ImageIO is not thread-safe

This post is wrong. See ImageIO is thread-safe after all for correction.

I can't find any documentation one way or another, and assumed that as a modern API it was OK to call ImageIO from multiple background threads. It appears that this is not the case. CGCreateImageSourceFromURL fails randomly if you do; always calling it from the main thread appears to work.

Radar 5759067 requests that this preferably be fixed, or if not, at least documented.