How to log backtrace to a file instead of popping it up and blocking following backtraces

Question

With emacs if there is a backtrace buffer open, new backtrace is ignored. I find it is unconvenient when I am trying to find out which function/hook changes a variable. I use debug-watch as described in https://stackoverflow.com/questions/62579200/find-out-when-a-variable-is-changed-in-emacs The problem is the variable changes happens after a backtrace buffer popup is impossible to know because of the backtrace blocking described above.

(debug-watch VARIABLE)

How can I make emacs popup as many backtrace buffers as possible or log the backtraces to a file?

Answer 1

I'm not sure whether this helps answer your request, but you can use function backtrace directly, to print a backtrace at any time to standard-output (which you can bind to a buffer etc.). C-h f backtrace:

backtrace is a compiled Lisp function in subr.el.

`(backtrace)1

Print a trace of Lisp function calls currently active.

Output stream used is value of standard-output.

See the Elisp manual, node Internals of Debugger, for more info about this, including and example information about variable debugger-stack-frame-as-list.

Answer 2

Enable debug-on-error and use at your own risk; I've given it the most rudimentary of testing, but wouldn't have the foggiest idea how robust this would be.

(defun my-debug-to-file (&rest _args)
  (let ((inhibit-debugger t)
        (backtrace (backtrace-get-frames 'debug))
        (print-length nil))
    (with-temp-buffer
      (insert (backtrace-to-string backtrace))
      (insert "\n------------------------------------------------------\n\n")
      (append-to-file nil nil "/tmp/backtrace.log"))))

(advice-add 'debug :override #'my-debug-to-file)
;; (advice-remove 'debug #'my-debug-to-file)