Print a backtrace on unhandled errors in batch mode (Bug#44942).

* src/eval.c (signal_or_quit): Print a backtrace in batch mode if no
error handler was found.

* test/src/eval-tests.el (eval-tests/backtrace-in-batch-mode)
(eval-tests/backtrace-in-batch-mode/demoted-errors): New unit tests.

* etc/NEWS: Document change.

1 files changed, 14 insertions, 1 deletions

diff --git a/src/eval.c b/src/eval.c
index d9a424b57a9..18df484aac0 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1709,6 +1709,7 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object data, bool keyboard_quit)
         break;
     }
 
+  bool debugger_called = false;
   if (/* Don't run the debugger for a memory-full error.
          (There is no room in memory to do that!)  */
       !NILP (error_symbol)
@@ -1722,7 +1723,7 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object data, bool keyboard_quit)
              if requested".  */
           || EQ (h->tag_or_ch, Qerror)))
     {
-      bool debugger_called
+      debugger_called
         = maybe_call_debugger (conditions, error_symbol, data);
       /* We can't return values to code which signaled an error, but we
          can continue code which has signaled a quit.  */
@@ -1730,6 +1731,18 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object data, bool keyboard_quit)
         return Qnil;
     }
 
+  /* 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.  */
+  if (!debugger_called && !NILP (error_symbol)
+      && (NILP (clause) || EQ (h->tag_or_ch, Qerror)) && noninteractive)
+    {
+      ptrdiff_t count = SPECPDL_INDEX ();
+      specbind (Vdebugger, Qdebug);
+      call_debugger (list2 (Qerror, Fcons (error_symbol, data)));
+      unbind_to (count, Qnil);
+    }
+
   if (!NILP (clause))
     {
       Lisp_Object unwind_data