[PATCH] copy_file_range: New function to copy file data

[Florian Weimer]

On 11/23/2017 04:46 PM, Andreas Schwab wrote:
> On Nov 23 2017, Florian Weimer <fweimer@redhat.com> wrote:
> 
>> On 11/23/2017 04:39 PM, Andreas Schwab wrote:
>>> On Nov 23 2017, Florian Weimer <fweimer@redhat.com> wrote:
>>>
>>>>> I don't think write can ever return 0 when writing more than zero bytes.
>>>>
>>>> I can drop the check.  With the Linux VFS layer, it is difficult to tell
>>>> whether this condition can ever happen, and if it does, we would likely
>>>> enter an infinite loop without the check.
>>>   > Or we get the real error in the next loop.
>>
>> Tradition has it that it's a replacement for the ENOSPC condition,
> 
> A short write, yes, but not a zero write.

Looking at _IO_new_file_write in libio/fileops.c (which is where a call 
to fwrite eventually ends up if the buffer is full):

   _IO_ssize_t to_do = n;
   while (to_do > 0)
     {
       _IO_ssize_t count = (__builtin_expect (f->_flags2
					     & _IO_FLAGS2_NOTCANCEL, 0)
			   ? __write_nocancel (f->_fileno, data, to_do)
			   : __write (f->_fileno, data, to_do));
       if (count < 0)
	{
	  f->_flags |= _IO_ERR_SEEN;
	  break;
	}
       to_do -= count;
       data = (void *) ((char *) data + count);
     }

I see that we do not have a count == 0 special case there, so I'm going 
to drop the check from copy_file_range, as you proposed.

Thanks,
Florian