Propagate EMACSCLIENT_TRAMP to remote hosts with Tramp

* doc/misc/tramp.texi (Remote processes):
Explain `tramp-propagate-emacsclient-tramp'.

* lisp/net/tramp.el (tramp-remote-process-environment): Adapt docstring.
(tramp-propagate-emacsclient-tramp): New defcustom.
(tramp-handle-make-process):
* lisp/net/tramp-sh.el (tramp-sh-handle-make-process)
(tramp-sh-handle-process-file): Use it.

* test/lisp/net/tramp-tests.el (tramp-test33-environment-variables):
Adapt test.

doc/misc/tramp.texi

@@ -4227,6 +4227,14 @@ called is local or remote, since @value{tramp} would add just the
 @env{HGPLAIN} setting and local processes would take whole value of
 @code{process-environment} along with the new value of @env{HGPLAIN}.
 
+@vindex tramp-propagate-emacsclient-tramp
+@vindex EMACSCLIENT_TRAMP@r{, environment variable}
+If you set the user option @code{tramp-propagate-emacsclient-tramp} to
+a non-@code{nil} value, the environment variable
+@env{EMACSCLIENT_TRAMP} will be set to a value which allows to call
+@command{emacsclient} from a process running on the remote
+host. @xref{emacsclient Options, , , emacs}.
+
 For integrating other Emacs packages so @value{tramp} can execute
 remotely, please file a bug report.  @xref{Bug Reports}.
 

lisp/net/tramp-sh.el

@@ -3091,6 +3091,11 @@ will be used."
 			(if (string-search "=" elt)
 			    (setq env (append env `(,elt)))
 			  (setq uenv (cons elt uenv))))))
+	     (env (if tramp-propagate-emacsclient-tramp
+		      (setenv-internal
+		       env "EMACSCLIENT_TRAMP"
+		       (tramp-make-tramp-file-name v 'noloc) 'keep)
+		    env))
 	     (env (setenv-internal
 		   env "INSIDE_EMACS" (tramp-inside-emacs) 'keep))
 	     ;; Environment is too large.  Keep it here.
@@ -3340,6 +3345,10 @@ will be used."
             (if (string-search "=" elt)
                 (setq env (append env `(,elt)))
               (setq uenv (cons elt uenv)))))
+      (when tramp-propagate-emacsclient-tramp
+	(setq env (setenv-internal
+		   env "EMACSCLIENT_TRAMP"
+		   (tramp-make-tramp-file-name v 'noloc) 'keep)))
       (setq env (setenv-internal env "INSIDE_EMACS" (tramp-inside-emacs) 'keep))
       (when env
 	(setq command

lisp/net/tramp.el

@@ -1528,12 +1528,21 @@ The PATH environment variable should be set via `tramp-remote-path'.
 
 The TERM environment variable should be set via `tramp-terminal-type'.
 
+The EMACSCLIENT_TRAMP environment variable will be set accordingly, if
+`tramp-propagate-emacsclient-tramp' is non-nil.
+
 The INSIDE_EMACS environment variable will automatically be set
 based on the Tramp and Emacs versions, and should not be set here."
   :version "26.1"
   :type '(repeat string)
   :link '(info-link :tag "Tramp manual" "(tramp) Remote processes"))
 
+(defcustom tramp-propagate-emacsclient-tramp nil
+  "Whether to propagate the EMACSCLIENT_TRAMP environment variable."
+  :version "31.1"
+  :type 'boolean
+  :link '(info-link :tag "Tramp manual" "(tramp) Remote processes"))
+
 ;;; Internal Variables:
 
 ;;;###tramp-autoload
@@ -5509,6 +5518,13 @@ processes."
 	   (env (if sh-file-name-handler-p
 		    (setenv-internal env "TERM" tramp-terminal-type 'keep)
 		  env))
+	   ;; Add EMACSCLIENT_TRAMP.
+	   (env (if (and tramp-propagate-emacsclient-tramp
+			 sh-file-name-handler-p)
+		    (setenv-internal
+		     env "EMACSCLIENT_TRAMP"
+		     (tramp-make-tramp-file-name v 'noloc) 'keep)
+		  env))
 	   ;; Add INSIDE_EMACS.
 	   (env (setenv-internal env "INSIDE_EMACS" (tramp-inside-emacs) 'keep))
 	   (env (mapcar #'tramp-shell-quote-argument (delq nil env)))

test/lisp/net/tramp-tests.el

@@ -6555,6 +6555,21 @@ INPUT, if non-nil, is a string sent to the process."
 	  (funcall
 	   this-shell-command-to-string "echo \"${INSIDE_EMACS:-bla}\""))))
 
+      ;; Check EMACSCLIENT_TRAMP.
+      (setenv "EMACSCLIENT_TRAMP")
+      (let ((tramp-propagate-emacsclient-tramp t))
+	(should
+	 (string-equal
+	  (format "%s\n" (tramp-make-tramp-file-name tramp-test-vec 'noloc))
+	  (funcall
+	   this-shell-command-to-string "echo \"${EMACSCLIENT_TRAMP:-bla}\""))))
+      (let (tramp-propagate-emacsclient-tramp)
+	(should
+	 (string-equal
+	  "bla\n"
+	  (funcall
+	   this-shell-command-to-string "echo \"${EMACSCLIENT_TRAMP:-bla}\""))))
+
       ;; Set a value.
       (let ((process-environment
 	     (cons (concat envvar "=foo") process-environment)))