Persistent logging limits for Emacs message buffer in headless environments

Question

When diagnosing failed configuration loads or deployments in headless environments, the volatile message buffer is often insufficient because it clears upon session termination. While the emacs-log-file variable provides a mechanism for persistent logging, there is uncertainty regarding the granularity of the data captured when a fatal Lisp exception occurs during startup.

Relying on debug-on-error to trigger backtrace buffers is ineffective in non-interactive sessions where no UI buffers can be inspected manually. It is unclear if the standard log file output includes the full call stack information typically provided by the backtrace command, or if it only records the final high-level error string.

Does setting emacs-log-file capture full backtrace details when debug-on-error is active, and is there a specific configuration to ensure the minibuffer-history is flushed to disk before a crash halts execution?

Tasadduq Burney · Answer

No, setting emacs-log-file does not guarantee that a full Lisp backtrace appears alongside the error string when debug-on-error triggers during headless startup. In non-interactive sessions, the debugger writes to stderr or the log file, but whether the complete call stack is captured depends on the Emacs version and the debug-verbose setting. Emacs 29+ introduces structured backtrace logging, but earlier versions may only record the high-level error message.

Confirmed facts

emacs-log-file uses log-file-name to redirect message buffer and error output to a disk file
  minibuffer-history is an in-memory feature with no built-in mechanism to flush to disk before a crash
  Emacs 29+ changes backtrace logging behavior; earlier versions (27, 28) differ in stack inclusion

Likely explanation
emacs-log-file captures what the message buffer outputs to the log stream at the moment of error, but debug-on-error backtrace inclusion is version-dependent and not automatic in headless mode.

Minimal verification steps

Start Emacs in daemon mode: emacs --daemon
  Add (setq emacs-log-file t) and (setq log-file-name ~/.emacs.log) to your init file
  Force a fatal Lisp error early in startup (e.g., (error crash)) hooked into early-init-hook or similar)
  Examine the log file for backtrace lines and compare with your Emacs version output via M-x emacs-version
  Check whether debug-verbose is non-nil; if not, backtrace completeness may be limited

If you can share your Emacs version, I can refine whether the logged file will contain a full call stack or only the error summary.

Persistent logging limits for Emacs message buffer in headless environments

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Confirmed facts

Likely explanation

Minimal verification steps

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