Re: Buffer-local process environments

[Michael Albinus <michael.albinus@gmx.de>]

Augusto Stoffel <arstoffel@gmail.com> writes:

> Hi Michael,

Hi Augusto,

>>> 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.

Yes, this is a bug (rather a missing feature) of Tramp. It is
implemented for synchronous processes, but not for asynchronous
processes. I'm aware of this, but I haven't found the mood to implement
it until now.

> 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.

Sure. Tramp shall not read the user's mind.

(As side remark, my wife expects always that I read her mind. I fail
miserably, usually.)

>> 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?

You can always shoot yourself in your feed.

> 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?

Set it only, when the buffer is not remote. Something like

(unless (file-remote-p default-directory) ...)

>>>    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.

No. See above.

> 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 ")"))))
>

I see. Interesting.

But this still means, that you use something global in
tramp-remote-process-environment for all remote processes.

What, if you want to set PYTHONPATH for hosta, and not for hostb?

Best regards, Michael.