hGetBuf sometimes reads fewer bytes than required

[rl]

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.

[rl]

Run with runghc Main.hs <file>

[simonmar]

I wasn't able to reproduce the bug (even with Cygwin), but your fix looks correct so I've applied it. Thanks!

Wed Oct 27 15:43:24 BST 2010  Simon Marlow <marlowsd@gmail.com>
  * hGetBuf: fix a case of a short read being returned (#4427)

[igloo]

Merged.