Add dynamic block function for backlinks

Use it like so:

    #+BEGIN: oni-backlinks

    #+END

oni-org/oni-org.el

@@ -4,7 +4,7 @@
 
 ;; Author: Tom Willemse <tom@ryuslash.org>
 ;; Keywords: local
-;; Version: 2020.0824.155403
+;; Version: 2020.0921.142856
 ;; Package-Requires: (oni-yasnippet oni-alert oni-hydra org-plus-contrib org-bullets org-edna diminish all-the-icons)
 
 ;; This program is free software; you can redistribute it and/or modify
@@ -177,6 +177,34 @@ _l_: Store link           ^^                         _j_: Journal entry
   ("A" (org-capture nil "a"))
   ("j" (org-capture nil "j")))
 
+(defun oni-org-in-dblock-p ()
+  "Non-nil when point belongs to a dynamic block."
+  (save-match-data
+    (let ((case-fold-search t)
+	      (lim-up (save-excursion (outline-previous-heading)))
+	      (lim-down (save-excursion (outline-next-heading))))
+	  (org-between-regexps-p
+	   "^[ \t]*#\\+begin"
+	   "^[ \t]*#\\+end"
+	   lim-up lim-down))))
+
+(defun oni-org-dblock-write-backlinks (_params)
+  "Generate backlinks to org headings."
+  (let ((current-heading (nth 4 (org-heading-components)))
+        backlinks)
+    (save-excursion
+      (goto-char (point-min))
+      (while (re-search-forward (rx-to-string `(and "*" ,current-heading "]")) nil t)
+        (unless (oni-org-in-dblock-p)
+          (push (nth 4 (org-heading-components)) backlinks))))
+    (insert (string-join
+             (mapcar (lambda (link)
+                       (concat "- [[*" link "][" link "]]"))
+                     (sort backlinks #'string<))
+             "\n"))))
+
+(defalias 'org-dblock-write:oni-backlinks 'oni-org-dblock-write-backlinks)
+
 (setq org-agenda-tags-todo-honor-ignore-options t)
 (setq org-agenda-todo-ignore-scheduled 'future)
 (setq org-catch-invisible-edits 'error)