Reinstate cwd setup, this time with locking

test/MonadSpec.hs

@@ -3,6 +3,8 @@ module MonadSpec where
 import Test.Hspec
 import TestUtils
 import Control.Monad.Error.Class
+import Control.Concurrent
+import Control.Exception
 
 spec :: Spec
 spec = do
@@ -15,3 +17,21 @@ spec = do
                          return "hello"
                      `catchError` (const $ fail "oh noes")
              a `shouldBe` (Left $ GMEString "oh noes")
+
+    describe "runGhcModT" $
+        it "throws an exception when run in multiple threads" $ do
+          mv1 :: MVar (Either SomeException ())
+              <- newEmptyMVar
+          mv2 :: MVar (Either SomeException ())
+              <- newEmptyMVar
+
+          _ <- forkOS $ putMVar mv1 =<< (try $ evaluate =<< (runD $ liftIO $ readMVar mv2 >> return ()))
+          _ <- forkOS $ putMVar mv2 =<< (try $ evaluate =<< (runD $ return ()))
+          e1 <- takeMVar mv1
+          e2 <- takeMVar mv2
+
+          (isLeft e1 || isLeft e2) `shouldBe` True
+
+isLeft :: Either a b -> Bool
+isLeft (Right _) = False
+isLeft (Left _) = True