
 oldincludedir='/usr/include'
 docdir='${datarootdir}/doc/${PACKAGE_TARNAME}'
@@ -1157,6 +1159,15 @@ do
   | -silent | --silent | --silen | --sile | --sil)
     silent=yes ;;
 
+  -runstatedir | --runstatedir | --runstatedi | --runstated \
+  | --runstate | --runstat | --runsta | --runst | --runs \
+  | --run | --ru | --r)
+    ac_prev=runstatedir ;;
+  -runstatedir=* | --runstatedir=* | --runstatedi=* | --runstated=* \
+  | --runstate=* | --runstat=* | --runsta=* | --runst=* | --runs=* \
+  | --run=* | --ru=* | --r=*)
+    runstatedir=$ac_optarg ;;
+
   -sbindir | --sbindir | --sbindi | --sbind | --sbin | --sbi | --sb)
     ac_prev=sbindir ;;
   -sbindir=* | --sbindir=* | --sbindi=* | --sbind=* | --sbin=* \
@@ -1294,7 +1305,7 @@ fi
 for ac_var in	exec_prefix prefix bindir sbindir libexecdir datarootdir \
 		datadir sysconfdir sharedstatedir localstatedir includedir \
 		oldincludedir docdir infodir htmldir dvidir pdfdir psdir \
-		libdir localedir mandir
+		libdir localedir mandir runstatedir
 do
   eval ac_val=\$$ac_var
   # Remove trailing slashes.
@@ -1447,6 +1458,7 @@ Fine tuning of the installation directories:
   --sysconfdir=DIR        read-only single-machine data [PREFIX/etc]
   --sharedstatedir=DIR    modifiable architecture-independent data [PREFIX/com]
   --localstatedir=DIR     modifiable single-machine data [PREFIX/var]
+  --runstatedir=DIR       modifiable per-process data [LOCALSTATEDIR/run]
   --libdir=DIR            object code libraries [EPREFIX/lib]
   --includedir=DIR        C header files [PREFIX/include]
   --oldincludedir=DIR     C header files for non-gcc [/usr/include]
@@ -8041,7 +8053,7 @@ sys/audioio.h sys/xattr.h sys/bsdtty.h sys/event.h sys/file.h sys/ioctl.h \
 sys/kern_control.h sys/loadavg.h sys/lock.h sys/mkdev.h sys/modem.h \
 sys/param.h sys/random.h sys/select.h sys/sendfile.h sys/socket.h sys/statvfs.h \
 sys/stat.h sys/syscall.h sys/sys_domain.h sys/termio.h sys/time.h \
-sys/times.h sys/types.h sys/uio.h sys/un.h sys/utsname.h sys/wait.h pty.h \
+sys/times.h sys/types.h sys/uio.h sys/un.h sys/utsname.h sys/wait.h pty.h utmp.h \
 libutil.h sys/resource.h netpacket/packet.h sysexits.h bluetooth.h \
 linux/tipc.h linux/random.h spawn.h util.h alloca.h endian.h \
 sys/endian.h sys/sysmacros.h linux/memfd.h linux/wait.h sys/memfd.h \
@@ -12735,7 +12747,7 @@ $as_echo "no" >&6; }
 fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 
-# check for openpty and forkpty
+# check for openpty, login_tty, and forkpty
 
 for ac_func in openpty
 do :
@@ -12831,6 +12843,103 @@ fi
 fi
 
 
+fi
+done
+
+for ac_func in login_tty
+do :
+  ac_fn_c_check_func "$LINENO" "login_tty" "ac_cv_func_login_tty"
+if test "x$ac_cv_func_login_tty" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_LOGIN_TTY 1
+_ACEOF
+
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for login_tty in -lutil" >&5
+$as_echo_n "checking for login_tty in -lutil... " >&6; }
+if ${ac_cv_lib_util_login_tty+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_check_lib_save_LIBS=$LIBS
+LIBS="-lutil  $LIBS"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char login_tty ();
+int
+main ()
+{
+return login_tty ();
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_lib_util_login_tty=yes
+else
+  ac_cv_lib_util_login_tty=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+LIBS=$ac_check_lib_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_util_login_tty" >&5
+$as_echo "$ac_cv_lib_util_login_tty" >&6; }
+if test "x$ac_cv_lib_util_login_tty" = xyes; then :
+  $as_echo "#define HAVE_LOGIN_TTY 1" >>confdefs.h
+ LIBS="$LIBS -lutil"
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for login_tty in -lbsd" >&5
+$as_echo_n "checking for login_tty in -lbsd... " >&6; }
+if ${ac_cv_lib_bsd_login_tty+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_check_lib_save_LIBS=$LIBS
+LIBS="-lbsd  $LIBS"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char login_tty ();
+int
+main ()
+{
+return login_tty ();
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_lib_bsd_login_tty=yes
+else
+  ac_cv_lib_bsd_login_tty=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+LIBS=$ac_check_lib_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_bsd_login_tty" >&5
+$as_echo "$ac_cv_lib_bsd_login_tty" >&6; }
+if test "x$ac_cv_lib_bsd_login_tty" = xyes; then :
+  $as_echo "#define HAVE_LOGIN_TTY 1" >>confdefs.h
+ LIBS="$LIBS -lbsd"
+fi
+
+
+fi
+
+
 fi
 done
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Soumendra Ganguly: New function os.login_tty() for Unix.