[Bug go/66378] New: libgo syscall.Sendfile() does not honor/use offset argument

["mvo at debian dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66378

            Bug ID: 66378
           Summary: libgo syscall.Sendfile() does not honor/use offset
                    argument
           Product: gcc
           Version: 5.1.1
               URL: https://bugs.launchpad.net/snappy/+bug/1460530
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: go
          Assignee: ian at airs dot com
          Reporter: mvo at debian dot org
                CC: cmang at google dot com
  Target Milestone: ---

Created attachment 35676
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35676&action=edit
Patch that make syscall.Sendfile() use offset.

The implementation in libgo/go/syscall/libcall_linux.go does not use the
"offset" argument. Instead it uses soff and poff which is never initialized
with the offset provided by the user (so always zero). This means that sendfile
will not be correct for any offset != 0.

Attached is a patch that fixes this issue, I verified the fix against our
failing snappy testsuite that has a test for sendfile with alternative offsets.