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+{-|
+Module : Servant.GitHub.Webhook
+Description : Easily write safe GitHub webhook handlers with Servant
+Copyright : (c) Jacob Thomas Errington, 2016
+License : MIT
+Maintainer : servant-github-webhook@mail.jerrington.me
+Stability : experimental
+
+The GitHub webhook machinery will attach three headers to the HTTP requests
+that it fires: @X-Github-Event@, @X-Hub-Signature@, and @X-Github-Delivery@.
+The former two headers correspond with the 'GitHubEvent' and
+'GitHubSignedReqBody''' routing combinators. This library ignores the
+@X-Github-Delivery@ header; if you would like to access its value, then use the
+builtin 'Header' combinator from Servant.
+
+Usage of the library is straightforward: protect routes with the 'GitHubEvent'
+combinator to ensure that the route is only reached for specific
+'RepoWebhookEvent's, and replace any 'ReqBody' combinators you would write
+under that route with 'GitHubSignedReqBody'. It is advised to always include a
+'GitHubSignedReqBody''', as this is the only way you can be sure that it is
+GitHub who is sending the request, and not a malicious user. If you don't care
+about the request body, then simply use Aeson\'s 'Object' type as the
+deserialization target -- @GitHubSignedReqBody' key '[JSON] Object@ -- and
+ignore the @Object@ in the handler.
+
+The 'GitHubSignedReqBody''' combinator makes use of the Servant 'Context' in
+order to extract the signing key. This is the same key that must be entered in
+the configuration of the webhook on GitHub. See 'GitHubKey'' for more details.
+
+In order to support multiple keys on a per-route basis, the basic combinator
+@GitHubSignedReqBody''@ takes as a type parameter as a key index. To use this,
+create a datatype, e.g. @KeyIndex@ whose constructors identify the different
+keys you will be using. Generally, this means one constructor per repository.
+Use the @DataKinds@ extension to promote this datatype to a kind, and write an
+instance of 'Reflect' for each promoted constructor of your datatype. Finally,
+create a 'Context' containing 'GitHubKey'' whose wrapped function's domain is
+the datatype you've built up. Thus, your function can determine which key to
+retrieve.
+-}
+
+{-# LANGUAGE CPP #-}
+
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+
+-- GHC 8 seems to have improved its decidability check for type family
+-- instances and class instances. In particular, without UndecidableInstances
+-- enabled, the Demote' instance for lists, which we need, will not compile.
+-- Similarly, the Reflect instance for Symbol, which just requires KnownSymbol,
+-- won't compile on GHC < 8 because the instance head is no smaller than the
+-- instance head.
+#if __GLASGOW_HASKELL__ < 800
+{-# LANGUAGE UndecidableInstances #-}
+#endif
+
+module Servant.GitHub.Webhook
+( -- * Servant combinators
+ GitHubSignedReqBody''
+, GitHubSignedReqBody'
+, GitHubSignedReqBody
+, GitHubEvent
+
+ -- ** Security
+, GitHubKey'(..)
+, GitHubKey
+, gitHubKey
+
+ -- * Reexports
+ --
+ -- | We reexport a few datatypes that are typically needed to use the
+ -- library.
+, RepoWebhookEvent(..)
+, KProxy(..)
+
+ -- * Implementation details
+
+ -- ** Type-level programming machinery
+, Demote
+, Demote'
+, Reflect(..)
+
+ -- ** Stringy stuff
+, parseHeaderMaybe
+, matchEvent
+
+ -- * Examples
+ --
+ -- $example1
+ --
+ -- $example2
+) where
+
+import Control.Monad.IO.Class ( liftIO )
+import Data.Aeson ( decode', encode )
+import qualified Data.ByteString as BS
+import Data.ByteString.Lazy ( fromStrict, toStrict )
+import qualified Data.ByteString.Base16 as B16
+import Data.HMAC ( hmac_sha1 )
+import Data.List ( intercalate )
+import Data.Maybe ( catMaybes, fromMaybe )
+import Data.Monoid ( (<>) )
+import Data.Proxy
+import Data.String.Conversions ( cs )
+import qualified Data.Text.Encoding as E
+import GHC.TypeLits
+import GitHub.Data.Webhooks
+import Network.HTTP.Types hiding (Header, ResponseHeaders)
+import Network.Wai ( requestHeaders, strictRequestBody )
+import Servant
+import Servant.API.ContentTypes ( AllCTUnrender(..) )
+import Servant.Server.Internal
+
+
+-- | A clone of Servant's 'ReqBody' combinator, except that it will also
+-- verify the signature provided by GitHub in the @X-Hub-Signature@ header by
+-- computing the SHA1 HMAC of the request body and comparing.
+--
+-- The use of this combinator will require that the router context contain an
+-- appropriate 'GitHubKey'' entry. Specifically, the type parameter of
+-- 'GitHubKey'' must correspond with @Demote k@ where @k@ is the kind of the
+-- index @key@ used here. Consequently, it will be necessary to use
+-- 'serveWithContext' instead of 'serve'.
+--
+-- Other routes are not tried upon the failure of this combinator, and a 401
+-- response is generated.
+--
+-- Use of this datatype directly is discouraged, since the choice of the index
+-- @key@ determines its kind @k@ and hence @proxy@, which is . Instead, use
+-- 'GitHubSignedReqBody'', which computes the @proxy@ argument given just
+-- @key@. The proxy argument is necessary to avoid @UndecidableInstances@ for
+-- the implementation of the 'HasServer' instance for the datatype.
+data GitHubSignedReqBody''
+ (proxy :: KProxy k)
+ (key :: k)
+ (list :: [Type])
+ (result :: Type) where
+
+-- | Convenient synonym for 'GitHubSignedReqBody''' that computes its first
+-- type argument given just the second one.
+--
+-- Use this type synonym if you are creating a webhook server to handle
+-- webhooks from multiple repositories, with different secret keys.
+type GitHubSignedReqBody' (key :: k)
+ = GitHubSignedReqBody'' ('KProxy :: KProxy k) key
+
+-- | A convenient alias for a trivial key index.
+--
+-- USe this type synonym if you are creating a webhook server to handle only
+-- webhooks from a single repository, or for mutliple repositories using the
+-- same secret key.
+type GitHubSignedReqBody = GitHubSignedReqBody' '()
+
+-- | A routing combinator that succeeds only for a webhook request that matches
+-- one of the given 'RepoWebhookEvent' given in the type-level list @events@.
+--
+-- If the list contains 'WebhookWildcardEvent', then all events will be
+-- matched.
+--
+-- The combinator will require that its associated handler take a
+-- 'RepoWebhookEvent' parameter, and the matched event will be passed to the
+-- handler. This allows the handler to determine which event triggered it from
+-- the list.
+--
+-- Other routes are tried if there is a mismatch.
+data GitHubEvent (events :: [RepoWebhookEvent]) where
+