I was debugging against a remote stub in async mode, using the TUI, and I
issued "quit" while still connected, in the hopes of bailing out of GDB
without much fuss, but, I ran into this SEGV:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fcf597fb6e0 (LWP 13869)]
0x0000000000552f5e in serial_can_async_p (scb=0x0) at ../../src/gdb/serial.c:504
504       return (scb->ops->async != NULL);
(top-gdb) bt
#0  0x0000000000552f5e in serial_can_async_p (scb=0x0) at ../../src/gdb/serial.c:504
During symbol reading, unsupported tag: 'DW_TAG_const_type'.
#1  0x000000000042ffa3 in remote_can_async_p () at ../../src/gdb/remote.c:8207
During symbol reading, DW_AT_type missing from DW_TAG_subrange_type.
#2  0x000000000040d740 in execute_command (p=0x7fff618212a0 "set width 118", from_tty=0) at ../../src/gdb/top.c:382
#3  0x0000000000457d1c in tui_update_gdb_sizes () at ../../src/gdb/tui/tui-win.c:468
#4  0x0000000000452002 in tui_disable () at ../../src/gdb/tui/tui.c:455
#5  0x0000000000452581 in tui_exit () at ../../src/gdb/tui/tui-interp.c:45
#6  0x00007fcf589e0110 in exit () from /lib/libc.so.6
#7  0x000000000040edeb in quit_force (args=0x0, from_tty=1) at ../../src/gdb/top.c:1292
#8  0x0000000000443f20 in quit_command (args=0x0, from_tty=1) at ../../src/gdb/cli/cli-cmds.c:315
#9  0x000000000043e660 in do_cfunc (c=0xa344b0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:60
#10 0x00000000004412a3 in cmd_func (cmd=0xa344b0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:1680
#11 0x000000000040d9df in execute_command (p=0x9e81c1 "", from_tty=1) at ../../src/gdb/top.c:465
#12 0x00000000004c19ea in command_handler (command=0x9e81c0 "q") at ../../src/gdb/event-top.c:514

As you can see from from #2, disabling the tui ends up
executing a command.  At this point, the remote target was still
pushed on the stack, but it had already been closed, hence the remote
connection was closed, and remote_desc was == NULL.

There are target_can_async_p call around every command invocation,
and since the remote target was still on the stack, remote_can_async_p
was called, but it isn't expecting to be reached with remote_desc == NULL,
as most of remote's code:

 static int
 remote_can_async_p (void)
 {
   if (!remote_async_permitted)
     /* We only enable async when the user specifically asks for it.  */
     return 0;
 
   /* We're async whenever the serial device is.  */
   return remote_async_mask_value && serial_can_async_p (remote_desc);
 }

The issue is that quit_target closes the current target, but doesn't
pop it:

 static int
 quit_target (void *arg)
 {
   struct qt_args *qt = (struct qt_args *)arg;
 
   if (! ptid_equal (inferior_ptid, null_ptid) && target_has_execution)
     {
       if (attach_flag)
         target_detach (qt->args, qt->from_tty);
       else
         target_kill ();
     }

   /* UDI wants this, to kill the TIP.  */
   target_close (&current_target, 1);

   /* Save the history information if it is appropriate to do so.  */
   if (write_history_p && history_filename)
     write_history (history_filename);

   do_final_cleanups (ALL_CLEANUPS);	/* Do any final cleanups before exiting */

   return 0;
 }


The UDI target seems to be gone from the sources, but, even if it's still
here and I missed it, this is broken.  It should at best be a pop_target.  But,
while I'm at it, if the current target is of stratum > process_stratum, this
will still leave the process_stratum target open, which is much
more likelly to need closing.  So, I've come up with a new
pop_all_targets method and used it instead.

Tested against x86_64-unknown-linux-gnu native and gdbserver.

OK?

-- 
Pedro Alves