feat(tazblog): Implement entry fetching from DNS

Not all error cases are properly handled yet, stay tuned.

services/tazblog/src/BlogStore.hs

@@ -15,6 +15,10 @@
 --
 -- This module implements logic for assembling a post out of these
 -- fragments and caching it based on the TTL of its `_meta` record.
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
 module BlogStore(
   BlogCache,
   EntryId(..),
@@ -22,54 +26,149 @@ module BlogStore(
   withCache,
   listEntries,
   getEntry,
+  show',
 ) where
 
-import Control.Monad.IO.Class (MonadIO)
-import Data.Text (Text)
-import Data.Time (UTCTime)
+import Data.Aeson ((.:), FromJSON(..), Value(Object), decodeStrict)
+import Control.Applicative ((<$>), (<*>))
+import Control.Monad (mzero)
+import Control.Monad.IO.Class (MonadIO, liftIO)
+import Data.Text as T (Text, concat, pack)
+import Data.Text.Encoding (encodeUtf8, decodeUtf8')
+import Data.Time (Day)
 import Locales (BlogLang (..))
-import Network.DNS.Lookup (lookupTXT)
+import Network.DNS (lookupTXT, DNSError)
 import qualified Network.DNS.Resolver as R
+import Data.ByteString.Base64 (decodeLenient)
+import Data.List (sortBy)
+import Data.Either (fromRight)
+import Debug.Trace (trace)
 
 newtype EntryId = EntryId {unEntryId :: Integer}
-  deriving (Eq, Ord)
+  deriving (Eq, Ord, FromJSON)
 
 instance Show EntryId where
-
   show = show . unEntryId
 
 data Entry
   = Entry
       { entryId :: EntryId,
-        lang :: BlogLang,
-        author :: Text,
-        title :: Text,
-        btext :: Text,
-        mtext :: Text,
-        edate :: UTCTime
+        lang    :: BlogLang,
+        author  :: Text,
+        title   :: Text,
+        btext   :: Text,
+        mtext   :: Text,
+        edate   :: Day
         }
   deriving (Eq, Ord, Show)
 
 -- | Wraps a DNS resolver with caching configured. For the initial
 -- version of this, all caching of entries is done by the resolver
 -- (i.e. no pre-assembled versions of entries are cached).
-data BlogCache
-  = BlogCache { resolver :: R.Resolver
-              , zone :: String }
+data BlogCache = BlogCache R.Resolver Text
 
-type Offset = Integer
+data StoreError
+  = PostNotFound EntryId
+  | DNS DNSError
+  | InvalidMetadata
+  | InvalidChunk
+  | InvalidPosts
+  deriving (Show)
 
-type Count = Integer
+type Offset = Int
 
-withCache :: String -> (BlogCache -> IO a) -> IO a
+type Count = Int
+
+withCache :: Text -> (BlogCache -> IO a) -> IO a
 withCache zone f = do
   let conf = R.defaultResolvConf { R.resolvCache = Just R.defaultCacheConf
                                  , R.resolvConcurrent = True }
   seed <- R.makeResolvSeed conf
   R.withResolver seed $ (\r -> f $ BlogCache r zone)
 
+
 listEntries :: MonadIO m => BlogCache -> Offset -> Count -> m [Entry]
-listEntries (BlogCache r z) offset count = undefined
+listEntries cache offset count = liftIO $ do
+  posts <- postList cache
+  entries <- mapM (entryFromDNS cache) $ take count $ drop offset $ fromRight (error "no posts") posts
+
+  -- TODO: maybe don't just drop broken entries
+  return
+    $ fromRight (error "no entries") $ sequence $ trace (show entries) entries
 
 getEntry :: MonadIO m => BlogCache -> EntryId -> m (Maybe Entry)
-getEntry (BlogCache r z) eId = undefined
+getEntry cache eid = liftIO $ (entryFromDNS cache eid) >>= \case
+  Left _ -> return Nothing -- TODO: ??
+  Right entry -> return $ Just entry
+
+show' :: Show a => a -> Text
+show' = pack . show
+
+-- * DNS fetching implementation
+
+type Chunk = Integer
+
+-- | Represents the metadata stored for each post in the _meta record.
+data Meta = Meta Integer Text Day
+  deriving (Show)
+
+instance FromJSON Meta where
+  parseJSON (Object v) = Meta <$>
+    v .: "c" <*>
+    v .: "t" <*>
+    v .: "d"
+  parseJSON _ = mzero
+
+entryMetadata :: BlogCache -> EntryId -> IO (Either StoreError Meta)
+entryMetadata (BlogCache r z) (EntryId eid) =
+  let domain = encodeUtf8 ("_meta." <> show' eid <> "." <> z)
+      record = lookupTXT r domain
+      toMeta rrdata = case decodeStrict $ decodeLenient rrdata  of
+        Nothing -> Left InvalidMetadata
+        Just m  -> Right m
+  in record >>= \case
+    (Left err) -> return $ Left $ DNS err
+    (Right [ bs ]) -> return $ toMeta bs
+    _ -> return $ Left InvalidMetadata
+
+entryChunk :: BlogCache -> EntryId -> Chunk -> IO (Either StoreError Text)
+entryChunk (BlogCache r z) (EntryId eid) c =
+  let domain = encodeUtf8 ("_" <> show' c <> "." <> show' eid <> "." <> z)
+      record = lookupTXT r domain
+      toChunk rrdata = case decodeUtf8' $ decodeLenient rrdata of
+        Left _ -> Left InvalidChunk
+        Right chunk -> Right chunk
+  in record >>= \case
+    (Left err) -> return $ Left $ DNS err
+    (Right [ bs ]) -> return $ toChunk bs
+    _ -> return $ Left InvalidChunk
+
+fetchAssembleChunks :: BlogCache -> EntryId -> Meta -> IO (Either StoreError Text)
+fetchAssembleChunks cache eid (Meta n _ _) = do
+  chunks <- mapM (entryChunk cache eid) [0..(n - 1)]
+  return $ either Left (Right . T.concat) $ sequence chunks
+
+entryFromDNS :: BlogCache -> EntryId -> IO (Either StoreError Entry)
+entryFromDNS cache eid = do
+  meta <- entryMetadata cache eid
+  case meta of
+    Left err -> return $ Left err
+    Right meta -> do
+      chunks <- fetchAssembleChunks cache eid meta
+      let (Meta _ t d) = meta
+      return $ either Left (\text -> Right $ Entry {
+                               entryId = eid,
+                               lang = EN,
+                               author = "tazjin",
+                               title = t,
+                               btext = text,
+                               mtext = "",
+                               edate = d}) chunks
+
+postList :: BlogCache -> IO (Either StoreError [EntryId])
+postList (BlogCache r z) =
+  let domain = encodeUtf8 ("_posts." <> z)
+      record = lookupTXT r domain
+      toPosts = fmap (sortBy (flip compare)) . sequence .
+        map (\r -> maybe (Left InvalidPosts) Right (decodeStrict r))
+  in record >>= return . either (Left . DNS) toPosts