Break on all errors in edebug

Question

How can I make Emacs break on an error, even if the error is being handled somewhere?

In my current scenario, the setup of a major mode is modifying the buffer and I want to find out why (and then fix it). I made a buffer read-only, ran edebug-defun on the major mode function and set debug-on-error to t, but running the major mode function still proceeds until it aborts with a “Buffer is read-only” error. How do I get it to break on error, so that I know where it is raised and can examine the environment? Or at the very least how do I get a backtrace?

Answer 1

Setting edebug options doesn't actually help here. Errors that are listed in debug-ignored-errors are ignored even while executing under Edebug. This isn't mentioned in the manual (as of Emacs 24.5) but it is mentioned in the docstring of debug-on-error (for which edebug-on-error is just a proxy). (I don't understand why debug-ignored-errors has any influence on Edebug: it only makes sense to me as errors for which the debugger should not trigger during normal operation, not while debugging.)

So remove the problematic error from debug-ignored-errors:

(setq debug-ignored-errors (delq 'buffer-read-only debug-ignored-errors))

or just set debug-ignored-errors to nil while doing any debugging. This plus setting debug-on-error to t is enough to break on the error and get a backtrace.