Extract setfs into UPDATE-CONTENT-METADATA.

src/coleslaw.lisp

   "Load all content stored in the blog's repo."
   (do-subclasses (ctype content)
     (discover ctype))
+  (update-content-metadata)
   (do-subclasses (itype index)
     (discover itype)))
 

src/indexes.lisp

 (defclass tag-index (index) ())
 
 (defmethod discover ((doc-type (eql (find-class 'tag-index))))
-  (setf *all-tags* (all-tags))
   (let ((content (by-date (find-all 'post))))
     (dolist (tag (all-tags))
       (add-document (index-by-tag tag content)))))
 (defclass month-index (index) ())
 
 (defmethod discover ((doc-type (eql (find-class 'month-index))))
-  (setf *all-months* (all-months))
   (let ((content (by-date (find-all 'post))))
     (dolist (month *all-months*)
       (add-document (index-by-month month content)))))
 
 ;;; Helper Functions
 
+(defun update-content-metadata ()
+  "Set *ALL-TAGS* and *ALL-MONTHS* to the union of all tags and months
+of content loaded in the DB."
+  (setf *all-tags* (all-tags))
+  (setf *all-months* (all-months)))
+
 (defun all-months ()
   "Retrieve a list of all months with published content."
   (let ((months (mapcar (lambda (x) (subseq (content-date x) 0 7))