[cairo] Implementing cairo_read_func_t/cairo_write_func_t properly

[Behdad Esfahbod]

On 16-01-02 09:34 PM, Bill Spitzak wrote:
> On 01/02/2016 01:27 PM, Bernhard Fischer wrote:
>> On Saturday 02 January 2016 13:01:39 Bill Spitzak wrote:
> 
>>> I think the api has to be changed to return number of bytes read, and a
>>> negative number for eof.
>>
>> The return value should be the number of bytes read and on eof it is something
>> between 0 and length. If it is called again at/after eof it returns 0 again. A
>> negative value (-1) is only returned in case of error (but not EOF). This is
>> exactly like read(3) behaves.
> 
> Yes you are correct, I made a typo, I meant to say "error" where I wrote
> "eof". I fully agree with your description.

Given that we cannot change the signature now, I suggest we hack it up to
interpret negative numbers returned from the callback to be interpreted as a
short read; the absolute value of the returned negative number is the number
of bytes read.

Cairo itself won't need to be updated, but clients should be encouraged to
implement the new behaviour.

b