1 files changed, 31 insertions, 0 deletions
diff --git a/testsuite/tests/rts/4850.hs b/testsuite/tests/rts/4850.hs
new file mode 100644
index 0000000000..72616d97eb
--- /dev/null
+++ b/testsuite/tests/rts/4850.hs
@@ -0,0 +1,31 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+module Main where
+
+import Foreign
+import Control.Concurrent
+
+type Fun = Int -> IO Int
+
+foreign import ccall "wrapper" mkF :: Fun -> IO (FunPtr Fun)
+
+foreign import ccall "dynamic" callF :: FunPtr Fun -> Fun
+
+-- This test should create 4 OS threads only:
+--   one for main
+--   worker 1 for the IO manager
+--   worker 2 to run the first forkIO
+--   worker 3 created when worker 2 makes its foreign call
+
+-- Due to #4850, an extra worker was being created because worker 2 was
+-- lost after returning from its foreign call.
+
+main = do
+  m <- newEmptyMVar
+  callback m >> takeMVar m >>= print
+  callback m >> takeMVar m >>= print
+
+callback m =
+  forkIO $ do
+    f <- mkF $ \x -> return (x+1)
+    r <- callF f 3
+    putMVar m r