I have a pretty large (~400kb) PNG file, and when I try gst-launch filesrc
location=test.png ! pngdec ! pngenc ! filesink location=out.png, I get lots of
warnings like this:

** (process:11950): WARNING **: reading past end of buffer

As far as I see, this happens because dec->buffer_in does not neccessarily
contain enough data, thus we trigger the reading past end of buffer thing.
Adding a blocksize=SIZE_OF_MY_FILE argument to filesrc works around the problem.

At the moment, pngdec expects the whole png data to arrive in one big buffer.
However, that is not always the case. I believe pngdec (and pngenc too.. I may
open a separate report about that) could be more intelligent and support input
that arrives in chunks.

Since I need this badly, I'll try to come up with a patch during the coming
days, time permitting.