gdb: allow gdb.Color to work correctly with pagination

This commit allows gdb.Color objects to be used to style output from
GDB commands written in Python, and the styled output should work
correctly with pagination.

There are two parts to fixing this:

First, GDB needs to be able to track the currently applied style
within the page_file class.  This means that style changes need to be
achieved with calls to pager_file::emit_style_escape.

Now usually, GDB does this by calling something like fprintf_styled,
which takes care to apply the style for us.  However, that's not
really an option here as a gdb.Color isn't a full style, and as the
gdb.Color object is designed to be converted directly into escape
sequences that can then be printed, we really need a solution that
works with this approach.

However pager_file::puts already has code in place to handle escape
sequences.  Right now all this code does is spot the escape sequence
and append it to the m_wrap_buffer.  But in this commit I propose that
we go one step further, parse the escape sequence back into a
ui_file_style object in pager_file::puts, and then we can call
pager_file::emit_style_escape.

If the parsing doesn't work then we can just add the escape sequence
to m_wrap_buffer as we did before.

But wait, how can this work if a gdb.Color isn't a full style?  Turns
out that's not a problem.  We only ever emit the escape sequence for
those parts of a style that need changing, so a full style that sets
the foreground color will emit the same escape sequence as a gdb.Color
for the foreground.  When we convert the escape sequence back into a
ui_file_style, then we get a style with everything set to default,
except the foreground color.

I had hoped that this would be all that was needed.  But unfortunately
this doesn't work because of the second problem...

... the implementation of the Python function gdb.write() calls
gdb_printf(), which calls gdb_vprintf(), which calls ui_file::vprintf,
which calls ui_out::vmessage, which calls ui_out::call_do_message, and
finally we reach cli_ui_out::do_message.  This final do_message
function does this:

  ui_file *stream = m_streams.back ();
  stream->emit_style_escape (style);
  stream->puts (str.c_str ());
  stream->emit_style_escape (ui_file_style ());

If we imagine the case where we are emitting a style, triggered from
Python like this:

  gdb.write(gdb.Color('red').escape_sequence(True))

the STYLE in this case will be the default ui_file_style(), and STR
will hold the escape sequence we are writing.

After the first change, where pager_file::puts now calls
pager_file::emit_style_escape, the current style of STREAM will have
been updated.  But this means that the final emit_style_escape will
now restore the default style.

The fix for this is to avoid using the high level gdb_printf from
gdb.write(), and instead use gdb_puts instead.  The gdb_puts function
doesn't restore the default style, which means our style modification
survives.

There's a new test included.  This test includes what appears like a
pointless extra loop (looping over a single value), but this makes
sense given the origin of this patch.  I've pulled this commit from a
longer series:

  https://inbox.sourceware.org/gdb-patches/cover.1755080429.git.aburgess@redhat.com

I want to get this bug fix merged before GDB 17 branches, but the
longer series is not getting reviews, so for now I'm just merging this
one fix.  Once the rest of the series gets merged, I'll be extending
the test, and the loop (mentioned above) will now loop over more
values.

4 files changed, 195 insertions, 12 deletions

diff --git a/gdb/python/python.c b/gdb/python/python.c
index cb0d642..1af7896 100644
--- a/gdb/python/python.c
+++ b/gdb/python/python.c
@@ -1570,21 +1570,21 @@ gdbpy_write (PyObject *self, PyObject *args, PyObject *kw)
 
   try
     {
+      ui_file *stream;
       switch (stream_type)
 	{
 	case 1:
-	  {
-	    gdb_printf (gdb_stderr, "%s", arg);
-	    break;
-	  }
+	  stream = gdb_stderr;
+	  break;
 	case 2:
-	  {
-	    gdb_printf (gdb_stdlog, "%s", arg);
-	    break;
-	  }
+	  stream = gdb_stdlog;
+	  break;
 	default:
-	  gdb_printf (gdb_stdout, "%s", arg);
+	  stream = gdb_stdout;
+	  break;
 	}
+
+      gdb_puts (arg, stream);
     }
   catch (const gdb_exception &except)
     {
diff --git a/gdb/testsuite/gdb.python/py-color-pagination.exp b/gdb/testsuite/gdb.python/py-color-pagination.exp
new file mode 100644
index 0000000..3235fff
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-color-pagination.exp
@@ -0,0 +1,122 @@
+# Copyright (C) 2025 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/>.
+
+# This file is part of the GDB testsuite.  It tests gdb.Color and how this
+# interacts with GDB's pagination system.
+
+load_lib gdb-python.exp
+
+require allow_python_tests
+
+standard_testfile
+
+set pyfile [gdb_remote_download host ${srcdir}/${subdir}/${testfile}.py]
+
+set str "<[string repeat - 78]>"
+
+# These define all the default attributes for a style: background
+# color, intensity, italics, and underlined.
+set other_attr ";49;22;23;24;27"
+
+# These colors set the foreground color only.  Everything else is the
+# default.
+set black "(?:\033\\\[30${other_attr}m)"
+set red "(?:\033\\\[31${other_attr}m)"
+set green "(?:\033\\\[32${other_attr}m)"
+set yellow "(?:\033\\\[33${other_attr}m)"
+set blue "(?:\033\\\[34${other_attr}m)"
+set magenta "(?:\033\\\[35${other_attr}m)"
+set cyan "(?:\033\\\[36${other_attr}m)"
+set white "(?:\033\\\[37${other_attr}m)"
+
+set any_color "(?:${black}|${red}|${green}|${yellow}|${blue}|${magenta}|${cyan}|${white})"
+
+# Run the command 'TYPE-fill MODE' which fills the screen with output and
+# triggers the pagination prompt.  Check that styling is applied correctly
+# to the output.
+proc test_pagination { type mode } {
+
+    # Start with a fresh GDB, but enable color support.
+    with_ansi_styling_terminal {
+	clean_restart
+    }
+
+    gdb_test_no_output "source $::pyfile" "source the script"
+
+    gdb_test_no_output "set width 80"
+    gdb_test_no_output "set height 15"
+
+    set saw_bad_color_handling false
+    set expected_restore_color ""
+    set last_color ""
+    gdb_test_multiple "$type-fill $mode" "" {
+	-re "^$type-fill $mode\r\n" {
+	    exp_continue
+	}
+
+	-re "^(${::any_color}?)(${::any_color})$::str" {
+	    # After a continuation prompt GDB will restore the previous
+	    # color, and then we immediately switch to a new color.
+	    set restored_color $expect_out(1,string)
+	    if { $restored_color ne ""
+		 && $restored_color ne $expected_restore_color } {
+		set saw_bad_color_handling true
+	    }
+	    set last_color $expect_out(2,string)
+	    exp_continue
+	}
+
+	-re "^\033\\\[${::decimal}m$::str" {
+	    # This catches the case where the color's escape sequence has
+	    # not been converted back into a full style.  This indicates
+	    # something went wrong in the pager_file::puts function.
+	    set saw_bad_color_handling true
+	    exp_continue
+	}
+
+	-re "^((?:\033\\\[m)?)$::pagination_prompt$" {
+	    # After a pagination prompt we expect GDB to restore the last
+	    # color.
+	    set expected_restore_color $last_color
+
+	    # If we didn't see a color reset sequence then the pagination
+	    # prompt will have been printed in the wrong color, this is a
+	    # GDB bug.
+	    set color_reset $expect_out(1,string)
+	    if { $color_reset eq "" } {
+		set saw_bad_color_handling true
+	    }
+
+	    # Send '\n' to view more output.
+	    send_gdb "\n"
+	    exp_continue
+	}
+
+	-re "^\r\n" {
+	    # The matches the newline sent to the continuation prompt.
+	    exp_continue
+	}
+
+	-re "^\033\\\[m\r\n$::gdb_prompt $" {
+	    gdb_assert { !$saw_bad_color_handling } $gdb_test_name
+	}
+    }
+}
+
+foreach_with_prefix type { color } {
+    foreach_with_prefix mode { write print } {
+	test_pagination $type $mode
+    }
+}
diff --git a/gdb/testsuite/gdb.python/py-color-pagination.py b/gdb/testsuite/gdb.python/py-color-pagination.py
new file mode 100644
index 0000000..efd501e
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-color-pagination.py
@@ -0,0 +1,46 @@
+# Copyright (C) 2025 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/>.
+
+import gdb
+
+basic_colors = ["black", "red", "green", "yellow", "blue", "magenta", "cyan", "white"]
+
+
+def write(mode, text):
+    if mode == "write":
+        gdb.write(text)
+    else:
+        print(text, end="")
+
+
+class ColorTester(gdb.Command):
+    def __init__(self):
+        super().__init__("color-fill", gdb.COMMAND_USER)
+
+    def invoke(self, args, from_tty):
+        mode = args
+        str = "<" + "-" * 78 + ">"
+        for i in range(0, 20):
+            for color_name in basic_colors:
+                c = gdb.Color(color_name)
+                write(mode, c.escape_sequence(True))
+                write(mode, str)
+
+        default = gdb.Color("none")
+        write(mode, default.escape_sequence(True))
+        write(mode, "\n")
+
+
+ColorTester()
diff --git a/gdb/utils.c b/gdb/utils.c
index 10d3d51..92e626a 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -1702,10 +1702,25 @@ pager_file::puts (const char *linebuffer)
 	  else if (*linebuffer == '\033'
 		   && skip_ansi_escape (linebuffer, &skip_bytes))
 	    {
-	      m_wrap_buffer.append (linebuffer, skip_bytes);
-	      /* Note that we don't consider this a character, so we
+	      /* We don't consider escape sequences as characters, so we
 		 don't increment chars_printed here.  */
-	      linebuffer += skip_bytes;
+
+	      size_t style_len;
+	      ui_file_style style;
+	      if (style.parse (linebuffer, &style_len)
+		  && style_len <= skip_bytes)
+		{
+		  this->emit_style_escape (style);
+
+		  linebuffer += style_len;
+		  skip_bytes -= style_len;
+		}
+
+	      if (skip_bytes > 0)
+		{
+		  m_wrap_buffer.append (linebuffer, skip_bytes);
+		  linebuffer += skip_bytes;
+		}
 	    }
 	  else if (*linebuffer == '\r')
 	    {