[PATCH 8/8] [gdb/tui] Fix TUI for TERM=ansi

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

With TERM=ansi, when resizing a TUI window from LINES/COLUMNS 31/118
(maximized) to 20/78 (de-maximized), I get a garbled screen 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 that 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.

Fix this by:
- detecting when readline reports 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.

Tested on x86_64-linux.

PR tui/30337
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30337
---
 gdb/testsuite/lib/tuiterm.exp | 10 ++--------
 gdb/tui/tui-win.c             |  3 +++
 gdb/utils.c                   |  9 +++++++++
 gdb/utils.h                   |  7 +++++++
 4 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/gdb/testsuite/lib/tuiterm.exp b/gdb/testsuite/lib/tuiterm.exp
index d8a99d2798a..f59a66a0958 100644
--- a/gdb/testsuite/lib/tuiterm.exp
+++ b/gdb/testsuite/lib/tuiterm.exp
@@ -1122,16 +1122,10 @@ 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"
+	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
+	stty columns $_cols < $::gdb_tty_name
 	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 3b17cb8dd29..7186fb97d68 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)
diff --git a/gdb/utils.c b/gdb/utils.c
index 6ec1cc0d48d..05a2b27bc65 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -1116,6 +1116,10 @@ static bool filter_initialized = false;
 
 \f
 
+/* See utils.h.  */
+
+int readline_hidden_cols = 0;
+
 /* Initialize the number of lines per page and chars per line.  */
 
 void
@@ -1144,6 +1148,11 @@ init_page_info (void)
 
       /* Get the screen size from Readline.  */
       rl_get_screen_size (&rows, &cols);
+      if (gdb_stdout->isatty ()) {
+	readline_hidden_cols = COLS - cols;
+	gdb_assert (readline_hidden_cols >= 0);
+	gdb_assert (readline_hidden_cols <= 1);
+      }
       lines_per_page = rows;
       chars_per_line = cols;
 
diff --git a/gdb/utils.h b/gdb/utils.h
index a383036bcfe..29ff376bb52 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 */
-- 
2.35.3