Introduce user option to translate absolute filenames

Fixes #268

README.md

@@ -843,6 +843,9 @@ provides an interface to all of the possible customizations:
     This is useful for compatibility with `org-mode`, which doesn't
     recognize the lowercase variant.
 
+  * `markdown-translate-filename-function` - A function to be used to
+    translate absolute filenames in links.
+
 Additionally, the faces used for syntax highlighting can be modified to
 your liking by issuing <kbd>M-x customize-group RET markdown-faces</kbd>
 or by using the "Markdown Faces" link at the bottom of the mode

markdown-mode.el

@@ -859,6 +859,9 @@
 ;;     This is useful for compatibility with `org-mode', which doesn't
 ;;     recognize the lowercase variant.
 ;;
+;;   * `markdown-translate-filename-function' - A function to be used to
+;;     translate absolute filenames in links.
+;;
 ;; Additionally, the faces used for syntax highlighting can be modified to
 ;; your liking by issuing `M-x customize-group RET markdown-faces`
 ;; or by using the "Markdown Faces" link at the bottom of the mode
@@ -1502,6 +1505,21 @@ or from the menu Markdown > Links & Images menu."
   :package-version '(markdown-mode . "2.3"))
 (make-variable-buffer-local 'markdown-hide-urls)
 
+(defcustom markdown-translate-filename-function #'identity
+  "Function to use to translate absolute filenames when following links.
+\\<markdown-mode-map>\\[markdown-follow-thing-at-point] and \\[markdown-follow-link-at-point]
+call this function with the filename as only argument whenever
+they encounter an absolute filename to be visited and use its
+return value instead of the filename in the link.  For example, if
+absolute filenames are actually relative to a server root
+directory, you can set `markdown-translate-filename-function' to
+a function that prepends the root directory to the given
+filename."
+  :group 'markdown
+  :type 'function
+  :risky t
+  :package-version '(markdown-mode . "2.4"))
+
 
 ;;; Regular Expressions =======================================================
 
@@ -8085,7 +8103,8 @@ returns nil."
 (defun markdown-follow-link-at-point ()
   "Open the current non-wiki link.
 If the link is a complete URL, open in browser with `browse-url'.
-Otherwise, open with `find-file' after stripping anchor and/or query string."
+Otherwise, open with `find-file' after stripping anchor and/or query string.
+Translate absolute filenames using `markdown-filename-translate-function'."
   (interactive)
   (if (markdown-link-p)
       (let* ((url (markdown-link-url))
@@ -8101,7 +8120,11 @@ Otherwise, open with `find-file' after stripping anchor and/or query string."
         ;; Open full URLs in browser, files in Emacs
         (if full
             (browse-url url)
-          (when (and file (> (length file) 0)) (find-file file))))
+          (when (and file (> (length file) 0))
+            (find-file
+             (if (file-name-absolute-p file)
+                 (funcall markdown-translate-filename-function file)
+               file)))))
     (user-error "Point is not at a Markdown link or URL")))
 
 (defun markdown-fontify-inline-links (last)

tests/markdown-test.el

@@ -4489,6 +4489,22 @@ like statement. Detail: https://github.com/jrblevin/markdown-mode/issues/75"
   (markdown-test-string "http://jblevins.org/projects/markdown-mode/"
     (should (equal (markdown-link-at-pos (point)) '(1 44 nil "http://jblevins.org/projects/markdown-mode/" nil nil nil)))))
 
+(ert-deftest test-markdown-link/follow-absolute-filename ()
+  "Test that `markdown-follow-thing-at-pos' uses `markdown-translate-filename-function' to translate absolute filenames."
+  (markdown-test-string "[text](/foo/bar/baz)"
+    (cl-letf* ((visited-files ())
+               ((symbol-function #'find-file)
+                (lambda (filename)
+                  (push filename visited-files)))
+               (translated-files ())
+               (markdown-translate-filename-function
+                (lambda (filename)
+                  (push filename translated-files)
+                  (format "/root%s.md" filename))))
+      (markdown-follow-thing-at-point nil)
+      (should (equal translated-files '("/foo/bar/baz")))
+      (should (equal visited-files '("/root/foo/bar/baz.md"))))))
+
 ;;; Wiki link tests:
 
 (ert-deftest test-markdown-wiki-link/file-local-variables ()