[PATCH v3] [gdb/remote] Use sevenbit_strings == true in 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.

Fix this by forcing sevenbit_strings to true in escape_buffer, getting us
instead:
...
[remote] Packet received: \
  l!\000\000\000\000\000\000\000\000`\374\367\377\177\000...
...

Note that this output 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 [3]:
- use c_isprint instead of std::isprint
- use string_appendf instead of std::ostringstream

Changes in v3:
- Rather than re-implementing escape_buffer, change it to use
  sevenbit_strings == true.
- update $subject to indicate new approach

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
[3] v2 https://sourceware.org/pipermail/gdb-patches/2026-March/226119.html
---
 gdb/remote.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/gdb/remote.c b/gdb/remote.c
index e84a0dc578b..3ec56a6d61b 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -10509,14 +10509,20 @@ 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. '\n' is converted to R"(\012)", '\t' to R"(\011)", etc.  */
 
 static std::string
 escape_buffer (const char *buf, int n)
 {
   string_file stb;
 
+  /* Temporarily set sevenbit_strings to true to make sure that we print
+     chars >= 0x80 as octal escape \ooo, independent of the current
+     sevenbit_strings setting.  This conservative choice is appropriate for
+     debug logs.  */
+  scoped_restore reset = make_scoped_restore (&sevenbit_strings, true);
   stb.putstrn (buf, n, '\\');
+
   return stb.release ();
 }
 

base-commit: 6fdfbd43bb85e3c5749a08494a5ae4af506dc4fb
-- 
2.51.0