Backlinking facilities for Org-mode entries

backlinks.el

(define-keymap
  :keymap org-mode-map
  "C-c n b" 'my/org-backlink-search
  "C-c n d" 'my/org-backlink-dblock)

;; TODO Flesh this out some more, so it can be used to find other
;; kinds of links, such as to non-Org-mode files, to emails, etc.
;; Maybe use whatever is the default link-type for the current
;; location.
(defun my/org-backlink-search ()
  "Search for backlinks to the current entry.
If the current entry has no ID property, use the ID property of a parent
heading.  If no ID can be found at all, then don't perform the search,
and don't add an ID property, but inform the user.

Unlike the `backlink-dblock' dynamic block, this will list entries which
link to this node but do not have an ID property of their own."
  (interactive)
  (let* ((id (org-id-get (point) nil nil :inherit))
         (link (concat "id:" id)))
    (if id
        (org-search-view nil link)
      (message
       "No ID property for this entry, inherited or otherwise."))))

;; Dynamic block for backlinks

(defun my/org-backlink-dblock ()
  "Insert a dynamic block for backlinks and update it."
  (interactive)
  (unless (looking-back "\n\n" (- (point) 2))
    (insert "\n"))
  (insert "#+BEGIN: backlinks\n#+END:")
  (while (looking-at "\n\n") (delete-char 1))
  (previous-line)
  (org-dblock-update))

(defun org-dblock-write:backlinks (_params)
  "Find backlinks to the current node from nodes in `org-agenda-files'.
Insert located backlinks at point.  Only backlinks from nodes that
have an ID property will be inserted.  If the node where backlinks are
to be inserted has no ID property, an error will issue."
  (let* ((id (or (org-id-get)
                 (error (concat
                         "Node without ID property "
                         "cannot have backlinks!"))))
         (link (concat "id:" id))
         (nodes (org-map-entries
                 (lambda ()
                   (let ((sourceid (org-id-get))
                         (sourcetitle (org-get-heading t t t t)))
                     (save-restriction
                       (org-narrow-to-subtree)
                       (when (search-forward
                            link
                            (save-excursion
                              (search-forward-regexp
                               "^#\\+BEGIN: backlinks$" nil t))
                            t)
                       (cons sourceid sourcetitle)))))
                 "ID>\"\"" 'agenda))
         node)
    (while nodes
      (setq node (pop nodes))
      (if node (insert "- [[id:" (car node) "][" (cdr node) "]]\n")))
    (delete-line)))

Aaaand of course this doesn't scale well. At least not with org-update-all-dblocks placed on after-save-hook.

If it gets too slow, I can have some code that adds a single link to a link-target's block when linking. Then, only run org-update-all-dblocks every once in a while.