WithResource combinator for Servant-managed resources (#1630)

Author: andremarianiello

cabal.project

@@ -47,6 +47,7 @@ packages:
   doc/cookbook/using-custom-monad
   doc/cookbook/using-free-client
   -- doc/cookbook/open-id-connect
+  doc/cookbook/managed-resource
 
 tests: True
 optimization: False

doc/cookbook/index.rst

@@ -37,3 +37,4 @@ you name it!
   sentry/Sentry.lhs
   testing/Testing.lhs
   open-id-connect/OpenIdConnect.lhs
+  managed-resource/ManagedResource.lhs

doc/cookbook/managed-resource/ManagedResource.lhs

@@ -0,0 +1,114 @@
+# Request-lifetime Managed Resources
+
+Let's see how we can write a handle that uses a resource managed by Servant. The resource is created automatically by Servant when the server recieves a request, and the resource is automatically destroyed when the server is finished handling a request.
+
+As usual, we start with a little bit of throat clearing.
+
+
+``` haskell
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE TypeOperators #-}
+import Control.Concurrent
+import Control.Exception (bracket, throwIO)
+import Control.Monad.IO.Class
+import Control.Monad.Trans.Resource
+import Data.Acquire
+import Network.HTTP.Client (newManager, defaultManagerSettings)
+import Network.Wai.Handler.Warp
+import Servant
+import Servant.Client
+import System.IO
+```
+
+Here we define an API type that uses the `WithResource` combinator. The server handler for an endpoint with a `WithResource res` component will receive a value of that type as an argument.
+
+``` haskell
+type API = WithResource Handle :> ReqBody '[PlainText] String :> Post '[JSON] NoContent
+
+api :: Proxy API
+api = Proxy
+```
+
+But this resource value has to come from somewhere. Servant obtains the value using an Acquire provided in the context. The Acquire knows how to both create and destroy resources of a particular type.
+
+``` haskell
+appContext :: Context '[Acquire Handle]
+appContext = acquireHandle :. EmptyContext
+
+acquireHandle :: Acquire Handle
+acquireHandle = mkAcquire newHandle closeHandle
+
+newHandle :: IO Handle
+newHandle = do
+  putStrLn "opening file"
+  h <- openFile "test.txt" AppendMode
+  putStrLn "opened file"
+  return h
+
+closeHandle :: Handle -> IO ()
+closeHandle h = do
+  putStrLn "closing file"
+  hClose h
+  putStrLn "closed file"
+```
+
+Now we create the handler which will use this resource. This handler will write the request message to the System.IO.Handle which was provided to us. In some situations the handler will succeed, but in some in will fail. In either case, Servant will clean up the resource for us.
+
+``` haskell
+server :: Server API
+server = writeToFile
+
+  where writeToFile :: (ReleaseKey, Handle) -> String -> Handler NoContent
+        writeToFile (_, h) msg = case msg of
+          "illegal" -> error "wait, that's illegal!"
+          legalMsg -> liftIO $ do
+            putStrLn "writing file"
+            hPutStrLn h legalMsg
+            putStrLn "wrote file"
+            return NoContent
+```
+
+Finally we run the server in the background while we post messages to it.
+
+``` haskell
+runApp :: IO ()
+runApp = run 8080 (serveWithContext api appContext $ server)
+
+postMsg :: String -> ClientM NoContent
+postMsg = client api
+
+main :: IO ()
+main = do
+  mgr <- newManager defaultManagerSettings
+  bracket (forkIO $ runApp) killThread $ \_ -> do
+    ms <- flip runClientM (mkClientEnv mgr (BaseUrl Http "localhost" 8080 "")) $ do
+      liftIO $ putStrLn "sending hello message"
+      _ <- postMsg "hello"
+      liftIO $ putStrLn "sending illegal message"
+      _ <- postMsg "illegal"
+      liftIO $ putStrLn "done"
+    print ms
+```
+
+This program prints
+
+```
+sending hello message
+opening file
+opened file
+writing file
+wrote file
+closing file
+closed file
+sending illegal message
+opening file
+opened file
+closing file
+closed file
+wait, that's illegal!
+CallStack (from HasCallStack):
+  error, called at ManagedResource.lhs:63:24 in main:Main
+Left (FailureResponse (Request {requestPath = (BaseUrl {baseUrlScheme = Http, baseUrlHost = "localhost", baseUrlPort = 8080, baseUrlPath = ""},""), requestQueryString = fromList [], requestBody = Just ((),text/plain;charset=utf-8), requestAccept = fromList [], requestHeaders = fromList [], requestHttpVersion = HTTP/1.1, requestMethod = "POST"}) (Response {responseStatusCode = Status {statusCode = 500, statusMessage = "Internal Server Error"}, responseHeaders = fromList [("Transfer-Encoding","chunked"),("Date","Thu, 24 Nov 2022 21:04:47 GMT"),("Server","Warp/3.3.23"),("Content-Type","text/plain; charset=utf-8")], responseHttpVersion = HTTP/1.1, responseBody = "Something went wrong"}))
+```
+
+and appends to a file called `test.txt`. We can see from the output that when a legal message is sent, the file is opened, written to, and closed. We can also see that when an illegal message is sent, the file is opened but not written to. Crucially, it is still closed even though the handler threw an exception.

doc/cookbook/managed-resource/managed-resource.cabal

@@ -0,0 +1,30 @@
+cabal-version:       2.2
+name:                cookbook-managed-resource
+version:             0.1
+synopsis:            Simple managed resource cookbook example
+homepage:            http://docs.servant.dev/
+license:             BSD-3-Clause
+license-file:        ../../../servant/LICENSE
+author:              Servant Contributors
+maintainer:          [email protected]
+build-type:          Simple
+tested-with:         GHC==9.4.2
+
+executable cookbook-managed-resource
+  main-is:             ManagedResource.lhs
+  build-depends:       base == 4.*
+                     , text >= 1.2
+                     , aeson >= 1.2
+                     , servant
+                     , servant-client
+                     , servant-server
+                     , warp >= 3.2
+                     , wai >= 3.2
+                     , http-types >= 0.12
+                     , markdown-unlit >= 0.4
+                     , http-client >= 0.5
+                     , transformers
+                     , resourcet
+  default-language:    Haskell2010
+  ghc-options:         -Wall -pgmL markdown-unlit
+  build-tool-depends: markdown-unlit:markdown-unlit

doc/requirements.txt

@@ -1,3 +1,4 @@
 recommonmark==0.5.0
 Sphinx==1.8.4
 sphinx_rtd_theme>=0.4.2
+jinja2<3.1.0

servant-client-core/src/Servant/Client/Core/HasClient.hs

@@ -77,7 +77,7 @@ import           Servant.API
                  NoContentVerb, QueryFlag, QueryParam', QueryParams, Raw,
                  ReflectMethod (..), RemoteHost, ReqBody', SBoolI, Stream,
                  StreamBody', Summary, ToHttpApiData, ToSourceIO (..), Vault,
-                 Verb, WithNamedContext, WithStatus (..), contentType, getHeadersHList,
+                 Verb, WithNamedContext, WithResource, WithStatus (..), contentType, getHeadersHList,
                  getResponse, toEncodedUrlPiece, toUrlPiece, NamedRoutes)
 import           Servant.API.Generic
                  (GenericMode(..), ToServant, ToServantApi
@@ -776,6 +776,14 @@ instance HasClient m subapi =>
 
   hoistClientMonad pm _ f cl = hoistClientMonad pm (Proxy :: Proxy subapi) f cl
 
+instance HasClient m subapi =>
+  HasClient m (WithResource res :> subapi) where
+
+  type Client m (WithResource res :> subapi) = Client m subapi
+  clientWithRoute pm Proxy = clientWithRoute pm (Proxy :: Proxy subapi)
+
+  hoistClientMonad pm _ f cl = hoistClientMonad pm (Proxy :: Proxy subapi) f cl
+
 instance ( HasClient m api
          ) => HasClient m (AuthProtect tag :> api) where
   type Client m (AuthProtect tag :> api)

servant-docs/golden/comprehensive.md

@@ -530,6 +530,24 @@
 
 ```
 
+## GET /resource
+
+### Response:
+
+- Status code 200
+- Headers: []
+
+- Supported content types are:
+
+    - `application/json;charset=utf-8`
+    - `application/json`
+
+- Example (`application/json;charset=utf-8`, `application/json`):
+
+```javascript
+
+```
+
 ## GET /streaming
 
 ### Request:

servant-docs/src/Servant/Docs/Internal.hs

@@ -1144,6 +1144,9 @@ instance HasDocs api => HasDocs (Vault :> api) where
 instance HasDocs api => HasDocs (WithNamedContext name context api) where
   docsFor Proxy = docsFor (Proxy :: Proxy api)
 
+instance HasDocs api => HasDocs (WithResource res :> api) where
+  docsFor Proxy = docsFor (Proxy :: Proxy api)
+
 instance (ToAuthInfo (BasicAuth realm usr), HasDocs api) => HasDocs (BasicAuth realm usr :> api) where
   docsFor Proxy (endpoint, action) =
     docsFor (Proxy :: Proxy api) (endpoint, action')

servant-foreign/src/Servant/Foreign/Internal.hs

@@ -487,6 +487,13 @@ instance HasForeign lang ftype api =>
 
   foreignFor lang ftype Proxy = foreignFor lang ftype (Proxy :: Proxy api)
 
+instance HasForeign lang ftype api =>
+  HasForeign lang ftype (WithResource res :> api) where
+
+  type Foreign ftype (WithResource res :> api) = Foreign ftype api
+
+  foreignFor lang ftype Proxy = foreignFor lang ftype (Proxy :: Proxy api)
+
 instance HasForeign lang ftype api
   => HasForeign lang ftype (HttpVersion :> api) where
   type Foreign ftype (HttpVersion :> api) = Foreign ftype api

servant-server/src/Servant/Server/Internal.hs

@@ -35,9 +35,10 @@ module Servant.Server.Internal
 import           Control.Monad
                  (join, when)
 import           Control.Monad.Trans
-                 (liftIO)
+                 (liftIO, lift)
 import           Control.Monad.Trans.Resource
-                 (runResourceT)
+                 (runResourceT, ReleaseKey)
+import           Data.Acquire
 import qualified Data.ByteString                            as B
 import qualified Data.ByteString.Builder                    as BB
 import qualified Data.ByteString.Char8                      as BC8
@@ -77,7 +78,7 @@ import           Servant.API
                  QueryParam', QueryParams, Raw, ReflectMethod (reflectMethod),
                  RemoteHost, ReqBody', SBool (..), SBoolI (..), SourceIO,
                  Stream, StreamBody', Summary, ToSourceIO (..), Vault, Verb,
-                 WithNamedContext, NamedRoutes)
+                 WithNamedContext, WithResource, NamedRoutes)
 import           Servant.API.Generic (GenericMode(..), ToServant, ToServantApi, GServantProduct, toServant, fromServant)
 import           Servant.API.ContentTypes
                  (AcceptHeader (..), AllCTRender (..), AllCTUnrender (..),
@@ -244,6 +245,42 @@ instance (KnownSymbol capture, FromHttpApiData a, Typeable a
       formatError = urlParseErrorFormatter $ getContextEntry (mkContextWithErrorFormatter context)
       hint = CaptureHint (T.pack $ symbolVal $ Proxy @capture) (typeRep (Proxy :: Proxy [a]))
 
+-- | If you use 'WithResource' in one of the endpoints for your API Servant
+-- will provide the handler for this endpoint an argument of the specified type.
+-- The lifespan of this resource will be automatically managed by Servant. This
+-- resource will be created before the handler starts and it will be destoyed
+-- after it ends. A new resource is created for each request to the endpoint.
+
+-- The creation and destruction are done using a 'Data.Acquire.Acquire'
+-- provided via server 'Context'.
+--
+-- Example
+--
+-- > type MyApi = WithResource Handle :> "writeToFile" :> Post '[JSON] NoContent
+-- >
+-- > server :: Server MyApi
+-- > server = writeToFile
+-- >   where writeToFile :: (ReleaseKey, Handle) -> Handler NoContent
+-- >         writeToFile (_, h) = hPutStrLn h "message"
+--
+-- In addition to the resource, the handler will also receive a 'ReleaseKey'
+-- which can be used to deallocate the resource before the end of the request
+-- if desired.
+
+instance (HasServer api ctx, HasContextEntry ctx (Acquire a))
+      => HasServer (WithResource a :> api) ctx where
+
+  type ServerT (WithResource a :> api) m = (ReleaseKey, a) -> ServerT api m
+
+  hoistServerWithContext _ pc nt s = hoistServerWithContext (Proxy @api) pc nt . s
+
+  route Proxy context d = route (Proxy @api) context (d `addParameterCheck` allocateResource)
+    where
+      allocateResource :: DelayedIO (ReleaseKey, a)
+      allocateResource = DelayedIO $ lift $ allocateAcquire (getContextEntry context)
+
+
+
 allowedMethodHead :: Method -> Request -> Bool
 allowedMethodHead method request = method == methodGet && requestMethod request == methodHead