[cairo] [PATCH] Generate better error message when loading invalid PNGs

[Cyril Roelandt]

On 03/25/2016 08:24 PM, Bill Spitzak wrote:
> I really don't see this as better, because now you are relying on local
> code arriving at exactly the same "is this a png file" conclusion as
> libpng. Also there are lots of other errors that are not no-memory.
> 
> Grepping libpng for png_error, I think perhaps this code will pick the most
> appropriate error:
> 
>     if (strstr(message, "alloc") || strstr(message, "OOM") ||
> strstr(message, "memory")
>          return CAIRO_STATUS_NO_MEMORY;
>     else
>          return CAIRO_STATUS_READ_ERROR;
> 

Why wouldn't we grep for "Not a PNG" as well? Also, Uli stated that it
might not be the best idea to rely on the error messages from libpng,
since they might change. WDYT?

Using png_sig_cmp allow us to be perfectly sure that the issue is with
the file/stream, and to return an appropriate error code to the user,
without gambling.

Cyril.