{-# LANGUAGE UnicodeSyntax #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternGuards #-}
{-# LANGUAGE CPP #-}

-- |
-- Module: Network.Wai.Middleware.Cors
-- Description: Cross-Origin resource sharing (CORS) for WAI
-- Copyright:
--     © 2015 Lars Kuhtz <lkuhtz@gmail.com,
--     © 2014 AlephCloud Systems, Inc.
-- License: MIT
-- Maintainer: Lars Kuhtz <lakuhtz@gmail.com>
-- Stability: stable
--
-- An implemenation of Cross-Origin resource sharing (CORS) for WAI that
-- aims to be compliant with <http://www.w3.org/TR/cors>.
--
-- The function 'simpleCors' enables support of simple cross-origin requests. More
-- advanced CORS policies can be enabled by passing a 'CorsResourcePolicy' to the
-- 'cors' middleware.
--
-- = Note On Security
--
-- This implementation doens't include any server side enforcement. By
-- complying with the CORS standard it enables the client (i.e. the web
-- browser) to enforce the CORS policy. For application authors it is strongly
-- recommended to take into account the security considerations in section 6.3
-- of <http://wwww.w3.org/TR/cors>. In particular the application should check
-- that the value of the @Origin@ header matches it's expectations.
--
-- = Websockets
--
-- Websocket connections don't support CORS and are ignored by this CORS
-- implementation. However Websocket requests usually (at least for some
-- browsers) include the @Origin@ header. Applications are expected to check
-- the value of this header and respond with an error in case that its content
-- doesn't match the expectations.
--
-- = Example
--
-- The following is an example how to enable support for simple cross-origin requests
-- for a <http://hackage.haskell.org/package/scotty scotty> application.
--
-- > {-# LANGUAGE UnicodeSyntax #-}
-- > {-# LANGUAGE OverloadedStrings #-}
-- >
-- > module Main
-- > ( main
-- > ) where
-- >
-- > import Network.Wai.Middleware.Cors
-- > import Web.Scotty
-- >
-- > main ∷ IO ()
-- > main = scotty 8080 $ do
-- >     middleware simpleCors
-- >     matchAny  "/" $ text "Success"
--
-- The result of following curl command will include the HTTP response header
-- @Access-Control-Allow-Origin: *@.
--
-- > curl -i http://127.0.0.1:8888 -H 'Origin: 127.0.0.1' -v
--
module Network.Wai.Middleware.Cors
( Origin
, CorsResourcePolicy(..)
, simpleCorsResourcePolicy
, cors
, simpleCors

-- * Utils
, isSimple
, simpleResponseHeaders
, simpleHeaders
, simpleContentTypes
, simpleMethods
) where

#ifndef MIN_VESION_base
#define MIN_VESION_base(x,y,z) 1
#endif

#ifndef MIN_VESION_wai
#define MIN_VESION_wai(x,y,z) 1
#endif

#if ! MIN_VERSION_base(4,8,0)
import Control.Applicative
#endif
import Control.Monad.Error.Class
import Control.Monad.Trans.Except
#if ! MIN_VERSION_wai(2,0,0)
import Control.Monad.Trans.Resource
#endif

import qualified Data.Attoparsec.ByteString.Char8 as P
import qualified Data.ByteString.Char8 as B8
import qualified Data.ByteString.Lazy.Char8 as LB8
import qualified Data.CaseInsensitive as CI
import Data.List (intersect, (\\), union)
import Data.Maybe (catMaybes)
#if ! MIN_VERSION_base(4,8,0)
import Data.Monoid (mempty)
#endif
import Data.Monoid.Unicode
import Data.String

import qualified Network.HTTP.Types as HTTP
import qualified Network.Wai as WAI

import Prelude.Unicode

{-
#if MIN_VERSION_wai(2,0,0)
type ReqMonad = IO
#else
type ReqMonad = ResourceT IO
#endif
-}

-- | Origins are expected to be formated as described in
-- <https://www.ietf.org/rfc/rfc6454.txt RFC6454> (section 6.2).
-- In particular the string @*@ is not a valid origin (but the string
-- @null@ is).
--
type Origin = B8.ByteString

data CorsResourcePolicy = CorsResourcePolicy
    {

    -- | HTTP origins that are allowed in CORS requests.
    --
    -- A value of 'Nothing' indicates unrestricted cross-origin sharing and
    -- results in @*@ as value for the @Access-Control-Allow-Origin@ HTTP
    -- response header.
    --
    -- A value other than 'Nothing' is a tuple that consists of a list of
    -- origins each with a Boolean flag that indicates if credentials are used
    -- to access the resource via CORS.
    --
    -- Origins must be formated as described in
    -- <https://www.ietf.org/rfc/rfc6454.txt RFC6454> (section 6.2). In
    -- particular the string @*@ is not a valid origin (but the string @null@
    -- is).
    --
       corsOrigins ∷ !(Maybe ([Origin], Bool))

    -- | HTTP methods that are allowed in CORS requests.
    --
    , corsMethods ∷ ![HTTP.Method]

    -- | Field names of HTTP request headers that are allowed in CORS requests.
    -- Header names that are included in 'simpleHeaders', except for
    -- @content-type@, are implicitely included an thus optional in this list.
    --
    , corsRequestHeaders ∷ ![HTTP.HeaderName]

    -- | Field names of HTTP headers that are exposed on the client.
    --
    , corsExposedHeaders ∷ !(Maybe [HTTP.HeaderName])

    -- | Number of seconds that the response may be cached by the client.
    --
    , corsMaxAge ∷ !(Maybe Int)

    -- | If the resource is shared by multiple origins but
    -- @Access-Control-Allow-Origin@ is not set to @*@ this may be set to
    -- 'True' to cause the server to include a @Vary: Origin@ header in the
    -- response, thus indicating that the value of the
    -- @Access-Control-Allow-Origin@ header may vary between different requests
    -- for the same resource. This prevents caching of the responses which may
    -- not apply accross different origins.
    --
    , corsVaryOrigin ∷ !Bool

    -- | If this is 'True' and the request does not include an @Origin@ header
    -- the response has HTTP status 400 (bad request) and the body contains
    -- a short error message.
    --
    -- If this is 'False' and the request does not include an @Origin@ header
    -- the request is passed on unchanged to the application.
    --
    -- @since 0.2
    , corsRequireOrigin ∷ !Bool

    -- | In the case that
    --
    -- * the request contains an @Origin@ header and
    --
    -- * the client does not conform with the CORS protocol
    --   (/request is out of scope/)
    --
    -- then
    --
    -- * the request is passed on unchanged to the application if this field is
    --   'True' or
    --
    -- * an response with HTTP status 400 (bad request) and short
    --   error message is returned if this field is 'False'.
    --
    -- @since 0.2
    --
    , corsIgnoreFailures ∷ !Bool
    }
    deriving (Show,Read,Eq,Ord)

-- | A 'CorsResourcePolicy' that supports /simple cross-origin requests/ as defined
-- in <http://www.w3.org/TR/cors/>.
--
-- * The HTTP header @Access-Control-Allow-Origin@ is set to @*@.
--
-- * Request methods are constraint to /simple methods/ (@GET@, @HEAD@, @POST@).
--
-- * Request headers are constraint to /simple request headers/
--   (@Accept@, @Accept-Language@, @Content-Language@, @Content-Type@).
--
-- * If the request is a @POST@ request the content type is constraint to
--    /simple content types/
--    (@application/x-www-form-urlencoded@, @multipart/form-data@, @text/plain@),
--
-- * Only /simple response headers/ may be exposed on the client
--   (@Cache-Control@, @Content-Language@, @Content-Type@, @Expires@, @Last-Modified@,  @Pragma@)
--
-- * The @Vary-Origin@ header is left unchanged (possibly unset).
--
-- * If the request doesn't include an @Origin@ header the request is passed unchanged to
--   the application.
--
-- * If the request includes an @Origin@ header but does not conform to the CORS
--   protocol (/request is out of scope/) an response with HTTP status 400 (bad request)
--   and a short error message is returned.
--
-- For /simple cross-origin requests/ a preflight request is not required. However, if
-- the client chooses to make a preflight request it is answered in accordance with
-- the policy for /simple cross-origin requests/.
--
simpleCorsResourcePolicy ∷ CorsResourcePolicy
simpleCorsResourcePolicy = CorsResourcePolicy
    { corsOrigins = Nothing
    , corsMethods = simpleMethods
    , corsRequestHeaders = []
    , corsExposedHeaders = Nothing
    , corsMaxAge = Nothing
    , corsVaryOrigin = False
    , corsRequireOrigin = False
    , corsIgnoreFailures = False
    }

-- | A Cross-Origin resource sharing (CORS) middleware.
--
-- The middleware is given a function that serves as a pattern to decide
-- whether a requested resource is available for CORS. If the match fails with
-- 'Nothing' the request is passed unmodified to the inner application.
--
-- The current version of this module does only aim at compliance with the CORS
-- protocol as specified in <http://www.w3.org/TR/cors/>. In accordance with
-- that standard the role of the server side is to support the client to
-- enforce CORS restrictions. This module does not implement any enforcement of
-- authorization policies that are possibly implied by the
-- 'CorsResourcePolicy'. It is up to the inner WAI application to enforce such
-- policy and make sure that it is in accordance with the configuration of the
-- 'cors' middleware.
--
-- Matches are done as follows: @*@ matches every origin. For all other cases a
-- match succeeds if and only if the ASCII serializations (as described in
-- RCF6454 section 6.2) are equal.
--
-- The OPTIONS method may return options for resources that are not actually
-- available. In particular for preflight requests the implementation returns
-- for the HTTP response headers @Access-Control-Allow-Headers@ and
-- @Access-Control-Allow-Methods@ all values specified in the
-- 'CorsResourcePolicy' together with the respective values for simple requests
-- (except @content-type@). This does not imply that the application actually
-- supports the respective values are for the requested resource. Thus,
-- depending on the application, an actual request may still fail with 404 even
-- if the preflight request /supported/ the usage of the HTTP method with CORS.
--
-- The implementation does not distinguish between simple requests and requests
-- that require preflight. The client is free to omit a preflight request or do
-- a preflight request in cases when it wouldn't be required.
--
-- For application authors it is strongly recommended to take into account the
-- security considerations in section 6.3 of <http://wwww.w3.org/TR/cors>.
--
-- /TODO/
--
-- * We may consider adding optional enforcment aspects to this module: we may
--   check if a request respects our origin restrictions and we may check that a
--   CORS request respects the restrictions that we publish in the preflight
--   responses.
--
-- * Even though slightly out of scope we may (optionally) check if
--   host header matches the actual host of the resource, since clients
--   using CORS may expect this, since this check is recommended in
--   <http://www.w3.org/TR/cors>.
--
-- * We may consider integrating CORS policy handling more closely with the
--   handling of the source, for instance by integrating with 'ActionM' from
--   scotty.
--
cors
    ∷ (WAI.Request → Maybe CorsResourcePolicy) -- ^ A value of 'Nothing' indicates that the resource is not available for CORS
    → WAI.Middleware
#if MIN_VERSION_wai(3,0,0)
cors policyPattern app r respond
#else
cors policyPattern app r
#endif
    -- We don't handle websockets, even if they include an @Origin@ header
    | isWebSocketsReq r = runApp

    | Just policy ← policyPattern r = case hdrOrigin of

        -- No origin header: requect request
        Nothing → if corsRequireOrigin policy
            then res $ corsFailure "Origin header is missing"
            else runApp

        -- Origin header: apply CORS policy to request
        Just origin → applyCorsPolicy policy origin

    | otherwise = runApp
  where

#if MIN_VERSION_wai(3,0,0)
    res = respond
    runApp = app r respond
#else
    res = return
    runApp = app r
#endif

    -- Lookup the HTTP origin request header
    --
    hdrOrigin = lookup "origin" (WAI.requestHeaders r)

    -- Process a CORS request
    --
    applyCorsPolicy policy origin = do

        -- The error continuation
        let err e = if corsIgnoreFailures policy
            then runApp
            else res $ corsFailure (B8.pack e)

        -- Match request origin with corsOrigins from policy
        let respOriginOrErr = case corsOrigins policy of
                Nothing → return Nothing
                Just (originList, withCreds) → if origin `elem` originList
                    then Right $ Just (origin, withCreds)
                    else Left $ "Unsupported origin: " ⊕ B8.unpack origin

        case respOriginOrErr of
            Left e → err e
            Right respOrigin → do

                -- Determine headers that are common to actuall responses and preflight responses
                let ch = commonCorsHeaders respOrigin (corsVaryOrigin policy)

                case WAI.requestMethod r of

                    -- Preflight CORS request
                    "OPTIONS" → runExceptT (preflightHeaders policy) >>= \case
                        Left e → err e
                        Right headers → res $ WAI.responseLBS HTTP.ok200 (ch ⊕ headers) ""

                    -- Actual CORS request
#if MIN_VERSION_wai(3,0,0)
                    _ → addHeaders (ch ⊕ respCorsHeaders policy) app r respond
#else
                    _ → addHeaders (ch ⊕ respCorsHeaders policy) app r
#endif

    -- Compute HTTP response headers for a preflight request
    --
    preflightHeaders ∷ (Functor μ, Monad μ) ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
    preflightHeaders policy = concat <$> sequence
        [ hdrReqMethod policy
        , hdrRequestHeader policy
        , hdrMaxAge policy
        ]

    hdrMaxAge ∷ Monad μ ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
    hdrMaxAge policy = case corsMaxAge policy of
        Nothing → return []
        Just secs → return [("Access-Control-Max-Age", sshow secs)]

    hdrReqMethod ∷ Monad μ ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
    hdrReqMethod policy = case lookup "Access-Control-Request-Method" (WAI.requestHeaders r) of
        Nothing → throwError "Access-Control-Request-Method header is missing in CORS preflight request"
        Just x → if  x `elem` supportedMethods
            then return [("Access-Control-Allow-Methods", hdrL supportedMethods)]
            else throwError
                $ "Method requested in Access-Control-Request-Method of CORS request is not supported; requested: "
                ⊕ B8.unpack x
                ⊕ "; supported are "
                ⊕ B8.unpack (hdrL supportedMethods)
                ⊕ "."
      where
         supportedMethods = corsMethods policy `union` simpleMethods

    hdrRequestHeader ∷ Monad μ ⇒ CorsResourcePolicy → ExceptT String μ HTTP.ResponseHeaders
    hdrRequestHeader policy = case lookup "Access-Control-Request-Headers" (WAI.requestHeaders r) of
        Nothing → return []
        Just hdrsBytes → do
            hdrs ← either throwError return $ P.parseOnly httpHeaderNameListParser hdrsBytes
            if hdrs `isSubsetOf` supportedHeaders
                then return [("Access-Control-Allow-Headers", hdrLI supportedHeaders)]
                else throwError
                    $ "HTTP header requested in Access-Control-Request-Headers of CORS request is not supported; requested: "
                    ⊕ B8.unpack (hdrLI hdrs)
                    ⊕ "; supported are "
                    ⊕ B8.unpack (hdrLI supportedHeaders)
                    ⊕ "."
      where
        supportedHeaders = corsRequestHeaders policy `union` simpleHeadersWithoutContentType

    simpleHeadersWithoutContentType = simpleHeaders \\ ["content-type"]

    -- HTTP response headers that are common to normal and preflight CORS responses
    --
    commonCorsHeaders ∷ Maybe (Origin, Bool) → Bool → HTTP.ResponseHeaders
    commonCorsHeaders Nothing _ = [("Access-Control-Allow-Origin", "*")]
    commonCorsHeaders (Just (o, creds)) vary = []
        ⊕ (True ?? ("Access-Control-Allow-Origin", o))
        ⊕ (creds ?? ("Access-Control-Allow-Credentials", "true"))
        ⊕ (vary ?? ("Vary", "Origin"))
      where
        (??) a b = if a then pure b else mempty

    -- HTTP response headers that are only used with normal CORS responses
    --
    respCorsHeaders ∷ CorsResourcePolicy → HTTP.ResponseHeaders
    respCorsHeaders policy = catMaybes
        [ fmap (\x → ("Access-Control-Expose-Headers", hdrLI x)) (corsExposedHeaders policy)
        ]

-- | A CORS middleware that supports simple cross-origin requests for all
-- resources.
--
-- This middleware does not check if the resource corresponds to the
-- restrictions for simple requests. This is in accordance with
-- <http://www.w3.org/TR/cors/>. The client (user-agent) is supposed to
-- enforcement CORS policy. The role of the server is to provide the client
-- with the respective policy constraints.
--
-- It is out of the scope of the this module if the server chooses to
-- enforce rules on its resources in relation to CORS policy itself.
--
simpleCors ∷ WAI.Middleware
simpleCors = cors (const $ Just simpleCorsResourcePolicy)

-- -------------------------------------------------------------------------- --
-- Definition from Standards

-- | Simple HTTP response headers as defined in <http://www.w3.org/TR/cors/>
--
simpleResponseHeaders ∷ [HTTP.HeaderName]
simpleResponseHeaders =
    [ "Cache-Control"
    , "Content-Language"
    , "Content-Type"
    , "Expires"
    , "Last-Modified"
    , "Pragma"
    ]

simpleHeaders ∷ [HTTP.HeaderName]
simpleHeaders =
    [ "Accept"
    , "Accept-Language"
    , "Content-Language"
    , "Content-Type"
    ]

simpleContentTypes ∷ [CI.CI B8.ByteString]
simpleContentTypes =
    [ "application/x-www-form-urlencoded"
    , "multipart/form-data"
    , "text/plain"
    ]


-- | Simple HTTP methods as defined in <http://www.w3.org/TR/cors/>
--
simpleMethods ∷ [HTTP.Method]
simpleMethods =
    [ "GET"
    , "HEAD"
    , "POST"
    ]

isSimple ∷ HTTP.Method → HTTP.RequestHeaders → Bool
isSimple method headers
    = method `elem` simpleMethods
    ∧ map fst headers `isSubsetOf` simpleHeaders
    ∧ case (method, lookup "content-type" headers) of
        ("POST", Just x) → CI.mk x `elem` simpleContentTypes
        _ → True

-- | Valid characters for HTTP header names according to RFC2616 (section 4.2)
--
isHttpHeaderNameChar ∷ Char → Bool
isHttpHeaderNameChar c = (c ≥ toEnum 33) && (c ≤ toEnum 126) && P.notInClass "()<>@,;:\\\"/[]?={}" c

httpHeaderNameParser ∷ P.Parser HTTP.HeaderName
httpHeaderNameParser = fromString <$> P.many1 (P.satisfy isHttpHeaderNameChar) P.<?> "HTTP Header Name"

-- -------------------------------------------------------------------------- --
-- Generic Tools

-- | A comma separated list of whitespace surounded HTTP header names.
--
-- Note that 'P.space' includes @SP@ (32), @HT@ (9), @LF@ (10), @VT@ (11),
-- @NP@ (12), and @CR@ (13). RFC 2616 (2.2) only defines @SP@ (32) and
-- @LWS = [CRLF] 1*(SP | HT)@ as whitespace. That's fine here since neither
-- of these characters is allowed in header names.
--
httpHeaderNameListParser ∷ P.Parser [HTTP.HeaderName]
httpHeaderNameListParser = spaces *> P.sepBy (httpHeaderNameParser <* spaces) (P.char ',') <* spaces
  where
    spaces = P.many' P.space

sshow ∷ (IsString α, Show β) ⇒ β → α
sshow = fromString ∘ show

isSubsetOf ∷ Eq α ⇒ [α] → [α] → Bool
isSubsetOf l1 l2 = intersect l1 l2 ≡ l1

-- | Add HTTP headers to a WAI response
--
#if MIN_VERSION_wai(3,0,0)
addHeaders ∷ HTTP.ResponseHeaders → WAI.Middleware
addHeaders hdrs app req respond = app req $ \response → do
    let (st, headers, streamHandle) = WAI.responseToStream response
    streamHandle $ \streamBody →
        respond $ WAI.responseStream st (headers ⊕ hdrs) streamBody

#elif MIN_VERSION_wai(2,0,0)

addHeaders ∷ HTTP.ResponseHeaders → WAI.Middleware
addHeaders hdrs app req = do
    (st, headers, src) ← WAI.responseToSource <$> app req
    WAI.responseSource st (headers ⊕ hdrs) <$> src return

#else

addHeaders ∷ HTTP.ResponseHeaders → WAI.Middleware
addHeaders hdrs app req = do
    (st, headers, src) ← WAI.responseSource <$> app req
    return $ WAI.ResponseSource st (headers ⊕ hdrs) src
#endif

-- | Format a list of 'HTTP.HeaderName's such that it can be used as
-- an HTTP header value
--
hdrLI ∷ [HTTP.HeaderName] → B8.ByteString
hdrLI l = B8.intercalate ", " (map CI.original l)

-- | Format a list of 'B8.ByteString's such that it can be used as
-- an HTTP header value
--
hdrL ∷ [B8.ByteString] → B8.ByteString
hdrL l = B8.intercalate ", " l

corsFailure
    ∷ B8.ByteString -- ^ body
    → WAI.Response
corsFailure msg = WAI.responseLBS HTTP.status400 [("Content-Type", "text/html; charset-utf-8")] (LB8.fromStrict msg)

-- Copied from the [wai-websocket package](https://github.com/yesodweb/wai/blob/master/wai-websockets/Network/Wai/Handler/WebSockets.hs#L21)
--
isWebSocketsReq
    ∷ WAI.Request
    → Bool
isWebSocketsReq req =
    fmap CI.mk (lookup "upgrade" $ WAI.requestHeaders req) == Just "websocket"