hGetBuf sometimes reads fewer bytes than required

The attached program repeatedly calls hGetBuf and prints how much it wanted to read and how many bytes hGetBuf returned. It should be run on a sufficiently large file. On Cygwin, the last 4 lines of the output are:

4 4
608 523
4 4
300 300

Note that we requested 608 bytes but hGetBuf only returned 523 even though we weren't at the end of file (since subsequent reads succeeded).

I suspect the bug is in GHC.IO.Handle.Text.bufReadEmpty, namely here:

  loop :: FD -> Int -> Int -> IO Int
  loop fd off bytes | bytes <= 0 = return off

It should probably return so_far + off.

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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