posix: Add terminal control setting support for posix_spawn

Currently there is no proper way to set the controlling terminal through
posix_spawn in race free manner [1].  This forces shell implementations
to keep using fork+exec when launching background process groups,
even when using posix_spawn yields better performance.

This patch adds a new GNU extension so the creating process can
configure the created process terminal group.  This is done with a new
flag, POSIX_SPAWN_TCSETPGROUP, along with two new attribute functions:
posix_spawnattr_tcsetpgrp_np, and posix_spawnattr_tcgetpgrp_np.
The function sets a new attribute, spawn-tcgroupfd, that references to
the controlling terminal.

The controlling terminal is set after the spawn-pgroup attribute, and
uses the spawn-tcgroupfd along with current creating process group
(so it is composable with POSIX_SPAWN_SETPGROUP).

To create a process and set the controlling terminal, one can use the
following sequence:

    posix_spawnattr_t attr;
    posix_spawnattr_init (&attr);
    posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP);
    posix_spawnattr_tcsetpgrp_np (&attr, tcfd);

If the idea is also to create a new process groups:

    posix_spawnattr_t attr;
    posix_spawnattr_init (&attr);
    posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP
				     | POSIX_SPAWN_SETPGROUP);
    posix_spawnattr_tcsetpgrp_np (&attr, tcfd);
    posix_spawnattr_setpgroup (&attr, 0);

The controlling terminal file descriptor is ignored if the new flag is
not set.

This interface is slight different than the one provided by QNX [2],
which only provides the POSIX_SPAWN_TCSETPGROUP flag.  The QNX
documentation does not specify how the controlling terminal is obtained
nor how it iteracts with POSIX_SPAWN_SETPGROUP.  Since a glibc
implementation is library based, it is more straightforward and avoid
requires additional file descriptor operations to request the caller
to setup the controlling terminal file descriptor (and it also allows
a bit less error handling by posix_spawn).

Checked on x86_64-linux-gnu and i686-linux-gnu.

[1] https://github.com/ksh93/ksh/issues/79
[2] https://www.qnx.com/developers/docs/7.0.0/index.html#com.qnx.doc.neutrino.lib_ref/topic/p/posix_spawn.html

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 15 insertions, 1 deletions

diff --git a/posix/spawn.h b/posix/spawn.h
index 58f17d1277..7779020250 100644
--- a/posix/spawn.h
+++ b/posix/spawn.h
@@ -34,7 +34,8 @@ typedef struct
   sigset_t __ss;
   struct sched_param __sp;
   int __policy;
-  int __pad[16];
+  int __ctty_fd;
+  int __pad[15];
 } posix_spawnattr_t;
 
 
@@ -59,6 +60,7 @@ typedef struct
 #ifdef __USE_GNU
 # define POSIX_SPAWN_USEVFORK		0x40
 # define POSIX_SPAWN_SETSID		0x80
+# define POSIX_SPAWN_TCSETPGROUP	0x100
 #endif
 
 
@@ -166,6 +168,18 @@ extern int posix_spawnattr_setschedparam (posix_spawnattr_t *__restrict __attr,
 					  __restrict __schedparam)
      __THROW __nonnull ((1, 2));
 
+#ifdef __USE_GNU
+/* Make the spawned process the foreground process group on the terminal
+   associated with FD (which must be a controlling terminal, and still be
+   associated with its session).  */
+extern int posix_spawnattr_tcsetpgrp_np (posix_spawnattr_t *__attr, int fd)
+     __THROW __nonnull ((1));
+
+/* Return the associated terminal FD in the attribute structure.  */
+extern int posix_spawnattr_tcgetpgrp_np (const posix_spawnattr_t *
+					 __restrict __attr, int *fd)
+     __THROW __nonnull ((1, 2));
+#endif
 
 /* Initialize data structure for file attribute for `spawn' call.  */
 extern int posix_spawn_file_actions_init (posix_spawn_file_actions_t *