(feat): add support for filtering backlinks. (#2247)

* Add support for filtering backlinks.

See #1043.

* Add documentation for backlinks filter

doc/org-roam.org

@@ -644,6 +644,19 @@ rendering reference links), set ~org-roam-mode-sections~ as follows:
           org-roam-reflinks-section))
 #+end_src
 
+The backlinks section ~org-roam-backlinks-section~ also supports a
+predicate to filter backlinks, ~:show-backlink-p~.  This can be used
+as follows:
+
+#+begin_src emacs-lisp
+  (defun my-org-roam-show-backlink-p (backlink)
+    (not (member "daily" (org-roam-node-tags (org-roam-backlink-source-node backlink)))))
+
+  (setq org-roam-mode-sections
+        '((org-roam-backlinks-section :unique t :show-backlink-p my/org-roam-show-backlink-p)
+          org-roam-reflinks-section))
+#+end_src
+
 ** Configuring the Org-roam buffer display
 
 Org-roam does not control how the pop-up buffer is displayed: this is left to

org-roam-mode.el

@@ -514,19 +514,25 @@ Sorts by title."
   (string< (org-roam-node-title (org-roam-backlink-source-node a))
            (org-roam-node-title (org-roam-backlink-source-node b))))
 
-(cl-defun org-roam-backlinks-section (node &key (unique nil))
+(cl-defun org-roam-backlinks-section (node &key (unique nil) (show-backlink-p nil))
   "The backlinks section for NODE.
 
 When UNIQUE is nil, show all positions where references are found.
-When UNIQUE is t, limit to unique sources."
+When UNIQUE is t, limit to unique sources.
+
+When SHOW-BACKLINK-P is not null, only show backlinks for which
+this predicate is not nil."
   (when-let ((backlinks (seq-sort #'org-roam-backlinks-sort (org-roam-backlinks-get node :unique unique))))
     (magit-insert-section (org-roam-backlinks)
       (magit-insert-heading "Backlinks:")
       (dolist (backlink backlinks)
-        (org-roam-node-insert-section
-         :source-node (org-roam-backlink-source-node backlink)
-         :point (org-roam-backlink-point backlink)
-         :properties (org-roam-backlink-properties backlink)))
+        (when (or (null show-backlink-p)
+                  (and (not (null show-backlink-p))
+                       (funcall show-backlink-p backlink)))
+          (org-roam-node-insert-section
+           :source-node (org-roam-backlink-source-node backlink)
+           :point (org-roam-backlink-point backlink)
+           :properties (org-roam-backlink-properties backlink))))
       (insert ?\n))))
 
 ;;;; Reflinks