bug#28655: 25.2; Problem with find-file-noselect

[Glenn Morris <rgm@gnu.org>]

Live System User wrote:

>         The partion where my /home directory is mounted became
>         unrelatedly readonly.
>
>         When I tried to visit a file, readonly "C-x C-r"
>         `(find-file-read-only),` I got the following
>         *Backtrace* (see below).
[...]
> Debugger entered--Lisp error: (wrong-type-argument arrayp nil)
>   file-truename(nil)
>   find-file-noselect-1(#<buffer dt.el<2>> "~/dt.el" nil nil "~/dt.el" (6637 64770))
>   find-file-noselect("~/dt.el" nil nil t)
>   find-file("~/dt.el" t)
>   find-file--read-only(find-file "~/dt.el" t)
>   find-file-read-only("~/dt.el" t)
>   funcall-interactively(find-file-read-only "~/dt.el" t)
>   call-interactively(find-file-read-only nil nil)
>   command-execute(find-file-read-only)

I can't reproduce this. I tried with:

mount -t tmpfs -o size=10m tmpfs /mnt/
HOME=/mnt emacs-25.2 -Q &
...
mount -r -o remount /mnt/

and C-x C-r continues to work fine.

The above backtrace isn't detailed enough to debug this issue.
I can only guess that buffer-file-name somehow became nil.
How, I have no idea.