{-
pgxhtml, an XSLT-based PostgreSQL web interface.
Copyright (C) 2018-2019 defanor <defanor@uberspace.net>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
-}

{- |
Description :  XSLT-based PostgreSQL web interface
Maintainer  :  defanor <defanor@uberspace.net>
Stability   :  unstable
Portability :  non-portable (uses GHC extensions)

This is a tool to make custom web (HTTP with XHTML) interfaces to
PostgreSQL databases, using XSLT for templating and SQL for querying,
HTTP basic authentication and PostgreSQL roles for authentication.
-}

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ForeignFunctionInterface #-}

#include <libxslt/xsltInternals.h>

import qualified Data.ByteString.Char8 as BS
import Database.PostgreSQL.LibPQ
  (getCancel, cancel, connectdb, finish, execParams, getvalue, invalidOid,
   escapeIdentifier, resultStatus, errorMessage, Connection, Format(..),
   ExecStatus(..))
import Data.Maybe (mapMaybe, fromMaybe, catMaybes, fromJust)
import System.Timeout (timeout)
import Control.Exception (bracket, finally)
import System.FilePath (takeFileName, (</>), (<.>))
import System.Environment (lookupEnv)
import Data.List (nubBy)
import Data.ByteString.Base64 (decodeLenient)
import Data.Char (ord, chr)
import Foreign
import Foreign.C


-- * libxml bindings and functions

-- http://xmlsoft.org/html/index.html
-- http://xmlsoft.org/XSLT/html/index.html

data XmlDoc
data XsltStylesheet
data XsltTransformContext

foreign import ccall "xmlReadMemory" xmlReadMemory ::
  CString -> CInt -> CString -> CString -> CInt -> IO (Ptr XmlDoc)
foreign import ccall "xmlFreeDoc" xmlFreeDoc :: Ptr XmlDoc -> IO ()
foreign import ccall "xsltParseStylesheetFile" xsltParseStylesheetFile ::
  CString -> IO (Ptr XsltStylesheet)
foreign import ccall "xsltFreeStylesheet" xsltFreeStylesheet ::
  Ptr XsltStylesheet -> IO ()
foreign import ccall "xsltNewTransformContext" xsltNewTransformContext ::
  Ptr XsltStylesheet -> Ptr XmlDoc -> IO (Ptr XsltTransformContext)
foreign import ccall "xsltFreeTransformContext" xsltFreeTransformContext ::
  Ptr XsltTransformContext -> IO ()
foreign import ccall "xsltQuoteUserParams" xsltQuoteUserParams ::
  Ptr XsltTransformContext -> Ptr CString -> IO CInt
foreign import ccall "xsltApplyStylesheetUser" xsltApplyStylesheetUser ::
  Ptr XsltStylesheet -> Ptr XmlDoc -> Ptr CString -> CString -> Ptr () ->
  Ptr XsltTransformContext -> IO (Ptr XmlDoc)
foreign import ccall "xsltSaveResultToString" xsltSaveResultToString ::
  Ptr CString -> Ptr CInt -> Ptr XmlDoc -> Ptr XsltStylesheet -> IO CInt
foreign import ccall "exsltRegisterAll" exsltRegisterAll :: IO ()

transform :: BS.ByteString
          -- ^ document
          -> String
          -- ^ base URI
          -> FilePath
          -- ^ path to stylesheet
          -> [(BS.ByteString, BS.ByteString)]
          -- ^ string params
          -> IO (BS.ByteString, Maybe BS.ByteString)
transform docBS baseStr pathStr stringParams =
  BS.useAsCStringLen docBS $ \(docCStr, docCStrLen) ->
  withCString baseStr $ \baseCStr ->
  withCString pathStr $ \pathCStr ->
  alloca $ \bufPtr ->
  alloca $ \lenPtr ->
  bracket
  (notNull $ xmlReadMemory docCStr (fromIntegral docCStrLen) baseCStr nullPtr 0)
  xmlFreeDoc $ \doc ->
  useAsCStrings (concatMap (\(x, y) -> [x, y]) $
                  nubBy (\x y -> fst x == fst y) stringParams) [] $ \params ->
  withArray0 nullPtr params $ \paramsArr ->
  withArray0 nullPtr [] $ \emptyArr ->
  bracket (notNull $ xsltParseStylesheetFile pathCStr) xsltFreeStylesheet $
  \stylesheet ->
  bracket (notNull $ xsltNewTransformContext stylesheet doc)
  xsltFreeTransformContext $
  \tc -> xsltQuoteUserParams tc paramsArr >>
  bracket
  (notNull $ xsltApplyStylesheetUser stylesheet doc emptyArr nullPtr nullPtr tc)
  xmlFreeDoc
  (\res -> do
      xsltSaveResultToString bufPtr lenPtr res stylesheet
      bracket (peek bufPtr) free $ \resultCStr -> do
        resultBS <- BS.packCString resultCStr
        mt <- mediaType stylesheet
        pure (resultBS, mt))
  where
    useAsCStrings :: [BS.ByteString] -> [CString] -> ([CString] -> IO a) -> IO a
    useAsCStrings [] a f = f a
    useAsCStrings (x:xs) a f = BS.useAsCString x $ \x' ->
      useAsCStrings xs (a ++ [x']) f
    notNull :: IO (Ptr a) -> IO (Ptr a)
    notNull a = a >>= \p -> if p == nullPtr
                            then error "Unexpected NULL pointer"
                            else pure p
    mediaType :: Ptr XsltStylesheet -> IO (Maybe BS.ByteString)
    mediaType pXslt = do
      mt <- (#peek xsltStylesheet, mediaType) pXslt
      if mt == nullPtr
        then pure Nothing
        else Just <$> BS.packCString mt
  -- TODO: improve error handling


-- * PostgreSQL-related functions

connString :: [(BS.ByteString, BS.ByteString)] -> BS.ByteString
connString [] = BS.empty
connString ((k,v):xs) =
  BS.concat [k, "='", BS.pack (val $ BS.unpack v), "' ", connString xs]
  where val [] = []
        val ('\\':cs) = '\\' : '\\' : val cs
        val ('\'':cs) = '\\' : '\'' : val cs
        val (c:cs) = c : val cs

-- https://www.postgresql.org/docs/11/libpq-exec.html
-- https://hackage.haskell.org/package/postgresql-libpq-0.9.4.2/docs/Database-PostgreSQL-LibPQ.html
prepareQuery :: Connection
             -> [(BS.ByteString, BS.ByteString)]
             -- ^ Form data
             -> [(BS.ByteString, BS.ByteString)]
             -- ^ URL query
             -> BS.ByteString
             -- ^ SQL query template
             -> IO (BS.ByteString, [BS.ByteString])
             -- ^ SQL query template and parameters
prepareQuery c f gq q = substWords [] [] $ BS.words q
  where
    placeholder :: Int -> BS.ByteString
    placeholder n = BS.cons '$' $ BS.pack $ show $ n + 1
    placeholders :: Int -> BS.ByteString
    placeholders cnt = BS.intercalate " , "
      [placeholder (cnt + n) | n <- [0 .. length f - 1]]
    substWords :: [BS.ByteString]
               -- ^ query bits
               -> [BS.ByteString]
               -- ^ parameters
               -> [BS.ByteString]
               -- ^ remaining words
               -> IO (BS.ByteString, [BS.ByteString])
               -- ^ query and parameters
    substWords qs ps [] = pure (BS.unwords qs, ps)
    substWords qs ps (":fields":rest) = do
      identifiers <- BS.intercalate ", " . catMaybes
                     <$> mapM (escapeIdentifier c . fst) f
      substWords (qs ++ [identifiers]) ps rest
    substWords qs ps (":values":rest) =
      substWords (qs ++ [placeholders $ length ps]) (ps ++ map snd f) rest
    substWords qs ps (other:rest) = case BS.splitAt 2 other of
      -- POST (form) parameter
      ("f:", fieldName) -> case lookup fieldName f of
        Nothing -> substWords (qs ++ [other]) ps rest
        Just v -> substWords (qs ++ [placeholder $ length ps]) (ps ++ [v]) rest
      -- GET (query/link or form) parameter
      ("q:", fieldName) -> case lookup fieldName gq of
        Nothing -> substWords (qs ++ [other]) ps rest
        Just v -> substWords (qs ++ [placeholder $ length ps]) (ps ++ [v]) rest
      _ -> substWords (qs ++ [other]) ps rest

cancelAndClose :: Connection -> IO (Maybe BS.ByteString)
cancelAndClose c = cancelConn `finally` finish c
  where cancelConn = do
          cl <- getCancel c
          case cl of
            Nothing -> pure $ Just "Failed to get a Cancel structure"
            Just cl' -> either Just (const Nothing) <$> cancel cl'


-- * CGI and HTTP utilities

-- https://tools.ietf.org/html/rfc3875
-- https://tools.ietf.org/html/rfc2396

-- https://www.w3.org/TR/html5/sec-forms.html
-- https://url.spec.whatwg.org/
parseFormUrlencoded :: BS.ByteString -> [(BS.ByteString, BS.ByteString)]
parseFormUrlencoded s = mapMaybe seq2nv $ BS.split '&' s
  where
    seq2nv sq
      | BS.null sq = Nothing
      | otherwise = let (n, v') = BS.break (== '=') sq
                        v = if BS.null v' then v' else BS.tail v'
                    in Just (unescape n, unescape v)
    unescape :: BS.ByteString -> BS.ByteString
    unescape bs = case BS.uncons bs of
      Nothing -> bs
      Just ('+', rest) -> BS.cons ' ' (unescape rest)
      Just ('%', rest) -> case BS.uncons rest of
        Nothing -> BS.pack ['%']
        Just (c1, rest') -> case BS.uncons rest' of
          Nothing -> BS.pack ['%', c1]
          Just (c2, rest'') -> case (parseChar c1, parseChar c2) of
            (Just c1', Just c2') ->
              BS.cons (chr $ c1' * 0x10 + c2') (unescape rest'')
            _ -> BS.cons '%' (unescape rest)
      Just (c, rest) -> BS.cons c (unescape rest)
    parseChar :: Char -> Maybe Int
    parseChar c
      | c >= 'A' && c <= 'F' = Just $ ord c - ord 'A' + 10
      | c >= '0' && c <= '9' = Just $ ord c - ord '0'
      | otherwise = Nothing

-- https://tools.ietf.org/html/rfc7235
-- https://tools.ietf.org/html/rfc7617
baCredentials :: BS.ByteString -> Maybe (BS.ByteString, BS.ByteString)
baCredentials cred = do
  (login, password) <-
    BS.break (== ':') . decodeLenient <$> BS.stripPrefix "Basic " cred
  if BS.length password > 0
    then Just (login, BS.tail password)
    else Nothing

respond' :: Int -> [String] -> BS.ByteString -> IO ()
respond' code headers content = do
  mapM_ putStrLn headers
  putStrLn $ concat ["Status:", show code, " ", reason, "\n"]
  BS.putStr content
  where
    reason = case code of
      200 -> "OK"
      401 -> "Unauthorized"
      418 -> "I'm a teapot"
      504 -> "Gateway Timeout"
      _ -> ""

respond :: Int -> BS.ByteString -> IO ()
respond c = respond' c []


-- * Main routines

errorXML :: [(BS.ByteString, BS.ByteString)] -> BS.ByteString
errorXML kv = xmlElem "error" " xmlns=\"urn:x-pgxhtml\"" $
  BS.concat $ map (\(k,v) -> xmlElem k "" v) kv
  where
    xmlElem n a s = BS.concat ["<", n, a, ">", s, "</", n, ">"]

serve :: FilePath -> IO Connection -> [(BS.ByteString, BS.ByteString)] -> IO ()
serve xsltDirectory ioc ps = case lookup "q" ps of
  Just q -> bracket ioc cancelAndClose $ \c -> do
    formData <- parseFormUrlencoded <$> BS.getContents
    (q', params) <- prepareQuery c formData ps q
    r <- execParams c q'
         (map (\p -> Just (invalidOid, p, Text)) params) Text
    case r of
      Just r' -> do
        rs <- resultStatus r'
        case rs of
          TuplesOk -> do
            -- TODO: add more checks and error messages
            val <- getvalue r' 0 0
            case val of
              Nothing ->
                respError 500 [("message", "Failed to read query result")]
              Just val' -> resp 200 val'
          CommandOk ->
            respError 500 [("message", "The command didn't return XML")]
          _ -> do
            errMsg <- maybe [] (\m -> [("message", m)]) <$> errorMessage c
            respError 500 $ ("exec_status", BS.pack (show rs)) : errMsg
      Nothing -> respError 500 [("message", "Failed to execute the query")]
  _ -> respError 418 [("message", "No query is provided")]
  where
    xsltPath = xsltDirectory </>
               takeFileName (BS.unpack $ fromMaybe "default" $ lookup "t" ps)
               <.> "xsl"
    resp st xml = do
      (doc, mt) <- transform xml "" xsltPath ps
      BS.putStrLn $ BS.append "Content-Type:" $
        fromMaybe "application/xhtml+xml" mt
      respond st doc
    respError st e = resp st (errorXML e)

main :: IO ()
main = do
  exsltRegisterAll
  -- It's okay to fail when not invoked properly.
  ps <- parseFormUrlencoded . BS.pack . fromJust <$> lookupEnv "QUERY_STRING"
  ha <- lookupEnv "HTTP_AUTHORIZATION"
  xsltDir <- fromMaybe "." <$> lookupEnv "XSLT_DIR"
  to <- maybe 10 read <$> lookupEnv "TIMEOUT"
  maybe (respond 504 "") pure =<< timeout (to * 10 ^ (6 :: Int))
    (case (lookup "auth" ps, baCredentials =<< BS.pack <$> ha) of
       (Just "on", Just (l, p)) ->
         serve xsltDir (connectdb (connString [("user", l), ("password", p)]))
         ps
       (Just "on", Nothing) ->
         respond' 401
         ["WWW-Authenticate:Basic realm=\"Protected area\", charset=\"UTF-8\""]
         ""
       _ -> serve xsltDir (connectdb "") ps)