Make tramp-archive fit for older Emacsen

* lisp/net/tramp-archive.el (tramp-archive-enabled)
(tramp-archive-file-name-handler-alist)
(tramp-archive-file-name-handler): Adapt docstring.
(tramp-register-archive-file-name-handler): Remove it from
`after-init-hook' when unloading.
(tramp-archive-gvfs-host): New defsubst.
(tramp-archive-dissect-file-name): Use it.

* lisp/net/tramp-cmds.el (tramp-cleanup-all-connections):
Check that `tramp-archive-enabled' is bound.

* test/lisp/net/tramp-archive-tests.el (tramp-archive-test42-auto-load):
Check also that tramp-archive is not loaded when Tramp is loaded.
(tramp-archive-test42-delay-load): Adapt test messages.

lisp/net/tramp-archive.el

@@ -116,8 +116,9 @@
 ;; would load Tramp. So we make a cheaper check.
 ;;;###autoload
 (defvar tramp-archive-enabled (featurep 'dbusbind)
-  "Non-nil when GVFS is available.")
+  "Non-nil when file archive support is available.")
 
+;; After loading tramp-gvfs.el, we know it better.
 (setq tramp-archive-enabled tramp-gvfs-enabled)
 
 ;; <https://github.com/libarchive/libarchive/wiki/LibarchiveFormats>
@@ -175,6 +176,9 @@ It must be supported by libarchive(3).")
     "\\)" ;; \1
     "\\(" "/" ".*" "\\)" "\\'"))) ;; \2
 
+;; In older Emacsen (prior 27.1), `tramp-archive-autoload-file-name-regexp'
+;; is not autoloaded.  So we cannot expect it to be known in
+;; tramp-loaddefs.el. But it exists, when tramp-archive.el is loaded.
 ;;;###tramp-autoload
 (defconst tramp-archive-file-name-regexp
   (ignore-errors (tramp-archive-autoload-file-name-regexp))
@@ -266,7 +270,7 @@ It must be supported by libarchive(3).")
     (vc-registered . ignore)
     (verify-visited-file-modtime . tramp-handle-verify-visited-file-modtime)
     (write-region . tramp-archive-handle-not-implemented))
-  "Alist of handler functions for GVFS archive method.
+  "Alist of handler functions for file archive method.
 Operations not mentioned here will be handled by the default Emacs primitives.")
 
 (defsubst tramp-archive-file-name-for-operation (operation &rest args)
@@ -288,7 +292,7 @@ pass to the OPERATION."
 
 ;;;###tramp-autoload
 (defun tramp-archive-file-name-handler (operation &rest args)
-  "Invoke the GVFS archive related OPERATION.
+  "Invoke the file archive related OPERATION.
 First arg specifies the OPERATION, second arg is a list of arguments to
 pass to the OPERATION."
   (let* ((filename (apply 'tramp-archive-file-name-for-operation
@@ -323,8 +327,16 @@ pass to the OPERATION."
     (put 'tramp-archive-file-name-handler 'safe-magic t))))
 
 ;;;###autoload
-(add-hook 'after-init-hook 'tramp-register-archive-file-name-handler)
+(progn
+  (add-hook 'after-init-hook 'tramp-register-archive-file-name-handler)
+  (add-hook
+   'tramp-archive-unload-hook
+   (lambda ()
+     (remove-hook
+      'after-init-hook 'tramp-register-archive-file-name-handler))))
 
+;; In older Emacsen (prior 27.1), the autoload above does not exist.
+;; So we call it again; it doesn't hurt.
 (tramp-register-archive-file-name-handler)
 
 ;; Mark `operations' the handler is responsible for.
@@ -343,12 +355,6 @@ pass to the OPERATION."
        (remove-hook
 	'url-handler-mode-hook 'tramp-register-file-name-handlers)))))
 
-;; Debug.
-;(trace-function-background 'tramp-archive-file-name-handler)
-;(trace-function-background 'tramp-gvfs-file-name-handler)
-;(trace-function-background 'tramp-file-name-archive)
-;(trace-function-background 'tramp-archive-dissect-file-name)
-
 
 ;; File name conversions.
 
@@ -374,6 +380,10 @@ The hash key is the archive name.  The value is a cons of the
 used `tramp-file-name' structure for tramp-gvfs, and the file
 name of a local copy, if any.")
 
+(defsubst tramp-archive-gvfs-host (archive)
+  "Return host name of ARCHIVE as used in GVFS for mounting"
+  (url-hexify-string (tramp-gvfs-url-file-name archive)))
+
 (defun tramp-archive-dissect-file-name (name)
   "Return a `tramp-file-name' structure.
 The structure consists of the `tramp-archive-method' method, the
@@ -397,8 +407,7 @@ name is kept in slot `hop'"
 	(let ((archive
 	       (tramp-make-tramp-file-name
 		(tramp-archive-dissect-file-name archive) nil 'noarchive)))
-	  (setf (tramp-file-name-host vec)
-		(url-hexify-string (tramp-gvfs-url-file-name archive))))
+	  (setf (tramp-file-name-host vec) (tramp-archive-gvfs-host archive)))
 	(puthash archive (list vec) tramp-archive-hash))
 
        ;; http://...
@@ -411,15 +420,13 @@ name is kept in slot `hop'"
 		 (url-type (url-generic-parse-url archive))
 		 url-tramp-protocols))
 	       (archive (url-tramp-convert-url-to-tramp archive)))
-	  (setf (tramp-file-name-host vec)
-		(url-hexify-string (tramp-gvfs-url-file-name archive))))
+	  (setf (tramp-file-name-host vec) (tramp-archive-gvfs-host archive)))
 	(puthash archive (list vec) tramp-archive-hash))
 
        ;; GVFS supported schemes.
        ((or (tramp-gvfs-file-name-p archive)
 	    (not (file-remote-p archive)))
-	(setf (tramp-file-name-host vec)
-	      (url-hexify-string (tramp-gvfs-url-file-name archive)))
+	(setf (tramp-file-name-host vec) (tramp-archive-gvfs-host archive))
 	(puthash archive (list vec) tramp-archive-hash))
 
        ;; Anything else.  Here we call `file-local-copy', which we
@@ -428,8 +435,7 @@ name is kept in slot `hop'"
 		 (inhibit-file-name-handlers
 		  (cons 'jka-compr-handler inhibit-file-name-handlers))
 		 (copy (file-local-copy archive)))
-	    (setf (tramp-file-name-host vec)
-		  (url-hexify-string (tramp-gvfs-url-file-name copy)))
+	    (setf (tramp-file-name-host vec) (tramp-archive-gvfs-host copy))
 	    (puthash archive (cons vec copy) tramp-archive-hash))))
 
       ;; So far, `vec' handles just the mount point.  Add `localname',

lisp/net/tramp-cmds.el

@@ -144,7 +144,8 @@ This includes password cache, file cache, connection cache, buffers."
   (clrhash tramp-cache-data)
 
   ;; Cleanup local copies of archives.
-  (tramp-archive-cleanup-hash)
+  (when (bound-and-true-p tramp-archive-enabled)
+    (tramp-archive-cleanup-hash))
 
   ;; Remove buffers.
   (dolist (name (tramp-list-tramp-buffers))

test/lisp/net/tramp-archive-tests.el

@@ -808,21 +808,29 @@ This tests also `file-executable-p', `file-writable-p' and `set-file-modes'."
   ;; Autoloading tramp-archive works since Emacs 27.1.
   (skip-unless (tramp-archive--test-emacs27-p))
 
+  ;; tramp-archive is neither loaded at Emacs startup, nor when
+  ;; loading a file like "/ssh::" (which loads Tramp).
   (let ((default-directory (expand-file-name temporary-file-directory))
 	(code
+	 "(progn \
+	    (message \"tramp-archive loaded: %%s %%s\" \
+              (featurep 'tramp) (featurep 'tramp-archive)) \
+	    (file-attributes %S \"/\") \
+	    (message \"tramp-archive loaded: %%s %%s\" \
+              (featurep 'tramp) (featurep 'tramp-archive)))"))
+    (dolist (file `("/ssh::foo" ,(concat tramp-archive-test-archive "foo")))
+      (should
+       (string-match
+	(format
+	 "tramp-archive loaded: nil nil[[:ascii:]]+tramp-archive loaded: t %s"
+	 (tramp-archive-file-name-p file))
+	(shell-command-to-string
 	 (format
-	  "(message \"Tramp loaded: %%s\" (and (file-exists-p %S) t))"
-	  tramp-archive-test-archive)))
-    (should
-     (string-match
-      "Tramp loaded: t[\n\r]+"
-      (shell-command-to-string
-       (format
-	"%s -batch -Q -L %s --eval %s"
-	(shell-quote-argument
-	 (expand-file-name invocation-name invocation-directory))
-	(mapconcat 'shell-quote-argument load-path " -L ")
-	(shell-quote-argument code)))))))
+	  "%s -batch -Q -L %s --eval %s"
+	  (shell-quote-argument
+	   (expand-file-name invocation-name invocation-directory))
+	  (mapconcat 'shell-quote-argument load-path " -L ")
+	  (shell-quote-argument (format code file)))))))))
 
 (ert-deftest tramp-archive-test42-delay-load ()
   "Check that `tramp-archive' is loaded lazily, only when needed."
@@ -836,18 +844,21 @@ This tests also `file-executable-p', `file-writable-p' and `set-file-modes'."
   (let ((default-directory (expand-file-name temporary-file-directory))
 	(code
 	 "(progn \
-          (setq tramp-archive-enabled %s) \
-	  (message \"Tramp loaded: %%s\" (featurep 'tramp-archive)) \
-	  (find-file %S \"/\") \
-	  (message \"Tramp loaded: %%s\" (featurep 'tramp-archive)) \
-	  (file-attributes %S \"/\") \
-	  (message \"Tramp loaded: %%s\" (featurep 'tramp-archive)))"))
+            (setq tramp-archive-enabled %s) \
+	    (message \"tramp-archive loaded: %%s\" \
+              (featurep 'tramp-archive)) \
+	    (file-attributes %S \"/\") \
+	    (message \"tramp-archive loaded: %%s\" \
+              (featurep 'tramp-archive)) \
+	    (file-attributes %S \"/\") \
+	    (message \"tramp-archive loaded: %%s\" \
+              (featurep 'tramp-archive)))"))
     ;; tramp-archive doesn't load when `tramp-archive-enabled' is nil.
     (dolist (tae '(t nil))
       (should
        (string-match
 	(format
-    "Tramp loaded: nil[[:ascii:]]+Tramp loaded: nil[[:ascii:]]+Tramp loaded: %s"
+	 "tramp-archive loaded: nil[[:ascii:]]+tramp-archive loaded: nil[[:ascii:]]+tramp-archive loaded: %s"
 	 tae)
         (shell-command-to-string
          (format