[6/6] gdb/tui: fix debuginfod prompt using 'C-x C-a' to enter TUI

diff --git a/gdb/testsuite/gdb.tui/debuginfod-query.exp b/gdb/testsuite/gdb.tui/debuginfod-query.exp
index 5baf3e24e1f..b7727090c3e 100644
--- a/gdb/testsuite/gdb.tui/debuginfod-query.exp
+++ b/gdb/testsuite/gdb.tui/debuginfod-query.exp
@@ -213,10 +213,7 @@  file copy -force $binfile $debugdir/
 # Create CACHE and DB directories ready for debuginfod to use.
 prepare_for_debuginfod cache db
 
-# Can add 'keys' to MODES list, but this doesn't currently
-# work due to PR gdb/33794.  This will be fixed in the next
-# commit, and this comment removed.
-set modes {command}
+set modes {command keys}
 
 with_debuginfod_env $cache {
     save_vars { env(DEBUGINFOD_URLS) } {
diff --git a/gdb/testsuite/gdb.tui/tui-enable-failure-lib.c b/gdb/testsuite/gdb.tui/tui-enable-failure-lib.c
new file mode 100644
index 00000000000..c267743fd2e
--- /dev/null
+++ b/gdb/testsuite/gdb.tui/tui-enable-failure-lib.c
@@ -0,0 +1,31 @@ 
+/* Copyright 2026 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Preload library that overrides ncurses newterm to always return NULL,
+   causing tui_enable to fail.  */
+
+#include <stddef.h>
+
+/* Define this type so we can override newterm.  We only plan to
+   return NULL, so the details of this type are not important.  */
+typedef struct screen_dummy SCREEN;
+
+SCREEN *
+newterm (const char *type, void *outfd, void *infd)
+{
+  return NULL;
+}
diff --git a/gdb/testsuite/gdb.tui/tui-enable-failure.c b/gdb/testsuite/gdb.tui/tui-enable-failure.c
new file mode 100644
index 00000000000..1dc4b7ca539
--- /dev/null
+++ b/gdb/testsuite/gdb.tui/tui-enable-failure.c
@@ -0,0 +1,22 @@ 
+/* Copyright 2026 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+main (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.tui/tui-enable-failure.exp b/gdb/testsuite/gdb.tui/tui-enable-failure.exp
new file mode 100644
index 00000000000..5ad2eca3d03
--- /dev/null
+++ b/gdb/testsuite/gdb.tui/tui-enable-failure.exp
@@ -0,0 +1,136 @@ 
+# Copyright 2026 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test that GDB handles tui_enable failure gracefully.
+#
+# Use an LD_PRELOAD library to override ncurses' newterm function,
+# making it always return NULL.  This causes tui_enable to fail with
+# an error.
+#
+# Test both the "tui enable" command and the multi-key combination
+# (C-x C-a) paths.  The multi-key path is particularly important as it
+# exercises the tui_rl_keybinding SCOPE_EXIT error handling.
+
+require allow_tui_tests
+require {!is_remote host}
+require {istarget *-linux*}
+
+tuiterm_env
+
+standard_testfile .c -lib.c
+
+set libfile ${testfile}-lib
+set libobj [standard_output_file ${libfile}.so]
+
+# Compile the preload library.
+if { [build_executable "build preload lib" $libobj $srcfile2 \
+	  {debug shlib}] == -1 } {
+    return
+}
+
+# Compile the test executable.
+if { [build_executable "build executable" $testfile $srcfile] == -1 } {
+    return
+}
+
+# Send 'print NUM' to GDB and check the output appear on LINE.
+proc check_responsiveness { num line } {
+    gdb_assert { [Term::command "print $num"] } \
+	"send print command"
+
+    gdb_assert { [Term::check_region_contents_p 0 $line 80 1 " = $num"] } \
+	"check print output is in expected location"
+}
+
+# Try to enable TUI mode using HOW, which is either 'command' or
+# 'keys'.  The preload library ensures that newterm returns NULL, so
+# tui_enable will fail.  Check that an error is produced and that GDB
+# remains responsive.
+#
+# The first failure sets tui_finish_init to TRIBOOL_UNKNOWN, so a
+# second attempt produces a different (generic) error message.  Try
+# the second attempt too, and check that GDB is still responsive.
+#
+# The 'keys' path (C-x C-a) is particularly important as it exercises
+# the tui_rl_keybinding SCOPE_EXIT error handling.
+proc_with_prefix run_test { how } {
+    Term::clean_restart 24 80 $::testfile
+
+    if {![Term::prepare_for_tui]} {
+	return
+    }
+
+    Term::gen_prompt
+
+    # First attempt to enable TUI.
+    if { $how eq "command" } {
+	send_gdb "tui enable\n"
+	set line_offset 1
+    } elseif { $how eq "keys" } {
+	send_gdb "\030\001"
+	set line_offset 0
+    } else {
+	perror "unknown test mode: $how"
+    }
+
+    gdb_assert { [Term::wait_for "Cannot enable the TUI: error opening terminal"] } \
+	"error from first tui enable attempt"
+
+    # Check that GDB is still responsive.
+    with_test_prefix "responsive after first failure" {
+	check_responsiveness 1 [expr {2 + $line_offset}]
+    }
+
+    # Second attempt using 'tui enable' command should also fail.
+    send_gdb "tui enable\n"
+    gdb_assert { [Term::wait_for "Cannot enable the TUI"] } \
+	"error from second tui enable attempt"
+
+    # Check that GDB is still responsive.
+    with_test_prefix "responsive after second failure" {
+	check_responsiveness 2 [expr {6 + $line_offset}]
+    }
+
+    # Third attempt using multi-key combo should also fail.
+    send_gdb "\030\001"
+    gdb_assert { [Term::wait_for "Cannot enable the TUI"] } \
+	"error from third tui enable attempt"
+
+    # Check that GDB is still responsive.
+    with_test_prefix "responsive after third failure" {
+	check_responsiveness 3 [expr {9 + $line_offset}]
+    }
+
+    verbose -log "\n\n\nAPB: Dump:\n\n"
+    Term::dump_screen
+}
+
+set modes {command keys}
+
+save_vars { env(LD_PRELOAD) env(ASAN_OPTIONS) } {
+    if { ![info exists env(LD_PRELOAD)]
+	 || $env(LD_PRELOAD) == "" } {
+	set env(LD_PRELOAD) "$libobj"
+    } else {
+	append env(LD_PRELOAD) ":$libobj"
+    }
+
+    # Prevent address sanitizer error about library ordering.
+    append_environment_default ASAN_OPTIONS verify_asan_link_order 0
+
+    foreach_with_prefix how $modes {
+	run_test $how
+    }
+}
diff --git a/gdb/tui/tui.c b/gdb/tui/tui.c
index 5da5a58a6fb..bf83e9c3fdf 100644
--- a/gdb/tui/tui.c
+++ b/gdb/tui/tui.c
@@ -42,6 +42,7 @@ 
 #include "top.h"
 #include "ui.h"
 #include "observable.h"
+#include "run-on-main-thread.h"
 
 #include <fcntl.h>
 
@@ -119,15 +120,20 @@  tui_rl_switch_mode (int notused1 = 0, int notused2 = 0)
 {
   gdb_assert (!gdb_in_secondary_prompt_p (current_ui));
 
-  /* Don't let exceptions escape.  We're in the middle of a readline
-     callback that isn't prepared for that.  */
+  /* This function is called through the run_on_main_thread event loop
+     callback mechanism.  That mechanism propagates
+     gdb_exception_forced_quit exceptions, but silently discards
+     gdb_exception exceptions.  We catch and print gdb_exception
+     exceptions before propagating them, the run_on_main_thread
+     mechanism will then discard these and return to the event loop.
+     Any RAII cleanup between here and there will have been done,
+     which is important.  For gdb_exception_forced_quit exceptions we
+     just propagate these up the stack without printing, these will be
+     handled when they are caught by the event loop.  */
   try
     {
       if (tui_active)
-	{
-	  tui_disable ();
-	  rl_prep_terminal (0);
-	}
+	tui_disable ();
       else
 	{
 	  /* If we type "foo", entering it into the readline buffer
@@ -143,43 +149,24 @@  tui_rl_switch_mode (int notused1 = 0, int notused2 = 0)
 	     TUI.  */
 	  rl_clear_visible_line ();
 
-	  /* If tui_enable throws, we'll re-prep below.  */
+	  /* Disable readline state ahead of enabling TUI mode.  If
+	     tui_enable fails then the next display_gdb_prompt will
+	     re-prep the terminal for us.  */
 	  rl_deprep_terminal ();
+
 	  tui_enable ();
 	}
     }
   catch (const gdb_exception_forced_quit &ex)
     {
-      /* Ideally, we'd do a 'throw' here, but as noted above, we can't
-	 do that, so, instead, we'll set the necessary flags so that
-	 a later QUIT check will restart the forced quit.  */
-      set_force_quit_flag ();
+      throw;
     }
   catch (const gdb_exception &ex)
     {
       exception_print (gdb_stderr, ex);
-
-      if (!tui_active)
-	rl_prep_terminal (0);
+      throw;
     }
 
-  /* Clear the readline in case switching occurred in middle of
-     something.  */
-  if (rl_end)
-    rl_kill_text (0, rl_end);
-
-  /* Since we left the curses mode, the terminal mode is restored to
-     some previous state.  That state may not be suitable for readline
-     to work correctly (it may be restored in line mode).  We force an
-     exit of the current readline so that readline is re-entered and
-     it will be able to setup the terminal for its needs.  By
-     re-entering in readline, we also redisplay its prompt in the
-     non-curses mode.  */
-  rl_newline (1, '\n');
-
-  /* Make sure the \n we are returning does not repeat the last
-     command.  */
-  dont_repeat ();
   return 0;
 }
 
@@ -334,7 +321,76 @@  tui_rl_keybinding (int count, int key)
   if (gdb_in_secondary_prompt_p (current_ui))
     return 0;
 
-  return FPTR (count, key);
+  run_on_main_thread ([=] () {
+    bool was_active = tui_active;
+
+    /* Cleanup required even on the exception path.  */
+    SCOPE_EXIT {
+      /* If we switched from CLI to TUI (or back) then we should
+	 reinitialize the pager in order to avoid spurious pagination
+	 prompts.  This is especially important going from CLI to TUI
+	 where the command window is usually smaller than the full
+	 terminal, so the pager might already think that we have more
+	 lines printed than will fit in the window, despite the window
+	 starting empty after a mode switch.  */
+      if (was_active != tui_active)
+	reinitialize_more_filter ();
+
+      /* The user was at a prompt and pressed a multi-key combination
+	 (e.g. C-x C-a).  As a result this callback was invoked from
+	 the event loop.  We're now exiting this callback and want to
+	 ensure that the prompt is drawn correctly.
+
+	 We have two approaches, full display_gdb_prompt, or a light
+	 weight tui_redisplay_readline.  The former will clear the
+	 readline input buffer and redisplay the prompt, while the
+	 second will redraw the prompt along with anything in the
+	 input buffer.  We want the light weight option where
+	 possible, but there are times when this isn't an option:
+
+	 1. A successful switch between CLI and TUI, in either
+	    direction, changes how we update the prompt.  We need to
+	    call display_gdb_prompt the first time to ensure
+	    everything is done correctly.  We also need to consider
+	    the case where tui_enable fails, leaving us in CLI mode,
+	    this also requires a call to display_gdb_prompt as the
+	    light weight tui_redisplay_readline is not appropriate for
+	    CLI use.
+
+	 2. Usually the TUI will have the RL_STATE_CALLBACK state flag
+	    set because of when this event callback is called.  If a
+	    secondary prompt has been displayed, then once the
+	    secondary prompt completed, the RL_STATE_CALLBACK flag
+	    will have been cleared.  This is good for us, because
+	    after a secondary prompt rl_prompt will still hold the
+	    secondary prompt string, so we need display_gdb_prompt to
+	    set the correct top-level prompt.  */
+      if (!was_active || !tui_active || !RL_ISSTATE (RL_STATE_CALLBACK))
+	{
+	  current_ui->prompt_state = PROMPT_NEEDED;
+	  display_gdb_prompt (nullptr);
+	}
+      else
+	{
+	  /* If we switched TUI to CLI then we took the above block.
+	     If we're in CLI mode then these callbacks will switch us
+	     back to TUI mode.  Either way, if we get here then the
+	     TUI must be active.  */
+	  gdb_assert (tui_active);
+
+	  /* The only time rl_prompt will be NULL is when we switch
+	     from CLI to TUI, but that will be handled by the block
+	     above.  */
+	  gdb_assert (rl_prompt != nullptr);
+
+	  tui_redisplay_readline ();
+	}
+    };
+
+    (void) FPTR (count, key);
+  });
+
+  return 0;
 }
 
 /* Initialize readline and configure the keymap for the switching