diff -r 7afee4bf89e8 -r e1a792312472 src/Pure/General/source.ML
--- a/src/Pure/General/source.ML	Mon Jul 23 16:45:00 2007 +0200
+++ b/src/Pure/General/source.ML	Mon Jul 23 16:45:01 2007 +0200
@@ -114,24 +114,26 @@
 
 (* stream source *)
 
-fun slurp_input instream =
+fun slurp_input instream = CRITICAL (fn () =>
   let
     fun slurp () =
       (case TextIO.canInput (instream, 1) handle IO.Io _ => NONE of
         NONE => []
       | SOME 0 => []
       | SOME _ => TextIO.input instream :: slurp ());
-  in maps explode (slurp ()) end;
+  in maps explode (slurp ()) end);
 
 fun drain_stream instream outstream prompt () =
   let val input = slurp_input instream in
     if exists (fn c => c = "\n") input then (input, ())
     else
-      (TextIO.output (outstream, Output.output prompt);
-        TextIO.flushOut outstream;
-        (case TextIO.inputLine instream of
+      (case
+        CRITICAL (fn () =>
+          (TextIO.output (outstream, Output.output prompt);
+            TextIO.flushOut outstream;
+            TextIO.inputLine instream)) of
           SOME line => (input @ explode line, ())
-        | NONE => (input, ())))
+        | NONE => (input, ()))
   end;
 
 fun of_stream instream outstream =