[qwerta]

I upvote, but disagree.

Studying source code is first step. There is difference between "full understanding", and fixing a bug when software crashes under very specific conditions. Modern IDEs (and hopefully unit tests with VCS history) are great, 1+ MLOC is not really a problem.

Good luck with contacting original author, I usually give up after a few weeks.

And even proprietary software can expose enough "source" without compromising its monopoly. Microsoft bundles debugging symbols, so you can debug their software and roughly understand whats going on.