[PATCH v2] [gdb/remote] Reimplement escape_buffer

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

With Fedora RH, using Tcl 9.0.2, I run into:
...
(gdb) break -q main^M
[remote] Sending packet: $qXfer:auxv:read::0,1000#6b^M
[remote] Packet received: l!\000\000\000\000\000\000\000\000 ERROR: \
  i_read(spawn_id fd=9): invalid or incomplete multibyte or wide character
  ...
UNRESOLVED: gdb.server/bkpt-other-inferior.exp: inf 2: set breakpoint
...

For reference, on openSUSE Leap 16.0, with Tcl 8.6.15 the initial part of the
problematic string looks like:
...
[remote] Packet received: l!\000\000\000\000\000\000\000\000`ü÷ÿ\177\000...
...

With Tcl 9.0, something (dejagnu/expect/gdb testsuite infrastructure)
has problems with gdb IO containing non-utf-8 chars.

Having said that, I don't think having chars like 'ü' in the debug output is
particularly helpful.

The easiest and most robust approach would be to just print octal escape
sequences, but that would mean we no longer print 'l', which indicates to us
what kind of qXfer reply we're looking at [1].

Reimplement function escape_buffer in gdb/remote.c, to print c_isprint chars
like the actual chars, and others like octal escape sequences, getting us
instead:
...
[remote] Packet received: \
  l!\000\000\000\000\000\000\000\000`\374\367\377\177\000...
...

Note that this is ambiguous, because one char '\0' and 4 chars '\\' '0' '0' '0'
are printed the same: \000, but I guess that's a pre-existing problem.

This also fixes test-case gdb.python/py-send-packet.exp, where we run into a
similar problem with the "Sending packet:" line.

Tested on x86_64-linux, with Tcl 9.0.2 and 8.6.15.

A v1 submitted was submitted here [2].

Changes in v2:
- use c_isprint instead of std::isprint
- use string_appendf instead of std::ostringstream

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34012

[1] https://sourceware.org/gdb/current/onlinedocs/gdb.html/General-Query-Packets.html#index-qXfer-packet
[2] v1 https://sourceware.org/pipermail/gdb-patches/2026-March/226110.html
---
 gdb/remote.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/gdb/remote.c b/gdb/remote.c
index e84a0dc578b..8c3d60bd326 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -10509,15 +10509,27 @@ remote_target::remote_serial_send_break ()
 }
 
 /* Return a string representing an escaped version of BUF, of len N.
-   E.g. \n is converted to \\n, \t to \\t, etc.  */
+   E.g. '\0' is converted to "\000", etc.  */
 
 static std::string
 escape_buffer (const char *buf, int n)
 {
-  string_file stb;
+  std::string str;
 
-  stb.putstrn (buf, n, '\\');
-  return stb.release ();
+  for (int i = 0; i < n; ++i)
+    {
+      char c = buf[i];
+      if (c_isprint (c))
+	{
+	  str += c;
+	  continue;
+	}
+
+      /* Append 3-wide leading zero octal \ooo.  */
+      string_appendf (str, "\\%03hho", c);
+    }
+
+  return str;
 }
 
 int

base-commit: c3d4baadd233635c13c306d9fdff3342710633ed
-- 
2.51.0