[PATCH] [gdb/tui] Make tui_setup_io more robust

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

The following sequence of events may happen when enabling TUI:
- tui_enable () is called,
- a SIGFPE happens, before tui_setup_io (1) is called,
- the SIGFPE triggers handle_fatal_signal, which calls tui_disable (),
- during tui_disable (), tui_setup_io (0) is called, and
- tui_setup_io (0) tries to restore things that were saved during a previous
  tui_setup_io (1) call, but tui_setup_io (1) was never called so the saving
  never happened.

This can cause current_ui.m_ui_stderr to be nullptr, which then can cause a
crash in sig_write (PR33918).

Fix this by:
- adding a variable tui_io_mode, and
- using the variable to bail out of tui_setup_io in case the current mode
  doesn't change.

Tested on x86_64-linux.
---
 gdb/tui/tui-io.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/gdb/tui/tui-io.c b/gdb/tui/tui-io.c
index f673fbf36f6..5a3bc604592 100644
--- a/gdb/tui/tui-io.c
+++ b/gdb/tui/tui-io.c
@@ -824,6 +824,10 @@ tui_rl_display_match_list (char **matches, int len, int max)
   gdb_display_match_list (matches, len, max, &displayer);
 }
 
+/* Whether the IO is setup for curses (1) or non-curses (0).  */
+
+static int tui_io_mode;
+
 /* Setup the IO for curses or non-curses mode.
    - In non-curses mode, readline and gdb use the standard input and
    standard output/error directly.
@@ -837,6 +841,14 @@ tui_setup_io (int mode)
 {
   extern int _rl_echoing_p;
 
+  if (tui_io_mode == mode)
+    {
+      /* Nothing to do.  Also, this makes sure that we don't try to restore
+	 things before having saved them. */
+      return;
+    }
+  tui_io_mode = mode;
+
   if (mode)
     {
       /* Ensure that readline has been initialized before saving any

base-commit: 646982f4295bf0a1e64867606d496c34c9a15a0c
-- 
2.51.0