Bring sanity to openTempFile

Test Plan: Run test of `openTempFile` under `strace` to verify
reasonably random filenames

Reviewers: austin, hvr, trofi, rwbarton

Reviewed By: trofi, rwbarton

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3188

libraries/base/System/IO.hs

@@ -233,6 +233,8 @@ import System.Posix.Types
 
 import GHC.Base
 import GHC.List
+import GHC.IORef
+import GHC.Num
 import GHC.IO hiding ( bracket, onException )
 import GHC.IO.IOMode
 import GHC.IO.Handle.FD
@@ -508,15 +510,16 @@ openTempFile' loc tmp_dir template binary mode = findTempName
                   | last a == pathSeparator = a ++ b
                   | otherwise = a ++ [pathSeparator] ++ b
 
--- int rand(void) from <stdlib.h>, limited by RAND_MAX (small value, 32768)
-foreign import capi "stdlib.h rand" c_rand :: IO CInt
+tempCounter :: IORef Int
+tempCounter = unsafePerformIO $ newIORef 0
+{-# NOINLINE tempCounter #-}
 
 -- build large digit-alike number
 rand_string :: IO String
 rand_string = do
-  r1 <- c_rand
-  r2 <- c_rand
-  return $ show r1 ++ show r2
+  r1 <- c_getpid
+  r2 <- atomicModifyIORef tempCounter (\n -> (n+1, n))
+  return $ show r1 ++ "-" ++ show r2
 
 data OpenNewFileResult
   = NewFileCreated CInt