Implement `org-export-custom-protocol-maybe' and use it

* lisp/ox.el (org-export-custom-protocol-maybe): New function.
* contrib/lisp/ox-groff.el (org-groff-link):
* lisp/ox-ascii.el (org-ascii-link):
* lisp/ox-beamer.el (org-beamer-link):
* lisp/ox-html.el (org-html-link):
* lisp/ox-latex.el (org-latex-link):
* lisp/ox-man.el (org-man-link):
* lisp/ox-md.el (org-md-link):
* lisp/ox-odt.el (org-odt-link):
* lisp/ox-texinfo.el (org-texinfo-link): Use new function.

* testing/lisp/test-ox.el (test-org-export/custom-protocol-maybe): New
  test.

contrib/lisp/ox-groff.el

@@ -1255,9 +1255,9 @@ INFO is a plist holding contextual information.  See
                  (concat type ":" raw-path))
                 ((and (string= type "file") (file-name-absolute-p raw-path))
                  (concat "file://" raw-path))
-                (t raw-path)))
-         protocol)
+                (t raw-path))))
     (cond
+     ((org-export-custom-protocol-maybe link desc info))
      ;; Image file.
      (imagep (org-groff-link--inline-image link info))
      ;; import groff files

lisp/ox-ascii.el

@@ -1520,6 +1520,7 @@ DESC is the description part of the link, or the empty string.
 INFO is a plist holding contextual information."
   (let ((type (org-element-property :type link)))
     (cond
+     ((org-export-custom-protocol-maybe link desc info))
      ((string= type "coderef")
       (let ((ref (org-element-property :path link)))
 	(format (org-export-get-coderef-format ref desc)
@@ -1545,10 +1546,6 @@ INFO is a plist holding contextual information."
 			  (org-export-data
 			   (org-element-property :title destination)
 			   info)))))))))
-     ((let ((protocol (nth 2 (assoc type org-link-protocols)))
-	    (path (org-element-property :path link)))
-	(and (functionp protocol)
-	     (funcall protocol (org-link-unescape path) desc 'ascii))))
      (t
       (let ((raw-link (org-element-property :raw-link link)))
 	(if (not (org-string-nw-p desc)) (format "[%s]" raw-link)

lisp/ox-beamer.el

@@ -689,8 +689,10 @@ CONTENTS is the description part of the link.  INFO is a plist
 used as a communication channel."
   (let ((type (org-element-property :type link))
 	(path (org-element-property :path link)))
-    ;; Use \hyperlink command for all internal links.
     (cond
+     ;; Link type is handled by a special function.
+     ((org-export-custom-protocol-maybe link contents info))
+     ;; Use \hyperlink command for all internal links.
      ((equal type "radio")
       (let ((destination (org-export-resolve-radio-link link info)))
 	(if (not destination) contents

lisp/ox-html.el

@@ -2765,9 +2765,10 @@ INFO is a plist holding contextual information.  See
 		 (org-export-read-attribute :attr_html parent))))
 	 (attributes
 	  (let ((attr (org-html--make-attribute-string attributes-plist)))
-	    (if (org-string-nw-p attr) (concat " " attr) "")))
-	 protocol)
+	    (if (org-string-nw-p attr) (concat " " attr) ""))))
     (cond
+     ;; Link type is handled by a special function.
+     ((org-export-custom-protocol-maybe link desc info))
      ;; Image file.
      ((and (plist-get info :html-inline-images)
 	   (org-export-inline-image-p
@@ -2856,9 +2857,6 @@ INFO is a plist holding contextual information.  See
 		attributes
 		(format (org-export-get-coderef-format path desc)
 			(org-export-resolve-coderef path info)))))
-     ;; Link type is handled by a special function.
-     ((functionp (setq protocol (nth 2 (assoc type org-link-protocols))))
-      (funcall protocol (org-link-unescape path) desc 'html))
      ;; External link with a description part.
      ((and path desc) (format "<a href=\"%s\"%s>%s</a>" path attributes desc))
      ;; External link without a description part.

lisp/ox-latex.el

@@ -1965,9 +1965,10 @@ INFO is a plist holding contextual information.  See
 		 (concat type ":" raw-path))
 		((and (string= type "file") (file-name-absolute-p raw-path))
 		 (concat "file:" raw-path))
-		(t raw-path)))
-	 protocol)
+		(t raw-path))))
     (cond
+     ;; Link type is handled by a special function.
+     ((org-export-custom-protocol-maybe link desc info))
      ;; Image file.
      (imagep (org-latex--inline-image link info))
      ;; Radio link: Transcode target's contents and use them as link's
@@ -2023,9 +2024,6 @@ INFO is a plist holding contextual information.  See
      ((string= type "coderef")
       (format (org-export-get-coderef-format path desc)
 	      (org-export-resolve-coderef path info)))
-     ;; Link type is handled by a special function.
-     ((functionp (setq protocol (nth 2 (assoc type org-link-protocols))))
-      (funcall protocol (org-link-unescape path) desc 'latex))
      ;; External link with a description part.
      ((and path desc) (format "\\href{%s}{%s}" path desc))
      ;; External link without a description part.

lisp/ox-man.el

@@ -657,6 +657,8 @@ INFO is a plist holding contextual information.  See
                 (t raw-path)))
          protocol)
     (cond
+     ;; Link type is handled by a special function.
+     ((org-export-custom-protocol-maybe link desc info))
      ;; External link with a description part.
      ((and path desc) (format "%s \\fBat\\fP \\fI%s\\fP" path desc))
      ;; External link without a description part.

lisp/ox-md.el

@@ -313,6 +313,8 @@ a communication channel."
 	      raw-path))))
 	(type (org-element-property :type link)))
     (cond
+     ;; Link type is handled by a special function.
+     ((org-export-custom-protocol-maybe link contents info))
      ((member type '("custom-id" "id"))
       (let ((destination (org-export-resolve-id-link link info)))
 	(if (stringp destination)	; External file.
@@ -358,13 +360,6 @@ a communication channel."
 		     ;; BUG: shouldn't headlines have a form like [ref](name) in md?
 		     (org-export-data
 		      (org-element-property :title destination) info))))))))
-     ;; Link type is handled by a special function.
-     ((let ((protocol (nth 2 (assoc type org-link-protocols))))
-	(and (functionp protocol)
-	     (funcall protocol
-		      (org-link-unescape (org-element-property :path link))
-		      contents
-		      'md))))
      (t (let* ((raw-path (org-element-property :path link))
 	       (path
 		(cond

lisp/ox-odt.el

@@ -2736,9 +2736,10 @@ INFO is a plist holding contextual information.  See
 		 (concat "file:" raw-path))
 		(t raw-path)))
 	 ;; Convert & to & for correct XML representation
-	 (path (replace-regexp-in-string "&" "&" path))
-	 protocol)
+	 (path (replace-regexp-in-string "&" "&" path)))
     (cond
+     ;; Link type is handled by a special function.
+     ((org-export-custom-protocol-maybe link desc info))
      ;; Image file.
      ((and (not desc) (org-export-inline-image-p
 		       link (plist-get info :odt-inline-image-rules)))
@@ -2820,9 +2821,6 @@ INFO is a plist holding contextual information.  See
 	 (format
 	  "<text:bookmark-ref text:reference-format=\"number\" text:ref-name=\"OrgXref.%s\">%s</text:bookmark-ref>"
 	  href line-no))))
-     ;; Link type is handled by a special function.
-     ((functionp (setq protocol (nth 2 (assoc type org-link-protocols))))
-      (funcall protocol (org-link-unescape path) desc 'odt))
      ;; External link with a description part.
      ((and path desc)
       (let ((link-contents (org-element-contents link)))

lisp/ox-texinfo.el

@@ -915,9 +915,9 @@ INFO is a plist holding contextual information.  See
 		 (concat type ":" raw-path))
 		((and (string= type "file") (file-name-absolute-p raw-path))
 		 (concat "file:" raw-path))
-		(t raw-path)))
-	 protocol)
+		(t raw-path))))
     (cond
+     ((org-export-custom-protocol-maybe link desc info))
      ((equal type "radio")
       (let ((destination (org-export-resolve-radio-link link info)))
 	(if (not destination) desc
@@ -976,9 +976,6 @@ INFO is a plist holding contextual information.  See
       (format "@email{%s}"
 	      (concat (org-texinfo--sanitize-content path)
 		      (and desc (concat "," desc)))))
-     ((let ((protocol (nth 2 (assoc type org-link-protocols))))
-	(and (functionp protocol)
-	     (funcall protocol (org-link-unescape path) desc 'texinfo))))
      ;; External link with a description part.
      ((and path desc) (format "@uref{%s,%s}" path desc))
      ;; External link without a description part.

lisp/ox.el

@@ -3862,6 +3862,9 @@ meant to be translated with `org-export-data' or alike."
 
 ;;;; For Links
 ;;
+;; `org-export-custom-protocol-maybe' handles custom protocol defined
+;; with `org-add-link-type', which see.
+;;
 ;; `org-export-solidify-link-text' turns a string into a safer version
 ;; for links, replacing most non-standard characters with hyphens.
 ;;
@@ -3888,6 +3891,28 @@ meant to be translated with `org-export-data' or alike."
   (save-match-data
     (mapconcat 'identity (org-split-string s "[^a-zA-Z0-9_.-:]+") "-")))
 
+(defun org-export-custom-protocol-maybe (link desc info)
+  "Try exporting LINK with a dedicated function.
+
+DESC is its description, as a string, or nil.  INFO is the plist
+containing export state.  Return output as a string, or nil if no
+protocol handles LINK.
+
+A custom protocol is expected to have precedence over regular
+back-end export.  The function ignores links with an implicit
+type (e.g., \"custom-id\")."
+  (let ((type (org-element-property :type link))
+	(backend (let ((b (plist-get info :back-end)))
+		   (and b (org-export-backend-name b)))))
+    (unless (or (member type '("coderef" "custom-id" "fuzzy" "radio"))
+		(not backend))
+      (let ((protocol (nth 2 (assoc type org-link-protocols))))
+	(and (functionp protocol)
+	     (funcall protocol
+		      (org-link-unescape (org-element-property :path link))
+		      desc
+		      backend))))))
+
 (defun org-export-get-coderef-format (path desc)
   "Return format string for code reference link.
 PATH is the link path.  DESC is its description."

testing/lisp/test-ox.el

@@ -2026,6 +2026,53 @@ Paragraph[fn:1]"
 
 ;;; Links
 
+(ert-deftest test-org-export/custom-protocol-maybe ()
+  "Test `org-export-custom-protocol-maybe' specifications."
+  (should
+   (string-match
+    "success"
+    (let ((org-link-types (copy-sequence org-link-types)))
+      (org-add-link-type "foo" nil (lambda (p d f) "success"))
+      (org-export-string-as
+       "[[foo:path]]"
+       (org-export-create-backend
+	:name 'test
+	:transcoders '((section . (lambda (s c i) c))
+		       (paragraph . (lambda (p c i) c))
+		       (link . (lambda (l c i)
+				 (or (org-export-custom-protocol-maybe l c i)
+				     "failure")))))))))
+  (should-not
+   (string-match
+    "success"
+    (let ((org-link-types (copy-sequence org-link-types)))
+      (org-add-link-type
+       "foo" nil (lambda (p d f) (and (eq f 'test) "success")))
+      (org-export-string-as
+       "[[foo:path]]"
+       (org-export-create-backend
+	:name 'no-test
+	:transcoders '((section . (lambda (s c i) c))
+		       (paragraph . (lambda (p c i) c))
+		       (link . (lambda (l c i)
+				 (or (org-export-custom-protocol-maybe l c i)
+				     "failure")))))))))
+  ;; Ignore anonymous back-ends.
+  (should-not
+   (string-match
+    "success"
+    (let ((org-link-types (copy-sequence org-link-types)))
+      (org-add-link-type
+       "foo" nil (lambda (p d f) (and (eq f 'test) "success")))
+      (org-export-string-as
+       "[[foo:path]]"
+       (org-export-create-backend
+	:transcoders '((section . (lambda (s c i) c))
+		       (paragraph . (lambda (p c i) c))
+		       (link . (lambda (l c i)
+				 (or (org-export-custom-protocol-maybe l c i)
+				     "failure"))))))))))
+
 (ert-deftest test-org-export/get-coderef-format ()
   "Test `org-export-get-coderef-format' specifications."
   ;; A link without description returns "%s"