Fix a number of flaws in the blktap userspace daemon when dealing

Fix a number of flaws in the blktap userspace daemon when dealing
with I/O errors.

There are a number of flaws in the blktap userspace daemon when dealing
with I/O errors.

 - The backends which use AIO check the io_events.res member to determine
  if an I/O error occurred. Which is good. But when calling the callback
  to signal completion of the I/O, they pass the io_events.res2 member

  Now this seems fine at first glance[1]

    "res is the usual result of an I/O operation: the number of bytes
     transfered, or a negative error code. res2 is a second status
     value which will be returned to the user"

  Except that

     "currently (2.6.0-test9), callers of aio_complete() within the
      kernel always set res2 to zero."

  And this hasn't changed anytime since 2.6.0, so by passing through
  the status from 'res2', the callback thinks the I/O operation succeeded
  even when it failed :-(

  The fix is simple instead of passing 'res2', just pass

     ep->res == io->u.c.nbytes ? 0 : 1

  This would solve the error reporting to the guest, except that there
  is a second flaw...

 - The tapdisk I/O completion callback checks the status parameter
  passed in, syslog's it and then returns. It never bothers to send
  the I/O completion response back to the blktap kernel driver when
  a failure occurrs.

  Fortunately the fix for this is also simple. Instead of returning
  from the callback when dealing with an error, we simply toggle the
  status field for the pending response to BLKIF_RSP_ERROR and then
  continue with the normal codepath. So the error eventually gets
  back to the guest.

The scenario I used to discover the problem and test the patch is thus:

 - In dom0  create a filesystem with only 200 MB of free space
 - Create a 1 GB sparse file on this volume.
 - Configure the guest so this sparse file appears as /dev/xvdb
 - In the domU create a single partition on /dev/xvdb and format
  it with ext3.
 - In the DomU, mount /dev/xvdb1 on /mnt and then run

     dd if=/dev/zero of=/mnt/data.bin bs=1GB count=1

Without this patch, the 'dd' command would succeed in writing 1 GB of data
even though the underlying disk in Dom0 was only 200 MB in size. More complex
tests of copying a whole directory heirarchy across resulted in catastrophic
data corruption of the filessytem itself. Manual fsck was needed to fixup
the filesystem & there were many very bad errors needing fixing.

With this patch applied the DomU sees the I/O failures and kernel  logs
messages

Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 722127
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 730327
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 738527
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 746727
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 754927
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 763127
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 771327
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 779527
Dec  1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector 792399

It will retry the I/O operation until it runs out of sectors to try, and then
fail the operation. The filesystem is not seriously damaged - ext3 journal
recovery will trivially cleanup if the guest is rebooted after the disk in
Dom0 is enlarged.

  Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
[1] http://lwn.net/Articles/24366/