Allow format specifiers in `tramp-login-program'

* doc/misc/tramp.texi (Extension packages): Explain how to use own
format specifiers.

* lisp/net/tramp.el (tramp-expand-args): Make DEFAULT argument
optional.  Handle also ARGS being an atom.
(tramp-handle-make-process):
* lisp/net/tramp-container.el (tramp-skeleton-completion-function):
* lisp/net/tramp-sh.el (tramp-ssh-or-plink-options)
(tramp-maybe-open-connection):
* lisp/net/tramp-sshfs.el (tramp-sshfs-handle-process-file):
Call `tramp-expand-args' for `tramp-login-program'.

doc/misc/tramp.texi

@@ -6728,6 +6728,45 @@ See the docstring of variable @code{tramp-methods} for possible
 @code{foo-tramp-executable} in this example would be a Lisp constant,
 which is the program name of @command{foo}.
 
+If a parameter doesn't have a static value but must be computed at
+runtime, a format specifier can be used, like @t{"%h"} in the example
+above.  See the docstring of @code{tramp-methods}, which patterns are
+expanded in which parameter.  Furthermore, other format specifiers can
+be added via the variable @code{tramp-extra-expand-args}.
+
+The following parameters expand format specifiers for the
+@code{tramp-sh} backend: @code{tramp-copy-args},
+@code{tramp-copy-env}, @code{tramp-copy-file-name},
+@code{tramp-login-args}, @code{tramp-login-program},
+@code{tramp-remote-copy-args}.
+
+The example above could use
+
+@lisp
+(tramp-login-program "%1")
+@end lisp
+
+And you could set @code{tramp-extra-expand-args} as connection-local value:
+
+@lisp
+@group
+(defun foo-tramp-get-login-program (vec)
+  "Return connection-local value of `tramp-login-program'."
+  @dots{})
+@end group
+
+@group
+(connection-local-set-profile-variables
+ 'foo-tramp-connection-local-default-profile
+ '((tramp-extra-expand-args
+    ?1 (foo-tramp-get-login-program (car tramp-current-connection)))))
+
+(connection-local-set-profiles
+ '(:application tramp :protocol "foo")
+ foo-tramp-connection-local-default-profile)
+@end group
+@end lisp
+
 Another initialization could tell @value{tramp} which are the default
 user and host name for method @option{foo}.  This is done by calling
 @code{tramp-set-completion-function}:

lisp/net/tramp-container.el

@@ -266,7 +266,7 @@ BODY is the backend specific code."
 		    tramp--last-hop-directory)
 	       tramp-compat-temporary-file-directory))
 	  (program (let ((tramp-verbose 0))
-		     (tramp-get-method-parameter
+		     (tramp-expand-args
 		      (make-tramp-file-name :method ,method)
 		      'tramp-login-program)))
 	  (vec (when (tramp-tramp-file-p default-directory)
@@ -656,10 +656,9 @@ see its function help for a description of the format."
    '((tramp-config-check . tramp-kubernetes--current-context-data)
      ;; This variable will be eval'ed in `tramp-expand-args'.
      (tramp-extra-expand-args
-      . (?a (tramp-kubernetes--container (car tramp-current-connection))
-	 ?h (tramp-kubernetes--pod (car tramp-current-connection))
-	 ?x (tramp-kubernetes--context-namespace
-	     (car tramp-current-connection)))))
+      ?a (tramp-kubernetes--container (car tramp-current-connection))
+      ?h (tramp-kubernetes--pod (car tramp-current-connection))
+      ?x (tramp-kubernetes--context-namespace (car tramp-current-connection))))
    "Default connection-local variables for remote kubernetes connections.")
 
  (connection-local-set-profile-variables

lisp/net/tramp-sh.el

@@ -5037,7 +5037,7 @@ Goes through the list `tramp-inline-compress-commands'."
    ;; Use plink options.
    ((string-match-p
      (rx "plink" (? ".exe") eol)
-     (tramp-get-method-parameter vec 'tramp-login-program))
+     (tramp-expand-args vec 'tramp-login-program))
     (concat
      (if (eq tramp-use-connection-share 'suppress)
 	 "-noshare" "-share")
@@ -5398,7 +5398,7 @@ connection if a previous connection has died for some reason."
 			   hop 'tramp-connection-timeout
 			   tramp-connection-timeout))
 			 (command
-			  (tramp-get-method-parameter
+			  (tramp-expand-args
 			   hop 'tramp-login-program))
 			 ;; We don't create the temporary file.  In
 			 ;; fact, it is just a prefix for the

lisp/net/tramp-sshfs.el

@@ -269,7 +269,7 @@ arguments to pass to the OPERATION."
       (setq ret
 	    (apply
 	     #'tramp-call-process
-	     v (tramp-get-method-parameter v 'tramp-login-program)
+	     v (tramp-expand-args v 'tramp-login-program)
 	     nil outbuf display
 	     (tramp-expand-args
 	      v 'tramp-login-args nil

lisp/net/tramp.el

@@ -5325,7 +5325,7 @@ Do not set it manually, it is used buffer-local in `tramp-get-lock-pid'.")
 (defvar tramp-extra-expand-args nil
   "Method specific arguments.")
 
-(defun tramp-expand-args (vec parameter default &rest spec-list)
+(defun tramp-expand-args (vec parameter &optional default &rest spec-list)
   "Expand login arguments as given by PARAMETER in `tramp-methods'.
 PARAMETER is a symbol like `tramp-login-args', denoting a list of
 list of strings from `tramp-methods', containing %-sequences for
@@ -5348,12 +5348,14 @@ a connection-local variable."
       (setq spec-list (cddr spec-list)))
     (setq spec (apply #'format-spec-make extra-spec-list))
     ;; Expand format spec.
-    (flatten-tree
-     (mapcar
-      (lambda (x)
-	(setq x (mapcar (lambda (y) (tramp-format-spec y spec)) x))
-	(unless (member "" x) x))
-      args))))
+    (if (atom args)
+	(tramp-format-spec args spec)
+      (flatten-tree
+       (mapcar
+	(lambda (x)
+	  (setq x (mapcar (lambda (y) (tramp-format-spec y spec)) x))
+	  (unless (member "" x) x))
+	args)))))
 
 (defun tramp-post-process-creation (proc vec)
   "Apply actions after creation of process PROC."
@@ -5476,7 +5478,7 @@ processes."
                  `(,(string-join command " ")))
 	      command))
 	   (login-program
-	    (tramp-get-method-parameter v 'tramp-login-program))
+	    (tramp-expand-args v 'tramp-login-program))
 	   ;; We don't create the temporary file.  In fact, it is just
 	   ;; a prefix for the ControlPath option of ssh; the real
 	   ;; temporary file has another name, and it is created and