*** Environment

- GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5) of
  2019-09-23, modified by Debian
- The code looks identical on the relevant parts in Emacs git master

*** Steps to reproduce

1. Run emacs
   $ emacs -q

2. Start remote debugging session
   M-x gdb RET gdb -i=mi /ssh:vagrant@debug:/vagrant/hello

3. Set a breakpoint in *gud-hello* buffer
   (gdb) break main
   Breakpoint 1 at 0x1149: file hello.c, line 6.

4. Run the program to the breakpoint
   (gdb) run

5. Switch back to source window
   C-x b hello.c RET

6. Add another breakpoint
   C-x C-a c-b

Emacs becomes unresponsive and the minibuffer is flooded with the
following error message:

~File /ssh:vagrant@debug:/vagrant/"/vagrant/hello.c" no longer exists!~

*** Fix

1. Apply patch gdb-tramp-fix.diff
   $ zcat /usr/share/emacs/26.1/lisp/progmodes/gdb-mi.el.gz >gdb-mi.el
   $ zcat /usr/share/emacs/26.1/lisp/progmodes/gud.el.gz >gud.el
   $ patch <gdb-tramp-fix.diff

2. run emacs with modified gdb-mi.el and gud.el
   $ emacs -q -l el/gdb-mi.el -l el/gud.el

3. Setting breakpoints with C-x C-a C-b should work now

The changes in ~gdb-mi.el~ fix the problem with the "no longer exist"
error message.

The change in ~gud.el~ makes the fringe mark work a bit better and also
the source buffer to pop up when a break point is hit.

The problem seems to be that not all filename information goes through
(gdb-jsonify-buffer) but gdb-mi.el and gud.el read them directly from
gdb's buffers. These patches check wheter a gdb buffer is remote, and
wrap the file names accordingly.

There is probably a better way to fix the problem, but my knowledge
of gdb-mi.el is limited.

Relates to bug #23608.

*** Affected functions
(gdb-place-breakpoints)
(gdb-get-location)
(gdb-goto-breakpoint)
(gdb-frame-handler)
(gud-file-name)

*** References
- https://debbugs.gnu.org/cgi/bugreport.cgi?bug=23608