Re: Buffer-local process environments

[Augusto Stoffel]

> Because there's a lot of functions out there, which work for the local
> host, and should work also for remote hosts. See for example
> vc-git-grep, which has
>
> (let ((default-directory dir)
>       (compilation-environment (cons "PAGER=" compilation-environment)))
>
> compilation-environment will be propagated to process-environment later
> on. And the setting for PAGER is needed for both local and remote
> instances of vc-git-grep. There's no special code in vc-git-grep for the
> remote case.

Some observations:

1. In this case, one could call "git --no-pager" instead of relying on
   an env var.

2. PAGER is overridden by `tramp-remote-process-environment' anyway,
   right?  And unlike something of the likes of PYTHONPATH, I see no
   reason to customize PAGER.

3. If Tramp checked for the buffer-local value of process-environment
   instead of the default value, then the patch for compile.el I
   attached yesterday wouldn't break for remote directories, and the
   let-binding trick in your example would still work.

So I wonder:

A. Are there more compelling examples showing that Lisp code needs
   fine-grained control over variables being exported to a remote host?

B. There is probably a small list of variables that should be preserved
   across machines, while there is an unbounded quantity of variables
   that probably only make sense machine-locally (e.g., any variable
   holding directory names).

In view of 3., one could introduce the convention that the buffer-local
value of `process-environment' is for "project-local" variables, and the
let-bound value is for variables that make sense even on remote machines.

I'm not sure this is a good proposal, though.  It's a subtle rule, and
it could be quite brittle and hard to use.

An alternative proposal is to introduce a variable
`remote-exported-variables', which anyone could set or let-bind or even
override on a connection-local basis.  The value of any variable whose
name appears in this list would be passed though a remote connection.
This proposal would make a lot of sense if assumption B. is true.