Re: Buffer-local process environments

[Augusto Stoffel]

On Sat,  8 May 2021 at 19:51, Michael Albinus <michael.albinus@gmx.de> wrote:

Hi Michael,

> We have already a white list, which is tramp-remote-process-environment.
> I'm reluctant to add package specific entries there by default, it would
> be an endless story. PAGER is included as being a general purpose
> environment variable, BZR_LOG and HGPLAIN are not. If packages believe
> they shall be added, this could be done in vc-bzr.el or vc-hg.el. Tramp
> is a stupid library, it shouldn't try to be clever.

Fair enough.  Let's cross out the first idea then.

>> 2. Introduce a blacklist of variables that are never exported to a
>>    remote.  This can be done by extending
>>    `tramp-remote-process-environment' to follow the same convention of
>>    `process-environment' that an entry of the form VAR, without the
>>    =VALUE, means removing the variable.
>
> There are already such variables to be unset. These are the variables
> without any value, like "HISTORY="

This only works for entries in the default toplevel value of
`process-environment'.  Not for things added buffer-locally or
in a let-binding.

The heuristic that every env var which is not in the default toplevel
value of `process-environment' will be exported to the remote works
pretty well.  But it needs a simple way to be overridden, since it can't
read the user's mind.

> TERM is handled special. All Tramp connections add "TERM=dumb",
> hard-coded. Since this shall not be changed by a user, it isn't
> configurable here.

Not all connections add "TERM=dumb".  Here's a counterexample:

    (let ((default-directory "/sudo::/")
          (process-environment `("TERM=dumber"
                                 ,@process-environment)))
      (shell-command "echo $TERM" t))
    => dumber

How do I tell Tramp not to export TERM=dumber to the remote in this
case?

To be clear: this may be a dumb example, but there are plenty of
interesting use-cases involving PATH, PYTHONPATH, and so on, which are
analogous.  How can I set PYTHONPATH buffer-locally, but disallow Tramp
to export this variable to a remote?

>>    As another example, python.el would append "PYTHONPATH" and
>>    "PYTHONHOME" globally to `tramp-remote-process-environment', since
>>    these variables hold directory names.
>
> Yes, if python.el developers prefer that. However, I doubt, that this
> value is always the same for all different remote hosts, the value might
> differ depending on the OS the remote host is running.

That's precisely what I meant: you *do not* want to export those variables
to a remote.  But currently Tramp effectively forbids anyone to set
PYTHONPATH buffer-locally, because this will cause them to be exported.

My suggestion is easier to implement than explain.  So let me repeat it
in the form of a little (possibly incomplete) patch:

diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index 015f458a63..3749f84e21 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -3938,21 +3938,23 @@ tramp-handle-make-process
                       (when (string-match-p "=" elt) elt))
                     tramp-remote-process-environment))
               ;; We use as environment the difference to toplevel
               ;; `process-environment'.
               (env (dolist (elt process-environment env)
                      (when
                          (and
                           (string-match-p "=" elt)
                           (not
                            (member
-                            elt (default-toplevel-value 
'process-environment))))
+                            elt (default-toplevel-value 'process-environment)))
+                           (not (member (car (split-string elt "="))
+                                        tramp-remote-process-environment)))
                        (setq env (cons elt env)))))
               (env (setenv-internal
                     env "INSIDE_EMACS" (tramp-inside-emacs) 'keep))
               (env (mapcar #'tramp-shell-quote-argument (delq nil env)))
               ;; Quote command.
               (command (mapconcat #'tramp-shell-quote-argument command " "))
               ;; Set cwd and environment variables.
               (command
                (append `("cd" ,localname "&&" "(" "env") env `(,command ")"))))