Fix buffering problem when GHCi is using the new IO library

Behind the scenes, the new IO library always does buffering for read
Handles regardless of NoBuffering.  Normally this isn't visible, but
it causes a problem in GHCi where there are two stdin Handles.

This should fix those ghci test failures that sprung up in full
testsuite runs recently.

ghc/InteractiveUI.hs

@@ -96,6 +96,7 @@ import GHC.Exts		( unsafeCoerce# )
 
 #if __GLASGOW_HASKELL__ >= 611
 import GHC.IO.Exception	( IOErrorType(InvalidArgument) )
+import GHC.IO.Handle    ( hFlushAll )
 #else
 import GHC.IOBase	( IOErrorType(InvalidArgument) )
 #endif
@@ -636,7 +637,16 @@ runStmt stmt step
  | null (filter (not.isSpace) stmt) = return False
  | ["import", mod] <- words stmt    = keepGoing' setContext ('+':mod)
  | otherwise
- = do result <- GhciMonad.runStmt stmt step
+ = do
+#if __GLASGOW_HASKELL__ >= 611
+      -- In the new IO library, read handles buffer data even if the Handle
+      -- is set to NoBuffering.  This causes problems for GHCi where there
+      -- are really two stdin Handles.  So we flush any bufferred data in
+      -- GHCi's stdin Handle here (only relevant if stdin is attached to
+      -- a file, otherwise the read buffer can't be flushed).
+      liftIO $ IO.try $ hFlushAll stdin
+#endif
+      result <- GhciMonad.runStmt stmt step
       afterRunStmt (const True) result
 
 --afterRunStmt :: GHC.RunResult -> GHCi Bool