bug#49229: 27.2; `M-x shell' fails over TRAMP from local MS Windows

[Jim Porter]

On Sat, Jun 26, 2021 at 7:33 AM Michael Albinus <michael.albinus@gmx.de> wrote:
>
> Occasionally, I've seen this problem on MS Windows already. Since I
> don't run anything on MS Windows unless for bug hunting, I couldn't
> locate it yet. With your recipe, it's reproducible now. It's not related
> to "M-x shell" only, but more general.

Looking at your patch, it makes sense to me. It's not where I
originally expected the fix to go, but I remember some previous corner
cases that were fixed with `tramp-drop-volume-letter', so this should
be good.

> I've pushed a fix to the master branches of Emacs and Tramp. Could you,
> pls, check?

I tested on MS Windows and it works correctly for me. (Note that I
just copied and eval'ed the new version of `tramp-file-name-handler',
since I don't have a proper build environment on MS Windows.)

While testing it, I discovered one other oddity though. This doesn't
necessarily need a fix, but it's a bit surprising, and I'll mention it
here in case someone thinks it's a problem worth fixing.

If I erase the default text and instead enter
"C:/Windows/System32/cmd.exe" into the `M-x shell' prompt, it treats
*that* as a remote path too. Now, the prompt does say to enter a
*remote* shell path, so if I enter a local path, I made a mistake.
However, the default shell path for `M-x shell' from a remote
directory is a TRAMP path ("/sshx:server:/path/to/some//bin/sh"), so
it's surprising that when I delete the TRAMP host prefix, I still end
up running a shell on the remote server.

Perhaps it would be nicer if, when `M-x shell' prompted for the remote
shell path, it didn't include the TRAMP prefix by default (e.g. the
default value would just be "/bin/sh"). That might not interact well
with `read-file-name' completion though; is it possible to use
file-name completion on a remote path without the TRAMP prefix?