remote: implement Logger_Write

Adds `setDataSink` which can be used to set
a function to be called when daemon
sned us data using `Logger_Write`.

`clearDataSink` should be used after
the operation using the data sink is finished.

hnix-store-remote/src/System/Nix/Store/Remote/Logger.hs

@@ -8,7 +8,7 @@ import Data.ByteString (ByteString)
 import Data.Serialize (Result(..))
 import System.Nix.Store.Remote.Serializer (LoggerSError, logger, runSerialT)
 import System.Nix.Store.Remote.Socket (sockGet8)
-import System.Nix.Store.Remote.MonadStore (MonadRemoteStore, RemoteStoreError(..), appendLog, getDataSource, getStoreSocket, getProtoVersion, setError)
+import System.Nix.Store.Remote.MonadStore (MonadRemoteStore, RemoteStoreError(..), appendLog, getDataSource, getDataSink, getStoreSocket, getProtoVersion, setError)
 import System.Nix.Store.Remote.Types.Logger (Logger(..))
 import System.Nix.Store.Remote.Types.ProtoVersion (ProtoVersion)
 
@@ -72,9 +72,14 @@ processOutput = do
             loop
 
           -- Write data to sink
-          -- used with tunnel sink in ExportPath operation
-          Logger_Write _out -> do
-            -- TODO: handle me
+          Logger_Write out -> do
+            mSink <- getDataSink
+            case mSink of
+              Nothing   ->
+                throwError RemoteStoreError_NoDataSinkProvided
+              Just sink -> do
+                liftIO $ sink out
+
             loop
 
           -- Following we just append and loop

hnix-store-remote/src/System/Nix/Store/Remote/MonadStore.hs

@@ -41,6 +41,10 @@ data RemoteStoreState = RemoteStoreState {
   -- as the daemon requests chunks of size @Word64@.
   -- If the function returns Nothing and daemon tries to read more
   -- data an error is thrown.
+  -- Used by @AddToStoreNar@ and @ImportPaths@ operations.
+  , remoteStoreState_mDataSink :: Maybe (ByteString -> IO ())
+  -- ^ Sink for @Logger_Write@, called repeatedly by the daemon
+  -- to dump us some data. Used by @ExportPath@ operation.
   , remoteStoreState_mNarSource :: Maybe (NarSource IO)
   }
 
@@ -61,6 +65,7 @@ data RemoteStoreError
   | RemoteStoreError_LoggerParserFail String ByteString -- when incremental parser returns ((Fail msg leftover) :: Result)
   | RemoteStoreError_NoDataSourceProvided -- remoteStoreState_mDataSource is required but it is Nothing
   | RemoteStoreError_DataSourceExhausted -- remoteStoreState_mDataSource returned Nothing but more data was requested
+  | RemoteStoreError_NoDataSinkProvided -- remoteStoreState_mDataSink is required but it is Nothing
   | RemoteStoreError_NoNarSourceProvided
   | RemoteStoreError_OperationFailed
   | RemoteStoreError_ProtocolMismatch
@@ -128,6 +133,7 @@ runRemoteStoreT r =
       { remoteStoreState_logs = mempty
       , remoteStoreState_gotError = False
       , remoteStoreState_mDataSource = Nothing
+      , remoteStoreState_mDataSink = Nothing
       , remoteStoreState_mNarSource = Nothing
       }
 
@@ -252,7 +258,33 @@ class ( MonadIO m
     => m ()
   clearDataSource = lift clearDataSource
 
+  setDataSink :: (ByteString -> IO ()) -> m ()
+  default setDataSink
+   :: ( MonadTrans t
+      , MonadRemoteStoreR r m'
+      , m ~ t m'
+      )
+   => (ByteString -> IO ())
+   -> m ()
+  setDataSink x = lift (setDataSink x)
 
+  getDataSink :: m (Maybe (ByteString -> IO ()))
+  default getDataSink
+   :: ( MonadTrans t
+      , MonadRemoteStoreR r m'
+      , m ~ t m'
+      )
+   => m (Maybe (ByteString -> IO ()))
+  getDataSink = lift getDataSink
+
+  clearDataSink :: m ()
+  default clearDataSink
+    :: ( MonadTrans t
+       , MonadRemoteStoreR r m'
+       , m ~ t m'
+       )
+    => m ()
+  clearDataSink = lift clearDataSink
 
 instance MonadRemoteStoreR r m => MonadRemoteStoreR r (StateT s m)
 instance MonadRemoteStoreR r m => MonadRemoteStoreR r (ReaderT r m)
@@ -282,6 +314,10 @@ instance ( MonadIO m
   getDataSource = remoteStoreState_mDataSource <$> RemoteStoreT get
   clearDataSource = RemoteStoreT $ modify $ \s -> s { remoteStoreState_mDataSource = Nothing }
 
+  setDataSink x = RemoteStoreT $ modify $ \s -> s { remoteStoreState_mDataSink = pure x }
+  getDataSink = remoteStoreState_mDataSink <$> RemoteStoreT get
+  clearDataSink = RemoteStoreT $ modify $ \s -> s { remoteStoreState_mDataSink = Nothing }
+
   setNarSource x = RemoteStoreT $ modify $ \s -> s { remoteStoreState_mNarSource = pure x }
   takeNarSource = RemoteStoreT $ do
     x <- remoteStoreState_mNarSource <$> get