How to debug Emacs lisp that makes Emacs crash/use 100% CPU?

Question

There's an external library I'm working with that frequently pegs my CPU. I'd like to help the author fix it (as I really like the library), but I don't know how to debug the crash properly.

Any tips for debugging Emacs lisp? Bear in mind when it crashes Emacs doesn't work anymore and I have to kill it (so solutions within Emacs itself might not be helpful).

Edit: I should clarify that it is byte-compiled, and this issue doesn't always happen for others, so it may be specific to my architecture/init files. It is definitely related to this library though.

Answer 1

First, always debug the uncompiled version of a Emacs-Lisp program, unless you're convinced the problem is introduced by the byte-compiler.

Second, if the code is hanging Emacs then the code is probably in an infinite loop with inhibit-quit bound non-nil. So the first thing to do is go through the source for the library and change all inhibit-quit references to something else so that C-g will work to stop the looping. After that, load up the library, set debug-on-quit to t and you should get a nice debug trace when you press C-g that shows you where the code is looping. From there, debugging the problem should be as straightforward as debugging any other infinite loop.