IO library locking doesn't count readers

We aren't keeping track of the number of readers of a file, so that the first one to close a Handle removes the lock.

> main = do
>         fc <- openFile "z" ReadMode   -- sacrificial handle
>         fr <- openFile "z" ReadMode
>         hClose fc
>         fa <- openFile "z" AppendMode
>         hPutStr fa "append this line\n"
>         hGetLine fr >>= print

this should fail.

Trac metadata

Trac field	Value
Version	6.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown