bug#64163: 29.0.92; Syntactic errors in early init when --debug-init is on

[Eli Zaretskii <eliz@gnu.org>]

> From: Sebastian Miele <iota@whxvd.name>
> Date: Mon, 19 Jun 2023 12:27:09 +0200
> 
> Create a $DIR with an early-init.el containing just '(' and an init.el
> with just '(error "Later")'.
> 
> Then running
> 
>   src/emacs --init-directory=$DIR
> 
> produces a warning buffer that lists both errors in order.  However,
> when running
> 
>   src/emacs --init-directory=$DIR --debug-init
> 
> the debugger triggers on the error "Later", without any mention of the
> error in the early init file.  This is unexpected.

First, the --init-directory=$DIR part is not relevant to the issue;
the same happens if you place such init files in your "normal" home
directory.

More to the point, that error in early-init.el does not trigger a
backtrace because 'end-of-file' error is in the list that is the value
of debug-ignored-errors.  So this problem is specific to that
particular kind of error in early-init.el.  If you trigger some other
error, which is not in the list of ignored ones, you will see a
backtrace.

IOW, this is the expected behavior.

I wonder whether the patch below could be the solution to this.
Stefan, WDYT?  Should we perhaps condition this binding by more
conditions, like only do this in interactive invocations or something?

diff --git a/lisp/startup.el b/lisp/startup.el
index b0f019d..bf16f14 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -1030,7 +1030,8 @@ startup--load-user-init-file
     (let ((debug-on-error debug-on-error-initial))
       (condition-case-unless-debug error
           (when init-file-user
-            (let ((init-file-name (funcall filename-function)))
+            (let ((init-file-name (funcall filename-function))
+                  (debug-ignored-errors nil))
 
               ;; If `user-init-file' is t, then `load' will store
               ;; the name of the file that it loads into