New file lisp/emacs-lisp/debug-early.el for backtraces in early bootstrap

This is also used in batch mode in general.

* lisp/debug-early.el (debug-early-backtrace, debug-early): New functions.

* lisp/loadup.el (top level): Load debug-early.el as first file.

* src/eval.c (signal_or_quit): Remove the condition in the batch mode section
of not being in dumping or bootstrap, since it is no longer needed.  Test that
'debug-early's symbol-function is bound.  Ensure there is enough working space
in specpdl and eval_depth.
(syms_of_eval): New DEFSYM for Qdebug_early.  Initialise Vdebugger to
Qdebug_early rather than Qnil.

lisp/emacs-lisp/debug-early.el

@@ -0,0 +1,77 @@
+;;; debug-early.el --- Dump a Lisp backtrace without frills  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022 Free Software Foundation, Inc.
+
+;; Author: Alan Mackenzie <acm@muc.de>
+;; Maintainer: emacs-devel@gnu.org
+;; Keywords: internal, backtrace, bootstrap.
+;; Package: emacs
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This file dumps a backtrace on stderr when an error is thrown.
+;; It has no dependencies on any Lisp libraries and is thus suitable
+;; for generating backtraces in the early parts of bootstrapping.  It
+;; is also good for generating backtraces in batch mode in general.
+
+(defalias 'debug-early-backtrace
+  #'(lambda ()
+  "Print a trace of Lisp function calls currently active.
+The output stream used is the value of `standard-output'.
+
+This is a simplified version of the standard `backtrace'
+function, intended for use in debugging the early parts
+of the build process."
+  (princ "\n")
+  (mapbacktrace
+   #'(lambda (evald func args _flags)
+       (let ((args args))
+	 (if evald
+	     (progn
+	       (princ "  ")
+	       (prin1 func)
+	       (princ "(")
+	       (while args
+		 (prin1 (car args))
+		 (setq args (cdr args))
+		 (if args
+		     (princ " ")))
+	       (princ ")\n"))
+	   (while args
+	     (princ "  ")
+	     (prin1 (car args))
+	     (princ "\n")
+	     (setq args (cdr args)))))))))
+
+(defalias 'debug-early
+  #'(lambda (&rest args)
+  "Print a trace of Lisp function calls currently active.
+The output stream used is the value of `standard-output'.
+
+There should be two ARGS, the symbol `error' and a cons of
+the error symbol and its data.
+
+This is a simplified version of `debug', intended for use
+in debugging the early parts of the build process."
+  (princ "\nError: ")
+  (prin1 (car (car (cdr args))))	; The error symbol.
+  (princ " ")
+  (prin1 (cdr (car (cdr args))))	; The error data.
+  (debug-early-backtrace)))
+
+;;; debug-early.el ends here.

lisp/loadup.el

@@ -128,6 +128,7 @@
 (set-buffer "*scratch*")
 (setq buffer-undo-list t)
 
+(load "emacs-lisp/debug-early")
 (load "emacs-lisp/byte-run")
 (load "emacs-lisp/backquote")
 (load "subr")

src/eval.c

@@ -1873,18 +1873,19 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object data, bool keyboard_quit)
     }
 
   /* If we're in batch mode, print a backtrace unconditionally to help
-     with debugging.  Make sure to use `debug' unconditionally to not
-     interfere with ERT or other packages that install custom
-     debuggers.  Don't try to call the debugger while dumping or
-     bootstrapping, it wouldn't work anyway.  */
+     with debugging.  Make sure to use `debug-early' unconditionally
+     to not interfere with ERT or other packages that install custom
+     debuggers.  */
   if (!debugger_called && !NILP (error_symbol)
       && (NILP (clause) || EQ (h->tag_or_ch, Qerror))
       && noninteractive && backtrace_on_error_noninteractive
-      && !will_dump_p () && !will_bootstrap_p ()
-      && NILP (Vinhibit_debugger))
+      && NILP (Vinhibit_debugger)
+      && !NILP (Ffboundp (Qdebug_early)))
     {
+      max_ensure_room (&max_lisp_eval_depth, lisp_eval_depth, 100);
+      max_ensure_room (&max_specpdl_size, SPECPDL_INDEX (), 200);
       ptrdiff_t count = SPECPDL_INDEX ();
-      specbind (Qdebugger, Qdebug);
+      specbind (Qdebugger, Qdebug_early);
       call_debugger (list2 (Qerror, Fcons (error_symbol, data)));
       unbind_to (count, Qnil);
     }
@@ -4399,6 +4400,7 @@ before making `inhibit-quit' nil.  */);
   DEFSYM (Qclosure, "closure");
   DEFSYM (QCdocumentation, ":documentation");
   DEFSYM (Qdebug, "debug");
+  DEFSYM (Qdebug_early, "debug-early");
 
   DEFVAR_LISP ("inhibit-debugger", Vinhibit_debugger,
 	       doc: /* Non-nil means never enter the debugger.
@@ -4453,7 +4455,7 @@ If due to frame exit, args are `exit' and the value being returned;
 If due to error, args are `error' and a list of the args to `signal'.
 If due to `apply' or `funcall' entry, one arg, `lambda'.
 If due to `eval' entry, one arg, t.  */);
-  Vdebugger = Qnil;
+  Vdebugger = Qdebug_early;
 
   DEFVAR_LISP ("signal-hook-function", Vsignal_hook_function,
 	       doc: /* If non-nil, this is a function for `signal' to call.