Subject: bug#49163: 28.0.50; Dynbind add-function and lambdas as file vars

From: Michael Heerdegen <[email-masked]>
Date: Tue, 22 Jun 2021 01:16:50 +0200

Hello,

this is the second time I have stumbled across this and it is annoying -
so here is the story:

The template of the code I was using is like this:

#+begin_src emacs-lisp
;; -*- lexical-binding: t -*-

(defun test ()
  (interactive)
  (funcall (or my-do-with-test-process-fun #'identity)
           (start-process "test-process" "foo" "xterm")))

(defvar my-do-with-test-process-fun nil)
#+end_src

I want to use `my-do-with-test-process-fun' as a file local variable.  I
try like this:

testfile.txt:
| File contents ...
| ...
|
| Local Variables:
| my-do-with-test-process-fun: (lambda (p) (add-function :before (process-sentinel p) (lambda (&rest _) (message "Test"))))
| End:

(I hope mailing stuff doesn't break the overlong line defining
`my-do-with-test-process-fun'.)

When I open that file and try M-x test I get this error:

| Debugger entered--Lisp error: (void-variable #:v)
|   (process-sentinel #:v)
|   (lambda nil (process-sentinel #:v))()
|   advice--add-function(:before ((lambda nil (process-sentinel #:v)) lambda (gv--val) (set-process-sentinel #:v gv--val)) (lambda (&rest _) (message "Test")) nil)
|   (add-function :before (process-sentinel p) (lambda (&rest _) (message "Test")))
|   (lambda (p) (add-function :before (process-sentinel p) (lambda (&rest _) (message "Test"))))(#<process test-process>)
|   funcall((lambda (p) (add-function :before (process-sentinel p) (lambda (&rest _) (message "Test")))) #<process test-process>)
|   test()

(1) I suspect that this happens because the lack of lexical binding
"somewhere".  Enabling lexical binding mode in `testfile.txt' alone
doesn't help.  OTOH, setting the file local variable like this:

| Local Variables:
| eval: (setq-local my-do-with-test-process-fun (eval (lambda (p) (add-function ...) t)))
| End:

makes `M-x test` work.  Is this all expected so far?

(2) If lack of lexical binding is the culprit: I see that `add-function'
uses `gv-ref' and that warns about uses in dynamically binding Elisp.
Should `add-function' warn about such a restriction as well?

(3) And I wonder: now that lexical binding Elisp gets more common,
should file local variables be set using a lexically binding environment
-- or at least when the file itself specifies lexical binding mode?

TIA,

Michael.

In GNU Emacs 28.0.50 (build 15, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2021-06-21 built on drachen
Repository revision: 50a59b9af1517f24fca60feab37140f2b35ea5ac
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux bullseye/sid

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP
SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LC_ALL: de_DE.utf8
  value of $LC_COLLATE: C
  value of $LC_TIME: C
  value of $LANG: de_DE.utf8
  locale-coding-system: utf-8-unix