{-|

This module defines a generic web application interface. It is a common
protocol between web servers and web applications.

The overriding design principles here are performance and generality. To
address performance, this library uses a streaming interface for request and
response bodies, paired with bytestring's 'Builder' type.  The advantages of a
streaming API over lazy IO have been debated elsewhere and so will not be
addressed here.  However, helper functions like 'responseLBS' allow you to
continue using lazy IO if you so desire.

Generality is achieved by removing many variables commonly found in similar
projects that are not universal to all servers. The goal is that the 'Request'
object contains only data which is meaningful in all circumstances.

Please remember when using this package that, while your application may
compile without a hitch against many different servers, there are other
considerations to be taken when moving to a new backend. For example, if you
transfer from a CGI application to a FastCGI one, you might suddenly find you
have a memory leak. Conversely, a FastCGI application would be well served to
preload all templates from disk when first starting; this would kill the
performance of a CGI application.

This package purposely provides very little functionality. You can find various
middlewares, backends and utilities on Hackage. Some of the most commonly used
include:

[warp] <http://hackage.haskell.org/package/warp>

[wai-extra] <http://hackage.haskell.org/package/wai-extra>

-}
-- Ignore deprecations, because this module needs to use the deprecated requestBody to construct a response.
{-# OPTIONS_GHC -fno-warn-deprecations #-}
module Network.Wai
    (
      -- * Types
      Application
    , Middleware
    , ResponseReceived
      -- * Request
    , Request
    , defaultRequest
    , RequestBodyLength (..)
      -- ** Request accessors
    , requestMethod
    , httpVersion
    , rawPathInfo
    , rawQueryString
    , requestHeaders
    , isSecure
    , remoteHost
    , pathInfo
    , queryString
    , getRequestBodyChunk
    , requestBody
    , vault
    , requestBodyLength
    , requestHeaderHost
    , requestHeaderRange
    , requestHeaderReferer
    , requestHeaderUserAgent
    -- $streamingRequestBodies
    , strictRequestBody
    , consumeRequestBodyStrict
    , lazyRequestBody
    , consumeRequestBodyLazy
      -- * Response
    , Response
    , StreamingBody
    , FilePart (..)
      -- ** Response composers
    , responseFile
    , responseBuilder
    , responseLBS
    , responseStream
    , responseRaw
      -- ** Response accessors
    , responseStatus
    , responseHeaders
      -- ** Response modifiers
    , responseToStream
    , mapResponseHeaders
    , mapResponseStatus
      -- * Middleware composition
    , ifRequest
    , modifyResponse
    ) where

import           Data.ByteString.Builder      (Builder, lazyByteString)
import           Data.ByteString.Builder      (byteString)
import           Control.Monad                (unless)
import qualified Data.ByteString              as B
import qualified Data.ByteString.Lazy         as L
import qualified Data.ByteString.Lazy.Internal as LI
import           Data.ByteString.Lazy.Internal (defaultChunkSize)
import           Data.ByteString.Lazy.Char8   ()
import           Data.Function                (fix)
import qualified Network.HTTP.Types           as H
import           Network.Socket               (SockAddr (SockAddrInet))
import           Network.Wai.Internal
import qualified System.IO                    as IO
import           System.IO.Unsafe             (unsafeInterleaveIO)

----------------------------------------------------------------

-- | Creating 'Response' from a file.
responseFile :: H.Status -> H.ResponseHeaders -> FilePath -> Maybe FilePart -> Response
responseFile = ResponseFile

-- | Creating 'Response' from 'Builder'.
--
-- Some questions and answers about the usage of 'Builder' here:
--
-- Q1. Shouldn't it be at the user's discretion to use Builders internally and
-- then create a stream of ByteStrings?
--
-- A1. That would be less efficient, as we wouldn't get cheap concatenation
-- with the response headers.
--
-- Q2. Isn't it really inefficient to convert from ByteString to Builder, and
-- then right back to ByteString?
--
-- A2. No. If the ByteStrings are small, then they will be copied into a larger
-- buffer, which should be a performance gain overall (less system calls). If
-- they are already large, then an insert operation is used
-- to avoid copying.
--
-- Q3. Doesn't this prevent us from creating comet-style servers, since data
-- will be cached?
--
-- A3. You can force a Builder to output a ByteString before it is an
-- optimal size by sending a flush command.
responseBuilder :: H.Status -> H.ResponseHeaders -> Builder -> Response
responseBuilder = ResponseBuilder

-- | Creating 'Response' from 'L.ByteString'. This is a wrapper for
--   'responseBuilder'.
responseLBS :: H.Status -> H.ResponseHeaders -> L.ByteString -> Response
responseLBS s h = ResponseBuilder s h . lazyByteString

-- | Creating 'Response' from a stream of values.
--
-- In order to allocate resources in an exception-safe manner, you can use the
-- @bracket@ pattern outside of the call to @responseStream@. As a trivial
-- example:
--
-- @
-- app :: Application
-- app req respond = bracket_
--     (putStrLn \"Allocating scarce resource\")
--     (putStrLn \"Cleaning up\")
--     $ respond $ responseStream status200 [] $ \\write flush -> do
--         write $ byteString \"Hello\\n\"
--         flush
--         write $ byteString \"World\\n\"
-- @
--
-- Note that in some cases you can use @bracket@ from inside @responseStream@
-- as well. However, placing the call on the outside allows your status value
-- and response headers to depend on the scarce resource.
--
-- Since 3.0.0
responseStream :: H.Status
               -> H.ResponseHeaders
               -> StreamingBody
               -> Response
responseStream = ResponseStream

-- | Create a response for a raw application. This is useful for \"upgrade\"
-- situations such as WebSockets, where an application requests for the server
-- to grant it raw network access.
--
-- This function requires a backup response to be provided, for the case where
-- the handler in question does not support such upgrading (e.g., CGI apps).
--
-- In the event that you read from the request body before returning a
-- @responseRaw@, behavior is undefined.
--
-- Since 2.1.0
responseRaw :: (IO B.ByteString -> (B.ByteString -> IO ()) -> IO ())
            -> Response
            -> Response
responseRaw = ResponseRaw

----------------------------------------------------------------

-- | Accessing 'H.Status' in 'Response'.
responseStatus :: Response -> H.Status
responseStatus (ResponseFile    s _ _ _) = s
responseStatus (ResponseBuilder s _ _  ) = s
responseStatus (ResponseStream  s _ _  ) = s
responseStatus (ResponseRaw _ res      ) = responseStatus res

-- | Accessing 'H.ResponseHeaders' in 'Response'.
responseHeaders :: Response -> H.ResponseHeaders
responseHeaders (ResponseFile    _ hs _ _) = hs
responseHeaders (ResponseBuilder _ hs _  ) = hs
responseHeaders (ResponseStream  _ hs _  ) = hs
responseHeaders (ResponseRaw _ res)        = responseHeaders res

-- | Converting the body information in 'Response' to a 'StreamingBody'.
responseToStream :: Response
                 -> ( H.Status
                    , H.ResponseHeaders
                    , (StreamingBody -> IO a) -> IO a
                    )
responseToStream (ResponseStream s h b) = (s, h, ($ b))
responseToStream (ResponseFile s h fp (Just part)) =
    ( s
    , h
    , \withBody -> IO.withBinaryFile fp IO.ReadMode $ \handle -> withBody $ \sendChunk _flush -> do
        IO.hSeek handle IO.AbsoluteSeek $ filePartOffset part
        let loop remaining | remaining <= 0 = return ()
            loop remaining = do
                bs <- B.hGetSome handle defaultChunkSize
                unless (B.null bs) $ do
                    let x = B.take remaining bs
                    sendChunk $ byteString x
                    loop $ remaining - B.length x
        loop $ fromIntegral $ filePartByteCount part
    )
responseToStream (ResponseFile s h fp Nothing) =
    ( s
    , h
    , \withBody -> IO.withBinaryFile fp IO.ReadMode $ \handle ->
       withBody $ \sendChunk _flush -> fix $ \loop -> do
            bs <- B.hGetSome handle defaultChunkSize
            unless (B.null bs) $ do
                sendChunk $ byteString bs
                loop
    )
responseToStream (ResponseBuilder s h b) =
    (s, h, \withBody -> withBody $ \sendChunk _flush -> sendChunk b)
responseToStream (ResponseRaw _ res) = responseToStream res

-- | Apply the provided function to the response header list of the Response.
mapResponseHeaders :: (H.ResponseHeaders -> H.ResponseHeaders) -> Response -> Response
mapResponseHeaders f (ResponseFile s h b1 b2) = ResponseFile s (f h) b1 b2
mapResponseHeaders f (ResponseBuilder s h b) = ResponseBuilder s (f h) b
mapResponseHeaders f (ResponseStream s h b) = ResponseStream s (f h) b
mapResponseHeaders _ r@(ResponseRaw _ _) = r

-- | Apply the provided function to the response status of the Response.
mapResponseStatus :: (H.Status -> H.Status) -> Response -> Response
mapResponseStatus f (ResponseFile s h b1 b2) = ResponseFile (f s) h b1 b2
mapResponseStatus f (ResponseBuilder s h b) = ResponseBuilder (f s) h b
mapResponseStatus f (ResponseStream s h b) = ResponseStream (f s) h b
mapResponseStatus _ r@(ResponseRaw _ _) = r

----------------------------------------------------------------

-- | The WAI application.
--
-- Note that, since WAI 3.0, this type is structured in continuation passing
-- style to allow for proper safe resource handling. This was handled in the
-- past via other means (e.g., @ResourceT@). As a demonstration:
--
-- @
-- app :: Application
-- app req respond = bracket_
--     (putStrLn \"Allocating scarce resource\")
--     (putStrLn \"Cleaning up\")
--     (respond $ responseLBS status200 [] \"Hello World\")
-- @
type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived


-- | A default, blank request.
--
-- Since 2.0.0
defaultRequest :: Request
defaultRequest = Request
    { requestMethod = H.methodGet
    , httpVersion = H.http10
    , rawPathInfo = B.empty
    , rawQueryString = B.empty
    , requestHeaders = []
    , isSecure = False
    , remoteHost = SockAddrInet 0 0
    , pathInfo = []
    , queryString = []
    , requestBody = return B.empty
    , vault = mempty
    , requestBodyLength = KnownLength 0
    , requestHeaderHost = Nothing
    , requestHeaderRange = Nothing
    , requestHeaderReferer = Nothing
    , requestHeaderUserAgent = Nothing
    }


-- | A @Middleware@ is a component that sits between the server and application.
--
-- It can modify both the 'Request' and 'Response',
-- to provide simple transformations that are required for all (or most of)
-- your web server’s routes.
--
-- = Users of middleware
--
-- If you are trying to apply one or more 'Middleware's to your 'Application',
-- just call them as functions.
--
-- For example, if you have @corsMiddleware@ and @authorizationMiddleware@,
-- and you want to authorize first, you can do:
--
-- @
-- let allMiddleware app = authorizationMiddleware (corsMiddleware app)
-- @
--
-- to get a new 'Middleware', which first authorizes, then sets, CORS headers.
-- The “outer” middleware is called first.
--
-- You can also chain them via '(.)':
--
-- @
-- let allMiddleware =
--         authorizationMiddleware
--       . corsMiddleware
--       . … more middleware here …
-- @
--
-- Then, once you have an @app :: Application@, you can wrap it
-- in your middleware:
--
-- @
-- let myApp = allMiddleware app :: Application
-- @
--
-- and run it as usual:
--
-- @
-- Warp.run port myApp
-- @
--
-- = Authors of middleware
--
-- When fully expanded, 'Middleware' has the type signature:
--
-- > (Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived) -> Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived
--
-- or if we shorten to @type Respond = Response -> IO ResponseReceived@:
--
-- > (Request -> Respond -> IO ResponseReceived) -> Request -> Respond -> IO ResponseReceived
--
-- so a middleware definition takes 3 arguments, an inner application, a request and a response callback.
--
-- Compare with the type of a simple `Application`:
--
-- > Request -> Respond -> IO ResponseReceived
--
-- It takes the 'Request' and @Respond@, but not the extra application.
--
-- Said differently, a middleware has the power of a normal 'Application'
-- — it can inspect the 'Request' and return a 'Response' —
-- but it can (and in many cases it /should/) also call the 'Application' which was passed to it.
--
-- == Modifying the 'Request'
--
-- A lot of middleware just looks at the request and does something based on its values.
--
-- For example, the @authorizationMiddleware@ from above could look at the @Authorization@
-- HTTP header and run <https://jwt.io/ JWT> verification logic against the database.
--
-- @
-- authorizationMiddleware app req respond = do
--   case verifyJWT ('requestHeaders' req) of
--     InvalidJWT err -> respond (invalidJWTResponse err)
--     ValidJWT -> app req respond
-- @
--
-- Notice how the inner app is called when the validation was successful.
-- If it was not, we can respond
-- e.g. with <https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/401 HTTP 401 Unauthorized>,
-- by constructing a 'Response' with 'responseLBS' and passing it to @respond@.
--
-- == Passing arguments to and from your 'Middleware'
--
-- Middleware must often be configurable.
-- Let’s say you have a type @JWTSettings@ that you want to be passed to the middleware.
-- Simply pass an extra argument to your middleware. Then your middleware type turns into:
--
-- @
-- authorizationMiddleware :: JWTSettings -> Application -> Request -> Respond -> IO ResponseReceived
-- authorizationMiddleware jwtSettings req respond =
--   case verifyJWT jwtSettings ('requestHeaders' req) of
--     InvalidJWT err -> respond (invalidJWTResponse err)
--     ValidJWT -> app req respond
-- @
--
-- or alternatively:
--
-- @
-- authorizationMiddleware :: JWTSettings -> Middleware
-- @
--
-- Perhaps less intuitively, you can also /pass on/ data from middleware to the wrapped 'Application':
--
-- @
-- authorizationMiddleware :: JWTSettings -> (JWT -> Application) -> Request -> Respond -> IO ResponseReceived
-- authorizationMiddleware jwtSettings req respond =
--   case verifyJWT jwtSettings ('requestHeaders' req) of
--     InvalidJWT err -> respond (invalidJWTResponse err)
--     ValidJWT jwt -> app jwt req respond
-- @
--
-- although then, chaining different middleware has to take this extra argument into account:
--
-- @
-- let finalApp =
--       authorizationMiddleware
--         (\\jwt -> corsMiddleware
--            (… more middleware here …
--              (app jwt)))
-- @
--
-- == Modifying the 'Response'
--
-- 'Middleware' can also modify the 'Response' that is returned by the inner application.
--
-- This is done by taking the @respond@ callback, using it to define a new @respond'@,
-- and passing this new @respond'@ to the @app@:
--
-- @
-- gzipMiddleware app req respond = do
--   let respond' resp = do
--         resp' <- gzipResponseBody resp
--         respond resp'
--   app req respond'
-- @
--
-- However, modifying the response (especially the response body) is not trivial,
-- so in order to get a sense of how to do it (dealing with the type of 'responseToStream'),
-- it’s best to look at an example, for example <https://hackage.haskell.org/package/wai-extra/docs/src/Network.Wai.Middleware.Gzip.html#gzip the GZIP middleware of wai-extra>.

type Middleware = Application -> Application


-- | apply a function that modifies a response as a 'Middleware'
modifyResponse :: (Response -> Response) -> Middleware
modifyResponse f app req respond = app req $ respond . f


-- | conditionally apply a 'Middleware'
ifRequest :: (Request -> Bool) -> Middleware -> Middleware
ifRequest rpred middle app req | rpred req = middle app req
                               | otherwise =        app req

-- $streamingRequestBodies
--
-- == Streaming Request Bodies
--
-- WAI is designed for streaming in request bodies, which allows you to process them incrementally.
-- You can stream in the request body using functions like 'getRequestBodyChunk',
-- the @wai-conduit@ package, or Yesod's @rawRequestBody@.
--
-- In the normal case, incremental processing is more efficient, since it
-- reduces maximum total memory usage.
-- In the worst case, it helps protect your server against denial-of-service (DOS) attacks, in which
-- an attacker sends huge request bodies to your server.
--
-- Consider these tips to avoid reading the entire request body into memory:
--
-- * Look for library functions that support incremental processing. Sometimes these will use streaming
-- libraries like @conduit@, @pipes@, or @streaming@.
-- * Any attoparsec parser supports streaming input. For an example of this, see the
-- "Data.Conduit.Attoparsec" module in @conduit-extra@.
-- * Consider streaming directly to a file on disk. For an example of this, see the
-- "Data.Conduit.Binary" module in @conduit-extra@.
-- * If you need to direct the request body to multiple destinations, you can stream to both those
-- destinations at the same time.
-- For example, if you wanted to run an HMAC on the request body as well as parse it into JSON,
-- you could use Conduit's @zipSinks@ to send the data to @cryptonite-conduit@'s 'sinkHMAC' and
-- @aeson@'s Attoparsec parser.
-- * If possible, avoid processing large data on your server at all.
-- For example, instead of uploading a file to your server and then to AWS S3,
-- you can have the browser upload directly to S3.
--
-- That said, sometimes it is convenient, or even necessary to read the whole request body into memory.
-- For these purposes, functions like 'strictRequestBody' or 'lazyRequestBody' can be used.
-- When this is the case, consider these strategies to mitigating potential DOS attacks:
--
-- * Set a limit on the request body size you allow.
-- If certain endpoints need larger bodies, whitelist just those endpoints for the large size.
-- Be especially cautious about endpoints that don't require authentication, since these are easier to DOS.
-- You can accomplish this with @wai-extra@'s @requestSizeLimitMiddleware@ or Yesod's @maximumContentLength@.
-- * Consider rate limiting not just on total requests, but also on total bytes sent in.
-- * Consider using services that allow you to identify and blacklist attackers.
-- * Minimize the amount of time the request body stays in memory.
-- * If you need to share request bodies across middleware and your application, you can do so using Wai's 'vault'.
-- If you do this, remove the request body from the vault as soon as possible.
--
-- Warning: Incremental processing will not always be sufficient to prevent a DOS attack.
-- For example, if an attacker sends you a JSON body with a 2MB long string inside,
-- even if you process the body incrementally, you'll still end up with a 2MB-sized 'Text'.
--
-- To mitigate this, employ some of the countermeasures listed above,
-- and try to reject such payloads as early as possible in your codebase.

-- | Get the request body as a lazy ByteString. However, do /not/ use any lazy
-- I\/O, instead reading the entire body into memory strictly.
--
-- Note: Since this function consumes the request body, future calls to it will return the empty string.
--
-- Since 3.0.1
strictRequestBody :: Request -> IO L.ByteString
strictRequestBody req =
    loop id
  where
    loop front = do
        bs <- getRequestBodyChunk req
        if B.null bs
            then return $ front LI.Empty
            else loop (front . LI.Chunk bs)

-- | Synonym for 'strictRequestBody'.
-- This function name is meant to signal the non-idempotent nature of 'strictRequestBody'.
--
-- @since 3.2.3
consumeRequestBodyStrict :: Request -> IO L.ByteString
consumeRequestBodyStrict = strictRequestBody

-- | Get the request body as a lazy ByteString. This uses lazy I\/O under the
-- surface, and therefore all typical warnings regarding lazy I/O apply.
--
-- Note: Since this function consumes the request body, future calls to it will return the empty string.
--
-- Since 1.4.1
lazyRequestBody :: Request -> IO L.ByteString
lazyRequestBody req =
    loop
  where
    loop = unsafeInterleaveIO $ do
        bs <- getRequestBodyChunk req
        if B.null bs
            then return LI.Empty
            else do
                bss <- loop
                return $ LI.Chunk bs bss

-- | Synonym for 'lazyRequestBody'.
-- This function name is meant to signal the non-idempotent nature of 'lazyRequestBody'.
--
-- @since 3.2.3
consumeRequestBodyLazy :: Request -> IO L.ByteString
consumeRequestBodyLazy = lazyRequestBody
