bug#54542: 29.0.50; dired can't visit a local directory if another buffer visits an unreachable remote (TRAMP) directory

[Dima Kogan]

Hi. This sounds convoluted, but it's a real-life bug I hit repeatedly.
Recipe:

1. emacs -Q

2. C-x C-f /ssh:host:directory

   Visit some remote directory via TRAMP. This opens a dired buffer

3. Unplug the network cable. Or walk away from wifi. Or turn off the
   remote host. Optionally, do other stuff with emacs. The bug is hit if
   the buffer in step 2 is open somewhere; doesn't have to be visible

4. C-x C-f /some/local/directory

One would expect step 4 to work just fine because it's a local
directory, so no TRAMP business should affect it, but it does. Step 4
will try to connect to the host in step 2. But that host is gone, so
it'll fail, and step 4 will fail too.

The problem is here:

  (defun dired-find-buffer-nocreate (dirname &optional mode)
      ....
      (let (found (blist dired-buffers))
        ...
        (while blist
            ...
            (with-current-buffer (cdr (car blist))
            ....
                              (expand-file-name
                               (if (consp dired-directory)
                                   (car dired-directory)
                                 dired-directory))))

Here dired is trying to figure out if we're already visiting the
requested directory. In doing so, it loops through all extant dired
buffers, and runs (expand-file-name) on each one. Running
(expand-file-name) on a remote directory will try to contact the host.

I'm not attaching a patch because I'm not 100% sure what's appropriate.
I guess we want to replace

  (expand-file-name dired-directory)

with something like this pseudo-code

  (concat (tramp-host dired-directory)
          (expand-file-name (tramp-filename dired-directory)))

Except, thinking about edge cases: making sure this works with/without
TRAMP, making sure relative directories work right, etc, etc.

Do we already have a function that does this?

Thanks!