bug#52196: 28.0.60; gdb-mi error message: "wrong type argument: stringp"

[sebastian]

I've attached a small C++ program that can be used to reproduce the issue. I 
compiled it like so

g++ -g3 main.cc -o test-program

Steps to reproduce:
- emacs -Q
- evaluate the forms:
(require 'gdb-mi)
(setq gdb-many-windows t)

- M-x gdb with argument "gdb -i=mi /path/to/test-program"
- Do "run" in the gdb command prompt
- M-x gdb-io-interrupt

Result: error message in minibuffer "Wrong type argument: stringp, nil" 
(although the command executes normally).

Furthermore, in this state any of the following will also cause the same 
message to flash:
- doing M-x gud-break in the source buffer
- toggling a breakpoint in the breakpoint buffer
- probably others

Although again none of the commands fail to execute properly.

Root cause:

In the function gdb-frame-handler the form:

(setq gdb-selected-file (file-local-name (gdb-mi--field frame 'fullname)))

causes the error message if gdb-mi--field returns nil. I don't know if this is 
an expected return value for that function or not. In any case, file-local-name 
signals the error in case of nil input. The file-local-name wrapping was 
introduced semi-recently with commit 019c70e731d0243ad3ea2d4ebb02dd4a343e868f.

Thanks.

----------------------------------------

In GNU Emacs 28.0.60 (build 8, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo 
version 1.15.12)
 of 2021-11-30 built on san
Repository revision: 30553d889d733613e8e5fd22358980baa7ee148e
Repository branch: emacs-28
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: CentOS Linux 8

Configured using:
 'configure --prefix=/home/snorgaard/emacs-master --with-cairo
 --with-dumping=pdumper --with-mailutils --with-json --with-x
 --with-xwidgets --with-native-compilation
 --enable-link-time-optimization 'CFLAGS=-g3 -O3 -mtune=native
 -march=native -fomit-frame-pointer''

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSELINUX LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG
RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM
XWIDGETS GTK3 ZLIB

Important settings:
  value of $LANG: en_DK.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: C++//l

Minor modes in effect:
  gdb-many-windows: 1
  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Load-path shadows:
None found.

Features:
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rmail-loaddefs auth-source eieio eieio-core eieio-loaddefs
password-cache json map text-property-search mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils time-date
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cc-vars cc-defs comp comp-cstr warnings subr-x rx cl-macs cl-extra
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rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
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xwidget-internal dbusbind inotify lcms2 dynamic-setting
system-font-setting font-render-setting cairo move-toolbar gtk x-toolkit
x multi-tty make-network-process native-compile emacs)

Memory information:
((conses 16 134661 11459)
 (symbols 48 10813 0)
 (strings 32 31925 1896)
 (string-bytes 1 1182054)
 (vectors 16 20650)
 (vector-slots 8 362625 14622)
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main.cc
Description: Binary data