[gdb/cli] Show readline wrapping mode for maint info screen

With the same trigger patch adding "set horizontal-scroll-mode on" to INPUTRC
as used in commit 250f1bbaf33 ("[gdb/testsuite] Fix gdb.tui/wrap-line.exp with
wrapping disabled"), we can easily reproduce a failure in
gdb.tui/wrap-line.exp mentioned in PR testsuite/31201:
...
(gdb) 78901234567890123456789012345678901234567890123456789012345678901234567^M<890123456789012345678901234567890123456789012345678                         ^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H^H9WFAIL: gdb.base/wrap-line.exp: term=ansi: width-hard-coded: wrap (timeout)
...

The test-case expects wrapping, but that's disabled by horizontal-scroll-mode.

Add a new line to "maint info screen", that describes the current readline
wrapping mode, and use it in the test-case to handle the different cases.

The reported values for the wrapping mode are as follows.

Unsupported because of running in batch mode:
...
$ gdb -q -batch -ex "maint info screen"
Readline wrapping mode: unsupported (gdb batch mode).
...

Unsupported because the terminal is not capable to move the cursor up:
...
$ TERM=dumb gdb -q -ex "maint info screen" -ex q
Readline wrapping mode: unsupported (terminal is not Cursor Up capable).
...

Disabled by horizontal-scroll-mode:
...
$ grep horizontal-scroll-mode ~/.inputrc
set horizontal-scroll-mode on
$ gdb -q -ex "maint info screen" -ex q
Readline wrapping mode: disabled (horizontal-scroll-mode).
...

Wrap done by readline because terminal is not auto wrap capable:
...
$ TERM=ansi gdb -q -ex "maint info screen" -ex q
Readline wrapping mode: readline (terminal is not auto wrap capable, last column reserved).
...

Wrap done by terminal autowrap:
...
$ TERM=xterm gdb -q -ex "maint info screen" -ex q
Readline wrapping mode: terminal (terminal is auto wrap capable).
...

Tested on x86_64-linux.

Co-Authored-By: Bernd Edlinger <bernd.edlinger@hotmail.de>
Approved-By: Tom Tromey <tom@tromey.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31201

2 files changed, 70 insertions, 6 deletions

diff --git a/gdb/testsuite/gdb.base/wrap-line.exp b/gdb/testsuite/gdb.base/wrap-line.exp
index b0931a3..323cd6c 100644
--- a/gdb/testsuite/gdb.base/wrap-line.exp
+++ b/gdb/testsuite/gdb.base/wrap-line.exp
@@ -19,8 +19,10 @@
 # build == host.
 require {!is_remote host}
 
-# Test both ansi (no auto-wrap) and xterm (auto-wrap).
-set terms {ansi xterm}
+# Test both ansi (no auto-wrap) and xterm (auto-wrap).  Also test dumb, which
+# shows the effective behaviour on stub-termcap systems, regardless of the
+# TERM setting.
+set terms {ansi xterm dumb}
 
 # Fill line, assuming we start after the gdb prompt.
 proc fill_line { width } {
@@ -44,9 +46,11 @@ proc get_screen_width { } {
     upvar gdb_width gdb_width
     upvar readline_width readline_width
     upvar env_width env_width
+    upvar wrap_mode wrap_mode
     set gdb_width 0
     set readline_width 0
     set env_width 0
+    set wrap_mode ""
     set re1 "Number of characters gdb thinks are in a line is ($::decimal)\[^\r\n\]*\\."
     set re2 \
 	"Number of characters readline reports are in a line is ($::decimal)\[^\r\n\]*\\."
@@ -54,6 +58,7 @@ proc get_screen_width { } {
 	"Number of characters curses thinks are in a line is $::decimal\\."
     set re4 \
 	"Number of characters environment thinks are in a line is ($::decimal) \\(COLUMNS\\)."
+    set re5 [string cat "Readline wrapping mode: (\[^\r\n]*\)\\."]
     set cmd "maint info screen"
     set re \
 	[multi_line \
@@ -61,12 +66,14 @@ proc get_screen_width { } {
 	     $re2 \
 	     "(?:$re3" \
 	     ")?$re4" \
-	     .*]
+	     .* \
+	     $re5]
     gdb_test_multiple $cmd  "" {
 	-re -wrap $re {
 	    set gdb_width $expect_out(1,string)
 	    set readline_width $expect_out(2,string)
 	    set env_width $expect_out(3,string)
+	    set wrap_mode $expect_out(4,string)
 	    pass $gdb_test_name
 	}
     }
@@ -79,10 +86,16 @@ proc test_wrap { width_auto_detected } {
 
     get_screen_width
 
-    if { $::term == "xterm" } {
+    set wrap_mode_terminal "terminal (terminal is auto wrap capable)"
+    set wrap_mode_readline \
+	"readline (terminal is not auto wrap capable, last column reserved)"
+    set have_wrap 1
+    if { $wrap_mode == $wrap_mode_terminal } {
 	gdb_assert { $gdb_width == $readline_width }
-    } else {
+    } elseif { $wrap_mode == $wrap_mode_readline } {
 	gdb_assert { $gdb_width == [expr $readline_width + 1] }
+    } else {
+	set have_wrap 0
     }
 
     gdb_assert { $gdb_width == $env_width } "width"
@@ -113,7 +126,10 @@ proc test_wrap { width_auto_detected } {
 
     gdb_test_multiple "" "wrap" {
 	-re $re {
-	    pass $gdb_test_name
+	    gdb_assert {$have_wrap} $gdb_test_name
+	}
+	-re "\r<.*" {
+	    gdb_assert {!$have_wrap} $gdb_test_name
 	}
     }
 
diff --git a/gdb/utils.c b/gdb/utils.c
index 07c5d3c..c503ff7 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -1282,6 +1282,14 @@ set_screen_width_and_height (int width, int height)
   set_width ();
 }
 
+/* Import termcap variable UP (instead of readline private variable
+   _rl_term_up, which we're trying to avoid, see PR build/10723).  The UP
+   variable doesn't seem be part of the regular termcap interface, but rather
+   curses-specific.  But if it's missing in the termcap library, then readline
+   provides a fallback version.  Let's assume the fallback is not part of the
+   private readline interface.  */
+extern "C" char *UP;
+
 /* Implement "maint info screen".  */
 
 static void
@@ -1340,6 +1348,46 @@ maintenance_info_screen (const char *args, int from_tty)
 	      _("Number of lines environment thinks "
 		"are in a page is %s (LINES).\n"),
 	      getenv ("LINES"));
+
+  bool have_up = UP != nullptr && *UP != '\0';
+
+  /* Fetch value of readline variable horizontal-scroll-mode.  */
+  const char *horizontal_scroll_mode_value
+    = rl_variable_value ("horizontal-scroll-mode");
+  bool force_horizontal_scroll_mode
+    = (horizontal_scroll_mode_value != nullptr
+       && strcmp (horizontal_scroll_mode_value, "on") == 0);
+
+  const char *mode = nullptr;
+  const char *reason = nullptr;
+  if (batch_flag)
+    {
+      mode = "unsupported";
+      reason = "gdb batch mode";
+    }
+  else if (!have_up)
+    {
+      mode = "unsupported";
+      reason = "terminal is not Cursor Up capable";
+    }
+  else if (force_horizontal_scroll_mode)
+    {
+      mode = "disabled";
+      reason = "horizontal-scroll-mode";
+    }
+  else if (readline_hidden_cols)
+    {
+      mode = "readline";
+      reason = "terminal is not auto wrap capable, last column reserved";
+    }
+  else
+    {
+      mode = "terminal";
+      reason = "terminal is auto wrap capable";
+    }
+
+  gdb_printf (gdb_stdout, _("Readline wrapping mode: %s (%s).\n"), mode,
+	      reason);
 }
 
 void