Add Tramp method "apptainer"

* doc/misc/tramp.texi (Top, Configuration): Add "Optional methods".
(Optional methods): New section.
(Inline methods) <androidsu, toolbox, flatpak>: These are optional
methods.
(Inline methods) <apptainer>: Add.

* etc/NEWS: New Tramp method "apptainer".
Some Tramp methods are optional.
Fix typos.

* lisp/net/tramp-androidsu.el (tramp-enable-androidsu-method):
New defun.  Call it when `system-type' is `android'.

* lisp/net/tramp-container.el (tramp-apptainer-program): New defcustom.
(tramp-apptainer-method): New defconst.
(tramp-apptainer--completion-function)
(tramp-enable-toolbox-method, tramp-enable-flatpak-method)
(tramp-enable-apptainer-method): New defuns.

* lisp/net/tramp.el (tramp-enable-method): New defun.

* test/lisp/net/tramp-tests.el (tramp--test-container-p):
Add "apptainer".
(tramp--test-supports-processes-p): Extend function.

3 files changed, 164 insertions, 80 deletions

diff --git a/lisp/net/tramp-androidsu.el b/lisp/net/tramp-androidsu.el
index aa7871e6a33..3dcee8e2f8a 100644
--- a/lisp/net/tramp-androidsu.el
+++ b/lisp/net/tramp-androidsu.el
@@ -77,19 +77,25 @@ may edit files belonging to any and all applications."
   "Name of the local temporary directory on Android.")
 
 ;;;###tramp-autoload
-(tramp--with-startup
- (add-to-list 'tramp-methods
-	      `(,tramp-androidsu-method
-                (tramp-login-program       "su")
-                (tramp-login-args          (("-") ("%u")))
-                (tramp-remote-shell        ,tramp-androidsu-local-shell-name)
-                (tramp-remote-shell-login  ("-l"))
-                (tramp-remote-shell-args   ("-c"))
-                (tramp-tmpdir              ,tramp-androidsu-local-tmp-directory)
-                (tramp-connection-timeout  10)
-                (tramp-shell-name	   ,tramp-androidsu-local-shell-name)))
- (add-to-list 'tramp-default-user-alist
-              `(,tramp-androidsu-method nil ,tramp-root-id-string)))
+(defun tramp-enable-androidsu-method ()
+  "Enable \"androidsu\" method."
+  (add-to-list 'tramp-methods
+	       `(,tramp-androidsu-method
+                 (tramp-login-program       "su")
+                 (tramp-login-args          (("-") ("%u")))
+                 (tramp-remote-shell        ,tramp-androidsu-local-shell-name)
+                 (tramp-remote-shell-login  ("-l"))
+                 (tramp-remote-shell-args   ("-c"))
+                 (tramp-tmpdir              ,tramp-androidsu-local-tmp-directory)
+                 (tramp-connection-timeout  10)
+                 (tramp-shell-name	   ,tramp-androidsu-local-shell-name)))
+
+  (add-to-list 'tramp-default-user-alist
+               `(,tramp-androidsu-method nil ,tramp-root-id-string)))
+
+;;;###tramp-autoload
+(when (eq system-type 'android)
+  (tramp-enable-androidsu-method))
 
 (defvar android-use-exec-loader) ; androidfns.c.
 
diff --git a/lisp/net/tramp-container.el b/lisp/net/tramp-container.el
index 30639cbeb85..7559f958838 100644
--- a/lisp/net/tramp-container.el
+++ b/lisp/net/tramp-container.el
@@ -83,6 +83,15 @@
 ;; Where:
 ;;     SANDBOX	is the running sandbox to connect to.
 ;;		It could be an application ID, an instance ID, or a PID.
+;;
+;;
+;;
+;; Open a file on a running Apptainer instance:
+;;
+;;     C-x C-f /apptainer:INSTANCE:/path/to/file
+;;
+;; Where:
+;;     INSTANCE	is the running instance to connect to.
 
 ;;; Code:
 
@@ -143,6 +152,14 @@ If it is nil, the default context will be used."
                  (string)))
 
 ;;;###tramp-autoload
+(defcustom tramp-apptainer-program "apptainer"
+  "Name of the Apptainer client program."
+  :group 'tramp
+  :version "30.1"
+  :type '(choice (const "apptainer")
+                 (string)))
+
+;;;###tramp-autoload
 (defconst tramp-docker-method "docker"
   "Tramp method name to use to connect to Docker containers.")
 
@@ -173,6 +190,10 @@ This is for out-of-band connections.")
   "Tramp method name to use to connect to Flatpak sandboxes.")
 
 ;;;###tramp-autoload
+(defconst tramp-apptainer-method "apptainer"
+  "Tramp method name to use to connect to Apptainer instances.")
+
+;;;###tramp-autoload
 (defmacro tramp-skeleton-completion-function (method &rest body)
   "Skeleton for `tramp-*-completion-function' with multi-hop support.
 BODY is the backend specific code."
@@ -374,6 +395,28 @@ see its function help for a description of the format."
       (mapcar (lambda (name) (list nil name)) (delq nil names)))))
 
 ;;;###tramp-autoload
+(defun tramp-apptainer--completion-function (method)
+  "List Apptainer instances available for connection.
+
+This function is used by `tramp-set-completion-function', please
+see its function help for a description of the format."
+  (tramp-skeleton-completion-function method
+    (when-let ((raw-list
+		(shell-command-to-string (concat program " instance list")))
+	       ;; Ignore header line.
+               (lines (cdr (split-string raw-list "\n" 'omit)))
+               (names (mapcar
+		       (lambda (line)
+			 (when (string-match
+				(rx bol (group (1+ (not space)))
+				    (1+ space) (1+ (not space))
+				    (1+ space) (1+ (not space)))
+				line)
+			   (match-string 1 line)))
+                       lines)))
+      (mapcar (lambda (name) (list nil name)) (delq nil names)))))
+
+;;;###tramp-autoload
 (defvar tramp-default-remote-shell) ;; Silence byte compiler.
 
 ;;;###tramp-autoload
@@ -453,29 +496,9 @@ see its function help for a description of the format."
                 (tramp-remote-shell-login ("-l"))
                 (tramp-remote-shell-args ("-i" "-c"))))
 
- (add-to-list 'tramp-methods
-	      `(,tramp-toolbox-method
-		(tramp-login-program ,tramp-toolbox-program)
-		(tramp-login-args (("run")
-				   ("-c" "%h")
-				   ("%l")))
-		(tramp-direct-async (,tramp-default-remote-shell "-c"))
-		(tramp-remote-shell ,tramp-default-remote-shell)
-		(tramp-remote-shell-login ("-l"))
-		(tramp-remote-shell-args ("-c"))))
-
- (add-to-list 'tramp-default-host-alist `(,tramp-toolbox-method nil ""))
-
- (add-to-list 'tramp-methods
-	      `(,tramp-flatpak-method
-		(tramp-login-program ,tramp-flatpak-program)
-		(tramp-login-args (("enter")
-				   ("%h")
-				   ("%l")))
-		(tramp-direct-async (,tramp-default-remote-shell "-c"))
-		(tramp-remote-shell ,tramp-default-remote-shell)
-		(tramp-remote-shell-login ("-l"))
-		(tramp-remote-shell-args ("-c"))))
+ (add-to-list 'tramp-completion-multi-hop-methods tramp-docker-method)
+ (add-to-list 'tramp-completion-multi-hop-methods tramp-podman-method)
+ (add-to-list 'tramp-completion-multi-hop-methods tramp-kubernetes-method)
 
  (tramp-set-completion-function
   tramp-docker-method
@@ -495,53 +518,99 @@ see its function help for a description of the format."
 
  (tramp-set-completion-function
   tramp-kubernetes-method
-  `((tramp-kubernetes--completion-function ,tramp-kubernetes-method)))
+  `((tramp-kubernetes--completion-function ,tramp-kubernetes-method))))
 
- (tramp-set-completion-function
-  tramp-toolbox-method
-  `((tramp-toolbox--completion-function ,tramp-toolbox-method)))
+;;;###tramp-autoload
+(defun tramp-enable-toolbox-method ()
+  "Enable connection to Toolbox containers."
+  (add-to-list 'tramp-methods
+	       `(,tramp-toolbox-method
+		 (tramp-login-program ,tramp-toolbox-program)
+		 (tramp-login-args (("run")
+				    ("-c" "%h")
+				    ("%l")))
+		 (tramp-direct-async (,tramp-default-remote-shell "-c"))
+		 (tramp-remote-shell ,tramp-default-remote-shell)
+		 (tramp-remote-shell-login ("-l"))
+		 (tramp-remote-shell-args ("-c"))))
+
+  (add-to-list 'tramp-default-host-alist `(,tramp-toolbox-method nil ""))
+  (add-to-list 'tramp-completion-multi-hop-methods tramp-toolbox-method)
+
+  (tramp-set-completion-function
+   tramp-toolbox-method
+   `((tramp-toolbox--completion-function ,tramp-toolbox-method))))
 
- (tramp-set-completion-function
-  tramp-flatpak-method
-  `((tramp-flatpak--completion-function ,tramp-flatpak-method)))
+;;;###tramp-autoload
+(defun tramp-enable-flatpak-method ()
+  "Enable connection to Flatpak sandboxes."
+  (add-to-list 'tramp-methods
+	       `(,tramp-flatpak-method
+		 (tramp-login-program ,tramp-flatpak-program)
+		 (tramp-login-args (("enter")
+				    ("%h")
+				    ("%l")))
+		 (tramp-direct-async (,tramp-default-remote-shell "-c"))
+		 (tramp-remote-shell ,tramp-default-remote-shell)
+		 (tramp-remote-shell-login ("-l"))
+		 (tramp-remote-shell-args ("-c"))))
+
+  (add-to-list 'tramp-completion-multi-hop-methods tramp-flatpak-method)
+
+  (tramp-set-completion-function
+   tramp-flatpak-method
+   `((tramp-flatpak--completion-function ,tramp-flatpak-method))))
 
- (add-to-list 'tramp-completion-multi-hop-methods tramp-docker-method)
- (add-to-list 'tramp-completion-multi-hop-methods tramp-podman-method)
- (add-to-list 'tramp-completion-multi-hop-methods tramp-kubernetes-method)
- (add-to-list 'tramp-completion-multi-hop-methods tramp-toolbox-method)
- (add-to-list 'tramp-completion-multi-hop-methods tramp-flatpak-method)
-
- ;; Default connection-local variables for Tramp.
-
- (defconst tramp-kubernetes-connection-local-default-variables
-   '((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)))))
-   "Default connection-local variables for remote kubernetes connections.")
-
- (connection-local-set-profile-variables
-  'tramp-kubernetes-connection-local-default-profile
-  tramp-kubernetes-connection-local-default-variables)
-
- (connection-local-set-profiles
-  `(:application tramp :protocol ,tramp-kubernetes-method)
-  'tramp-kubernetes-connection-local-default-profile)
-
- (defconst tramp-flatpak-connection-local-default-variables
-   `((tramp-remote-path  . ,(cons "/app/bin" tramp-remote-path)))
-   "Default connection-local variables for remote flatpak connections.")
-
- (connection-local-set-profile-variables
-  'tramp-flatpak-connection-local-default-profile
-  tramp-flatpak-connection-local-default-variables)
-
- (connection-local-set-profiles
-  `(:application tramp :protocol ,tramp-flatpak-method)
-  'tramp-flatpak-connection-local-default-profile))
+;;;###tramp-autoload
+(defun tramp-enable-apptainer-method ()
+  "Enable connection to Apptainer instances."
+  (add-to-list 'tramp-methods
+	       `(,tramp-apptainer-method
+		 (tramp-login-program ,tramp-apptainer-program)
+		 (tramp-login-args (("shell")
+				    ("instance://%h")
+				    ("%h"))) ; Needed for multi-hop check.
+		 (tramp-remote-shell ,tramp-default-remote-shell)
+		 (tramp-remote-shell-login ("-l"))
+		 (tramp-remote-shell-args ("-c"))))
+
+  (add-to-list 'tramp-completion-multi-hop-methods tramp-apptainer-method)
+
+  (tramp-set-completion-function
+   tramp-apptainer-method
+   `((tramp-apptainer--completion-function ,tramp-apptainer-method))))
+
+;; Default connection-local variables for Tramp.
+
+(defconst tramp-kubernetes-connection-local-default-variables
+  '((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)))))
+  "Default connection-local variables for remote kubernetes connections.")
+
+(connection-local-set-profile-variables
+ 'tramp-kubernetes-connection-local-default-profile
+ tramp-kubernetes-connection-local-default-variables)
+
+(connection-local-set-profiles
+ `(:application tramp :protocol ,tramp-kubernetes-method)
+ 'tramp-kubernetes-connection-local-default-profile)
+
+(defconst tramp-flatpak-connection-local-default-variables
+  `((tramp-remote-path  . ,(cons "/app/bin" tramp-remote-path)))
+  "Default connection-local variables for remote flatpak connections.")
+
+(connection-local-set-profile-variables
+ 'tramp-flatpak-connection-local-default-profile
+ tramp-flatpak-connection-local-default-variables)
+
+(connection-local-set-profiles
+ `(:application tramp :protocol ,tramp-flatpak-method)
+ 'tramp-flatpak-connection-local-default-profile)
 
 (add-hook 'tramp-unload-hook
 	  (lambda ()
diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index 4c62d1d9f39..f6d02847927 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -1460,6 +1460,15 @@ calling HANDLER.")
 
 ;;; Internal functions which must come first:
 
+(defun tramp-enable-method (method)
+  "Enable optional METHOD if possible."
+  (interactive "Mmethod: ")
+  (when-let (((not (assoc method tramp-methods)))
+	     (fn (intern (format "tramp-enable-%s-method" method)))
+	     ((functionp fn)))
+    (funcall fn)
+    (message "Tramp method \"%s\" enabled" method)))
+
 ;; Conversion functions between external representation and
 ;; internal data structure.  Convenience functions for internal
 ;; data structure.