Re: tramp (2.7.1.4 nil/nil); Tramp may attempt to create ControlPath sockets that exceed maximum path length

[Michael Albinus]

Alex Hutcheson <alexhutcheson@google.com> writes:

Hi Alex,

> I think ideally we should make Tramp work "out-of-the-box", even if
> the user
> hasn't changed any of the default in their Emacs or SSH config. This
> would
> require picking somewhere reasonable to put the ControlPath domain
> socket
> in cases where the full expansion of a user's home directory might be
> too
> long.

I doubt that there is a user in the wild who uses Tramp w/o even a
single configuration line in ~/.emacs :-)

> If the manual still suggests configuring OpenSSH to put the
> ControlPath
> under /tmp/, then it seems like it would be fine to default to that
> behavior?
> At least as a fallback in the case where the ~/.cache/emacs/ directory
> would
> be too long.

Agreed. We shouldn't recommend use of /tmp.

> The value of tramp-compat-temporary-file-directory could be kept
> as-is, since
> the length isn't a problem for other use-cases. We'd just need to
> update the
> behavior for the ControlPath domain socket.

Reading the sources, it looks like Tramp is already prepared for
that. The relevant setting of ControlPath is in tramp-sh.el:

--8<---------------cut here---------------start------------->8---
             (expand-file-name
              (if (tramp-ssh-option-exists-p vec "ControlPath=tramp.%C")
                  "tramp.%%C" "tramp.%%r@%%h:%%p")
              (or small-temporary-file-directory
                  tramp-compat-temporary-file-directory))
--8<---------------cut here---------------end--------------->8---

small-temporary-file-directory is nil in vanilla Emacs. So I would
change the FAQ entry to something like this:

--8<---------------cut here---------------start------------->8---
   • I get an error ‘unix_listener: path
     "/very/long/path/.cache/emacs/tramp.XXX" too long for Unix domain
     socket’ when connectiong via ‘ssh’ to a remote host.

     By default, TRAMP uses the directory ‘~/.cache/emacs/’ for
     creation of OpenSSH Unix domain sockets.  On GNU/Linux, domain
     sockets have a much lower maximum path length (currently 107
     characters) than normal files.

     You can change this directory by setting the user option
     ‘small-temporary-file-directory’ to another name, like

          (unless small-temporary-file-directory
            (customize-set-variable
             'small-temporary-file-directory
             (format "/run/user/%d/emacs/" (user-uid)))
            (make-directory small-temporary-file-directory t))

     ‘"/run/user/UID"’ is the value of the environment variable
     ‘XDG_RUNTIME_DIR’, which you can use instead via ‘(getenv
     "XDG_RUNTIME_DIR")’.
--8<---------------cut here---------------end--------------->8---

Best regards, Michael.