[gdb/tui] Fix TUI resizing for TERM=ansi

With TERM=ansi, when resizing a TUI window from LINES/COLUMNS 31/118
(maximized) to 20/78 (de-maximized), I get a garbled screen (that ^L doesn't
fix) and a message:
...
@@ resize done 0, size = 77x20
...
with the resulting width being 77 instead of the expected 78.

[ The discrepancy also manifests in CLI, filed as PR30346. ]

The discrepancy comes from tui_resize_all, where we ask readline for the
screen size:
...
   rl_get_screen_size (&screenheight, &screenwidth);
...

As it happens, when TERM is set to ansi, readline decides that the terminal
cannot auto-wrap lines, and reserves one column to deal with that, and as a
result reports back one less than the actual screen width:
...
$ echo $COLUMNS
78
$ TERM=xterm gdb -ex "show width" -ex q
Number of characters gdb thinks are in a line is 78.
$ TERM=ansi  gdb -ex "show width" -ex q
Number of characters gdb thinks are in a line is 77.
...

In tui_resize_all, we need the actual screen width, and using a screenwidth of
one less than the actual value garbles the screen.

This is currently not causing trouble in testing because we have a workaround
in place in proc Term::resize.  If we disable the workaround:
...
-       stty columns [expr {$_cols + 1}] < $::gdb_tty_name
+       stty columns $_cols < $::gdb_tty_name
...
and dump the screen we get the same type of screen garbling:
...
    0 +---------------------------------------+|
    1                                         ||
    2                                         ||
    3                                         ||
...

Another way to reproduce the problem is using command "maint info screen".
After starting gdb with TERM=ansi, entering TUI, and issuing the command, we
get:
...
Number of characters curses thinks are in a line is 78.
...
and after maximizing and demaximizing the window we get:
...
Number of characters curses thinks are in a line is 77.
...
If we use TERM=xterm, we do get the expected 78.

Fix this by:
- detecting when readline will report back less than the actual screen width,
- accordingly setting a new variable readline_hidden_cols,
- using readline_hidden_cols in tui_resize_all to fix the resize problem, and
- removing the workaround in Term::resize.

The test-case gdb.tui/empty.exp serves as regression test.

I've applied the same fix in tui_async_resize_screen, the new test-case
gdb.tui/resize-2.exp serves as a regression test for that change.  Without
that fix, we have:
...
FAIL: gdb.tui/resize-2.exp: again: gdb width 80
...

Tested on x86_64-linux.

PR tui/30337
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30337

5 files changed, 129 insertions, 10 deletions

diff --git a/gdb/testsuite/gdb.tui/resize-2.exp b/gdb/testsuite/gdb.tui/resize-2.exp
new file mode 100644
index 0000000..ebe8216
--- /dev/null
+++ b/gdb/testsuite/gdb.tui/resize-2.exp
@@ -0,0 +1,89 @@
+# Copyright 2023 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 TUI resizing using maint screen info command.
+
+require allow_tui_tests
+
+tuiterm_env
+
+Term::clean_restart 24 80
+set screen_dim { 0 0 80 24 }
+
+# Use a layout with just a command window.
+gdb_test "tui new-layout command-layout cmd 1"
+
+if {![Term::prepare_for_tui]} {
+    unsupported "TUI not supported"
+    return 0
+}
+
+# Enter TUI.
+Term::command_no_prompt_prefix "layout command-layout"
+
+proc check_width { what n } {
+    set re "Number of characters $what thinks are in a line is $n"
+    Term::check_region_contents "$what width $n" {*}$::screen_dim $re
+}
+
+# Check that curses has the correct notion of screen width.
+Term::command "maint info screen"
+check_width curses 80
+check_width gdb 80
+
+# Resize with TUI enabled, wait for the resize message.
+Term::resize 40 90
+set screen_dim { 0 0 90 40 }
+
+# Check that curses has the correct notion of screen width after resize.
+Term::command "maint info screen"
+check_width curses 90
+check_width gdb 90
+
+# Temporarily disable TUI.
+gdb_test_multiple "tui disable" "" {
+    -re "$gdb_prompt $" {
+	pass $gdb_test_name
+    }
+}
+
+# Resize with TUI disabled, so don't wait for the resize message.
+Term::resize 24 80 0
+set screen_dim { 0 0 80 24 }
+gdb_test_multiple "" "two prompt redisplays after resize" {
+    -re "\r.*$gdb_prompt \r.*$gdb_prompt $" {
+	pass $gdb_test_name
+    }
+}
+
+# At this point, curses still thinks the width is 90.  This doesn't look
+# harmful because TUI is disabled.
+gdb_test "maint info screen" \
+    "\r\nNumber of characters curses thinks are in a line is 90.\\r\n.*" \
+    "curses width after resize with TUI disabled"
+
+# Re-enable TUI.
+send_gdb "tui enable\n"
+# The "tui enable" command is issued on the CLI screen, on the TUI we have the
+# last command issued there: "tui disable".
+Term::wait_for "tui disable"
+
+# Check that curses has the correct notion of screen width after screen resize
+# with TUI disabled.
+Term::command "maint info screen"
+with_test_prefix again {
+    check_width curses 80
+    check_width gdb 80
+}
diff --git a/gdb/testsuite/lib/tuiterm.exp b/gdb/testsuite/lib/tuiterm.exp
index d8a99d2..8f78c32 100644
--- a/gdb/testsuite/lib/tuiterm.exp
+++ b/gdb/testsuite/lib/tuiterm.exp
@@ -1111,7 +1111,7 @@ namespace eval Term {
 	}
     }
 
-    proc resize {rows cols} {
+    proc resize {rows cols {wait_for_msg 1}} {
 	variable _rows
 	variable _cols
 	variable _resize_count
@@ -1122,17 +1122,15 @@ namespace eval Term {
 	# explicit here.  This also simplifies waiting for the redraw.
 	_do_resize $rows $_cols
 	stty rows $_rows < $::gdb_tty_name
-	# Due to the strange column resizing behavior, and because we
-	# don't care about this intermediate resize, we don't check
-	# the size and the "@@ " prefix here.
-	wait_for "resize done $_resize_count"
+	if { $wait_for_msg } {
+	    wait_for "@@ resize done $_resize_count, size = ${_cols}x${rows}"
+	}
 	incr _resize_count
-	# Somehow the number of columns transmitted to gdb is one less
-	# than what we request from expect.  We hide this weird
-	# details from the caller.
 	_do_resize $_rows $cols
-	stty columns [expr {$_cols + 1}] < $::gdb_tty_name
-	wait_for "@@ resize done $_resize_count, size = ${_cols}x${rows}"
+	stty columns $_cols < $::gdb_tty_name
+	if { $wait_for_msg } {
+	    wait_for "@@ resize done $_resize_count, size = ${_cols}x${rows}"
+	}
 	incr _resize_count
     }
 }
diff --git a/gdb/tui/tui-win.c b/gdb/tui/tui-win.c
index 3b17cb8..7eac03f 100644
--- a/gdb/tui/tui-win.c
+++ b/gdb/tui/tui-win.c
@@ -36,6 +36,7 @@
 #include "gdbsupport/event-loop.h"
 #include "gdbcmd.h"
 #include "async-event.h"
+#include "utils.h"
 
 #include "tui/tui.h"
 #include "tui/tui-io.h"
@@ -528,6 +529,8 @@ tui_resize_all (void)
   int screenheight, screenwidth;
 
   rl_get_screen_size (&screenheight, &screenwidth);
+  screenwidth += readline_hidden_cols;
+
   width_diff = screenwidth - tui_term_width ();
   height_diff = screenheight - tui_term_height ();
   if (height_diff || width_diff)
@@ -576,6 +579,7 @@ tui_async_resize_screen (gdb_client_data arg)
       int screen_height, screen_width;
 
       rl_get_screen_size (&screen_height, &screen_width);
+      screen_width += readline_hidden_cols;
       set_screen_width_and_height (screen_width, screen_height);
 
       /* win_resized is left set so that the next call to tui_enable()
diff --git a/gdb/utils.c b/gdb/utils.c
index 002a588..e10198a 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -1116,6 +1116,14 @@ static bool filter_initialized = false;
 
 
 
+/* See readline's rlprivate.h.  */
+
+EXTERN_C int _rl_term_autowrap;
+
+/* See utils.h.  */
+
+int readline_hidden_cols = 0;
+
 /* Initialize the number of lines per page and chars per line.  */
 
 void
@@ -1144,6 +1152,19 @@ init_page_info (void)
 
       /* Get the screen size from Readline.  */
       rl_get_screen_size (&rows, &cols);
+
+      /* Readline:
+	 - ignores the COLUMNS variable when detecting screen width
+	   (because rl_prefer_env_winsize defaults to 0)
+	 - puts the detected screen width in the COLUMNS variable
+	   (because rl_change_environment defaults to 1)
+	 - may report one less than the detected screen width in
+	   rl_get_screen_size (when _rl_term_autowrap == 0).
+	 We could set readline_hidden_cols by comparing COLUMNS to cols as
+	 returned by rl_get_screen_size, but instead simply use
+	 _rl_term_autowrap.  */
+      readline_hidden_cols = _rl_term_autowrap ? 0 : 1;
+
       lines_per_page = rows;
       chars_per_line = cols;
 
diff --git a/gdb/utils.h b/gdb/utils.h
index a383036..29ff376 100644
--- a/gdb/utils.h
+++ b/gdb/utils.h
@@ -335,4 +335,11 @@ extern void copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
 			  const gdb_byte *source, ULONGEST source_offset,
 			  ULONGEST nbits, int bits_big_endian);
 
+/* When readline decides that the terminal cannot auto-wrap lines, it reduces
+   the width of the reported screen width by 1.  This variable indicates
+   whether that's the case or not, allowing us to add it back where
+   necessary.  See _rl_term_autowrap in readline/terminal.c.  */
+
+extern int readline_hidden_cols;
+
 #endif /* UTILS_H */