Re: [PATCH 1/2] [gdb] Fix sig_write for null gdb_stderr

[Tom de Vries <tdevries@suse.de>]

On 4/29/25 17:34, Tom Tromey wrote:
>>>>>> "Tom" == Tom de Vries <tdevries@suse.de> writes:
> 
> Tom> - gdbpy_flush attempts to flush gdb_stdout, which is nullptr
> Tom> - that causes a segfault
> 
> When can this be nullptr?
> That seems surprising to me.

I managed to recognize the situation using:
...
diff --git a/gdb/tui/tui-io.c b/gdb/tui/tui-io.c
index 1b4cc82cce8..fa4186aea3b 100644
--- a/gdb/tui/tui-io.c
+++ b/gdb/tui/tui-io.c
@@ -869,6 +869,7 @@ tui_setup_io (int mode)
        /* Restore gdb output.  */
        gdb_stdout = tui_old_stdout;
        gdb_stderr = tui_old_stderr;
+      gdb_assert (gdb_stderr != nullptr);
        gdb_stdlog = tui_old_stdlog;
        gdb_stdtarg = gdb_stderr;
        current_uiout = tui_old_uiout;
...

And the backtrace (in an unusual form because of recursion) is:
...
#19000 0x000000000041a3b2 in main (argc=14, argv=0x7ffc5c28c168)
     at /data/vries/gdb/src/gdb/gdb.c:38
38	  return gdb_main (&args);
(gdb) down
#18999 0x0000000000b68937 in gdb_main (args=0x7ffc5c28c030)
     at /data/vries/gdb/src/gdb/main.c:1363
1363	      captured_main (args);
(gdb)
#18998 0x0000000000b6888e in captured_main (data=0x7ffc5c28c030)
     at /data/vries/gdb/src/gdb/main.c:1344
1344		  captured_command_loop ();
(gdb)
#18997 0x0000000000b66dad in captured_command_loop ()
     at /data/vries/gdb/src/gdb/main.c:466
466	  start_event_loop ();
(gdb)
#18996 0x0000000000b66c21 in start_event_loop ()
     at /data/vries/gdb/src/gdb/main.c:402
402		  result = gdb_do_one_event ();
(gdb)
#18995 0x0000000001985e31 in gdb_do_one_event (mstimeout=-1)
     at /data/vries/gdb/src/gdbsupport/event-loop.cc:263
263	  return gdb_wait_for_event (1);
(gdb)
#18994 0x0000000001986f70 in gdb_wait_for_event (block=1)
     at /data/vries/gdb/src/gdbsupport/event-loop.cc:672
672	      handle_file_event (file_ptr, mask);
(gdb)
#18993 0x0000000001986995 in handle_file_event (file_ptr=0x3b26aff0,
     ready_mask=25) at /data/vries/gdb/src/gdbsupport/event-loop.cc:551
551	      file_ptr->proc (file_ptr->error, file_ptr->client_data);
(gdb)
#18992 0x00000000010563ad in stdin_event_handler (error=1,
     client_data=0x3aec7120) at /data/vries/gdb/src/gdb/ui.c:128
128		  quit_command ((char *) 0, 0);
(gdb)
#18991 0x00000000006b2e63 in quit_command (args=0x0, from_tty=0)
     at /data/vries/gdb/src/gdb/cli/cli-cmds.c:483
483	  quit_force (args ? &exit_code : NULL, from_tty);
(gdb)
#18990 0x0000000000fd94dc in quit_force (exit_arg=0x0, from_tty=0)
     at /data/vries/gdb/src/gdb/top.c:1750
1750	  undo_terminal_modifications_before_exit ();
(gdb)
#18989 0x0000000000fd942f in undo_terminal_modifications_before_exit ()
     at /data/vries/gdb/src/gdb/top.c:1718
1718	  tui_disable ();
(gdb)
#18988 0x000000000103fa11 in tui_disable ()
     at /data/vries/gdb/src/gdb/tui/tui.c:562
562	  tui_setup_io (0);
(gdb)
#18987 0x00000000010129d4 in tui_setup_io (mode=0)
     at /data/vries/gdb/src/gdb/tui/tui-io.c:872
872	      gdb_assert (gdb_stderr != nullptr);
(gdb)
...

Thanks,
- Tom