gdb/tui: install SIGWINCH only when connected to a TTY

PR26056 reports that when GDB is connected to non-TTY stdin/stdout, it
crashes when it receives a SIGWINCH signal.

This can be reproduced as follows:

    $ gdb/gdb -nx -batch -ex 'run' --args sleep 60 </dev/null 2>&1 | cat

    # from another terminal:
    $ kill -WINCH %(pidof gdb)

When doing so, the process crashes in a call to rl_resize_terminal:

    void
    rl_resize_terminal (void)
    {
      _rl_get_screen_size (fileno (rl_instream), 1);
      ...
    }

The problem is that at this point rl_instream has the value NULL.

The rl_instream variable is supposed to be initialized during a call to
readline_initialize_everything, which in a normal startup sequence is
called under this call chain:

    tui_interp::init
      tui_ensure_readline_initialized
        rl_initialize
          readline_initialize_everything

In tui_interp::init, we have the following sequence:

    tui_initialize_io ();
    tui_initialize_win ();                // <- Installs SIGWINCH
    if (gdb_stdout->isatty ())
      tui_ensure_readline_initialized (); // <- Initializes rl_instream

This function unconditionally installs the SIGWINCH signal handler (this
is done by tui_initialize_win), and then if gdb_stdout is a TTY it
initializes readline.  Therefore, if stdout is not a TTY, SIGWINCH is
installed but readline is not initialized.  In such situation
rl_instream stays NULL, and when GDB receives a SIGWINCH it calls its
handler and in fine tries to access rl_instream leading to the crash.

This patch proposes to fix this issue by installing the SIGWINCH signal
handler only if GDB is connected to a TTY.  Given that this
initialization it the only task of tui_initialize_win, this patch moves
tui_initialize_win just after the call to
tui_ensure_readline_initialized.

Tested on x86_64-linux.

Co-authored-by: Pedro Alves <pedro@palves.net>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26056
Change-Id: I6458acef7b0d9beda2a10715d0345f02361076d9

1 files changed, 8 insertions, 2 deletions

diff --git a/gdb/tui/tui-interp.c b/gdb/tui/tui-interp.c
index 0cd5b12..16859cb 100644
--- a/gdb/tui/tui-interp.c
+++ b/gdb/tui/tui-interp.c
@@ -242,9 +242,15 @@ tui_interp::init (bool top_level)
   atexit (tui_exit);
 
   tui_initialize_io ();
-  tui_initialize_win ();
   if (gdb_stdout->isatty ())
-    tui_ensure_readline_initialized ();
+    {
+      tui_ensure_readline_initialized ();
+
+      /* This installs the SIGWINCH signal handler.  The handler needs to do
+	 readline calls (to rl_resize_terminal), so it must not be installed
+	 unless readline is properly initialized.  */
+      tui_initialize_win ();
+    }
 }
 
 /* Used as the command handler for the tui.  */