feat(web/tazblog_lisp): Implement retrieval of blog posts from DNS

This is mostly equivalent to the Haskell implementation, with the
primary difference that the Lisp DNS library does not support caching
yet.

web/tazblog_lisp/default.nix

@@ -0,0 +1,21 @@
+{ pkgs, ... }:
+
+pkgs.nix.buildLisp.library {
+  name = "tazblog";
+
+  deps =
+  # Local dependencies
+  (with pkgs.lisp; [ dns ])
+
+  # Third-party dependencies
+  ++ (with pkgs.third_party.lisp; [
+    cl-base64
+    cl-json
+    hunchentoot
+    iterate
+  ]);
+
+  srcs = [
+    ./store.lisp
+  ];
+}

web/tazblog_lisp/store.lisp

@@ -0,0 +1,79 @@
+(defpackage #:tazblog/store
+  (:documentation
+   "This module implements fetching of individual blog entries from DNS.
+   Yes, you read that correctly.
+
+   Each blog post is stored as a set of records in a designated DNS
+   zone. For the production blog, this zone is `blog.tazj.in.`.
+
+   A top-level record at `_posts` contains a list of all published
+   post IDs.
+
+   For each of these post IDs, there is a record at `_meta.$postID`
+   that contains the title and number of post chunks.
+
+   For each post chunk, there is a record at `_$chunkID.$postID` that
+   contains a base64-encoded post fragment.
+
+   This module implements logic for assembling a post out of these
+   fragments and caching it based on the TTL of its `_meta` record.")
+
+  (:use #:cl #:dns #:iterate)
+  (:import-from #:cl-base64 #:base64-string-to-string))
+(in-package :tazblog/store)
+
+;; TODO:
+;;
+;; - implement DNS caching
+
+(defvar *tazblog-zone* ".blog.tazj.in."
+  "DNS zone in which blog posts are persisted.")
+
+(deftype entry-id () 'string)
+
+(defun list-entries (&key (offset 0) (count 4) (zone *tazblog-zone*))
+  "Retrieve COUNT entry IDs from ZONE at OFFSET."
+  (let ((answers (lookup-txt (concatenate 'string "_posts" zone))))
+    (map 'vector #'dns-rr-rdata (subseq answers offset (+ offset count)))))
+
+(defun get-entry-meta (entry-id zone)
+  (let* ((name (concatenate 'string "_meta." entry-id zone))
+         (answer (lookup-txt name))
+         (encoded (dns-rr-rdata (alexandria:first-elt answer)))
+         (meta-json (base64-string-to-string encoded)))
+    (json:decode-json-from-string meta-json)))
+
+(defun base64-add-padding (string)
+  "Adds padding to the base64-encoded STRING if required."
+  (let ((rem (- 4 (mod (length string) 4))))
+    (if (= 0 rem) string
+        (format nil "~A~v@{~A~:*~}" string rem "="))))
+
+(defun collect-entry-fragments (entry-id count zone)
+  (let* ((fragments
+           (iter (for i from 0 below count)
+             (for name = (format nil "_~D.~A~A" i entry-id zone))
+             (collect (alexandria:first-elt (lookup-txt name)))))
+         (decoded (map 'list (lambda (f)
+                               (base64-string-to-string
+                                (base64-add-padding (dns-rr-rdata f))))
+                       fragments)))
+    (apply #'concatenate 'string decoded)))
+
+(defstruct entry
+  (id "" :type string)
+  (title "" :type string)
+  (content "" :type string)
+  (date "" :type string))
+
+(defun get-entry (entry-id &optional (zone *tazblog-zone*))
+  "Retrieve the entry at ENTRY-ID from ZONE."
+  (let* ((meta (get-entry-meta entry-id zone))
+         (count (cdr (assoc :c meta)))
+         (title (cdr (assoc :t meta)))
+         (date (cdr (assoc :d meta)))
+         (content (collect-entry-fragments entry-id count zone)))
+    (make-entry :id entry-id
+                :date date
+                :title title
+                :content content)))