using ReadWriteMode in Windows corrupts files

In Windows, running the following program makes test.txt corrupted.

import System.IO
main = do
  h <- openFile "test.txt" ReadWriteMode
  hGetLine h
  hPutStrLn h "yz"
  hClose h

Initial content of test.txt is:

ab
cd
ef
gh

Expected content of test.txt after the excution is:

ab
yz
ef
gh

Actual result is(in hex):

61 62 0d 0a 63 64 0d 79 7a 0d 0a 0a 67 68 0d 0a

I think that the problem is that GHC.Handle.flushReadBuffer does not calculate the correct amount of seek to be done.

Trac metadata

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