Subject: 26.1; Load order of custom-autoload, custom-load-symbol, custom-loads is backward?
From: Allen Li <[email-masked]>
To: [email-masked]
Date: Wed, 27 Mar 2019 12:45:42 -0700
List-Id: <debbugs-submit.debbugs.gnu.org>

I just spent some time tracking down an unexpected behavior in custom
autoloads.

The custom-autoload function is used in autoloads/loaddef files to set
up custom variables.  custom-autoload adds a load path to the
custom-loads property of a variable.  This property is then used in
custom-load-symbol to load each of the load paths.

The problem is that custom-autoload adds load paths to the front of
custom-loads, and custom-load-symbol loads the paths in forward order.

This is unexpected because if the site distributes a package foo and the
user installs their own version of foo, the site custom-autoload will be
called first:

;; Called from site-start
(custom-autoload 'foo-some-variable "path/to/site/foo" nil)
;; Called from init.el/package-initialize
(custom-autoload 'foo-some-variable "path/to/user/foo" nil)

In this case the custom-loads property will be

("path/to/user/foo" "path/to/site/foo")

The user version will be loaded first, and the site version will be
loaded later, overriding the user version.

This behavior is unexpected; the user would expect that their version of
foo overrides the site version of foo.  Furthermore, this behavior is
somewhat non-deterministic; site overrides foo *only if* foo is loaded
via setting foo-some-variable with customize.  If foo is loaded normally
(e.g. with require), then the user version of foo will be loaded in
preference to the site verison of foo.

I have attached a patch fixing this.

Note that this is technically a breaking change; however I really doubt
any one is intentionally relying on this behavior.  I consider this a
bugfix and I think there are probably users who don't realize that they
are suffering from this bug because it's so subtle.

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.22.24), modified by Debian
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Debian GNU/Linux rodete (upgraded from: Ubuntu 14.04 LTS)
From 719573c43ecb6e37e2f7c813dfcab09c8707d59d Mon Sep 17 00:00:00 2001
From: Allen Li <[email-masked]>
Date: Wed, 27 Mar 2019 12:39:28 -0700
Subject: [PATCH] Load custom variable dependencies in FIFO order

The custom-autoload function is used in autoloads/loaddef files to set
up custom variables.  custom-autoload adds a load path to the
custom-loads property of a variable.  This property is then used in
custom-load-symbol to load each of the load paths.

The problem is that custom-autoload adds load paths to the front of
custom-loads, and custom-load-symbol loads the paths in forward order.

This is unexpected because if the site distributes a package foo and the
user installs their own version of foo, the site custom-autoload will be
called first:

;; Called from site-start
(custom-autoload 'foo-some-variable "path/to/site/foo" nil)
;; Called from init.el/package-initialize
(custom-autoload 'foo-some-variable "path/to/user/foo" nil)

In this case the custom-loads property will be

("path/to/user/foo" "path/to/site/foo")

The user version will be loaded first, and the site version will be
loaded later, overriding the user version.

This behavior is unexpected; the user would expect that their version of
foo overrides the site version of foo.  Furthermore, this behavior is
somewhat non-deterministic; site overrides foo *only if* foo is loaded
via setting foo-some-variable with customize.  If foo is loaded normally
(e.g. with require), then the user version of foo will be loaded in
preference to the site verison of foo.

* lisp/custom.el (custom-load-symbol): Load variable deps in reverse order.
---
 lisp/custom.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/custom.el b/lisp/custom.el
index f0125742d1..da407bee83 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -639,7 +639,7 @@ custom-load-symbol
       (condition-case nil
 	  (require 'cus-start)
 	(error nil))
-      (dolist (load (get symbol 'custom-loads))
+      (dolist (load (reverse (get symbol 'custom-loads)))
 	(cond ((symbolp load) (condition-case nil (require load) (error nil)))
 	      ;; This is subsumed by the test below, but it's much faster.
 	      ((assoc load load-history))
-- 
2.21.0.392.gf8f6787159e-goog