Tramp: Refactor environment variable filtering to a separate function

* lisp/net/tramp.el (tramp-local-environment-variable-p):
New function.  (Bug#79413)
(tramp-handle-make-process):
* lisp/net/tramp-sh.el (tramp-sh-handle-make-process)
(tramp-sh-handle-process-file):
* lisp/net/tramp-androidsu.el
(tramp-androidsu-handle-make-process):
Use `tramp-local-environment-variable-p'.

3 files changed, 16 insertions, 9 deletions

diff --git a/lisp/net/tramp-androidsu.el b/lisp/net/tramp-androidsu.el
index a593833a836..6cc3f14381d 100644
--- a/lisp/net/tramp-androidsu.el
+++ b/lisp/net/tramp-androidsu.el
@@ -311,9 +311,7 @@ FUNCTION."
                   (when
                       (and
                        (string-search "=" elt)
-                       (not
-                        (member
-                         elt (default-toplevel-value 'process-environment))))
+                       (not (tramp-local-environment-variable-p elt)))
                     (setq env (cons elt env)))))
            ;; Add remote path if exists.
            (env (let ((remote-path (string-join (tramp-get-remote-path v) ":")))
diff --git a/lisp/net/tramp-sh.el b/lisp/net/tramp-sh.el
index 9d13cdc3a2d..0a454fed69b 100644
--- a/lisp/net/tramp-sh.el
+++ b/lisp/net/tramp-sh.el
@@ -3052,8 +3052,7 @@ will be used."
              ;; `process-environment'.
              env uenv
              (env (dolist (elt (cons prompt process-environment) env)
-                    (or (member
-                         elt (default-toplevel-value 'process-environment))
+                    (or (tramp-local-environment-variable-p elt)
                         (if (string-search "=" elt)
                             (setq env (append env `(,elt)))
                           (setq uenv (cons elt uenv))))))
@@ -3288,7 +3287,7 @@ will be used."
                                (cons program args) " "))
       ;; We use as environment the difference to toplevel `process-environment'.
       (dolist (elt process-environment)
-        (or (member elt (default-toplevel-value 'process-environment))
+        (or (tramp-local-environment-variable-p elt)
             (if (string-search "=" elt)
                 (setq env (append env `(,elt)))
               (setq uenv (cons elt uenv)))))
diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index ad768f9e038..a27e8b8fbe8 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -5347,6 +5347,18 @@ should be set connection-local.")
          (or (not (stringp buffer)) (not (tramp-tramp-file-p buffer)))
          (or (not (stringp stderr)) (not (tramp-tramp-file-p stderr))))))
 
+;; This function is used by tramp-*-handle-make-process and
+;; tramp-sh-handle-process-file to filter local environment variables
+;; that should not be propagated remotely.  Users can override this
+;; function if necessary, for example, to ensure that a specific
+;; environment variable is applied to remote processes, whether it
+;; exists locally or not.
+(defun tramp-local-environment-variable-p (arg)
+  "Return non-nil if ARG exists in default `process-environment'.
+Tramp does not propagate local environment variables in remote
+processes."
+  (member arg (default-toplevel-value 'process-environment)))
+
 (defun tramp-handle-make-process (&rest args)
   "An alternative `make-process' implementation for Tramp files."
   (tramp-skeleton-make-process args nil nil
@@ -5365,9 +5377,7 @@ should be set connection-local.")
            (env (dolist (elt process-environment env)
                   (when (and
                          (string-search "=" elt)
-                         (not
-                          (member
-                           elt (default-toplevel-value 'process-environment))))
+                         (not (tramp-local-environment-variable-p elt)))
                     (setq env (cons elt env)))))
            ;; Add remote path if exists.
            (env (if-let* ((sh-file-name-handler-p)