hLookAhead + hSetBuffering = unsupported operation (Illegal seek)

If you compile and run this program, and then press 'a' while it is running:

module Main (main) where

import System.IO

main :: IO ()
main = do hSetBuffering stdin NoBuffering
          hLookAhead stdin >>= print
          hSetBuffering stdin LineBuffering

then you get (amd64/Linux):

$ ./y
a'a'
y: <stdin>: hSetBuffering: unsupported operation (Illegal seek)

The docs at http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html#v%3AhSetBuffering say what happens when setting a buffer to !NoBuffering, but don't give any clue that this can happen when changing to !LineBuffering. I don't know if it is the behaviour or the docs that should be changed.

Also happens in the 6.10 branch.

Trac metadata

Trac field	Value
Version	6.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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