[PATCH] do not auto-remove external packages (fixes #27822)

[Yuri D'Elia]

User-agent: mu4e 1.1.0; emacs 27.0.50
The package-autoremove machinery assumes all packages which are not
user-selected are removable (see 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27822)

However this is blatantly untrue. For example, in debian unstable, emacs
packages can be installed system-wide via apt as well (see all the elpa-*
packages).

Such packages _cannot_ and shouldn't be auto-removed.

The following patch checks whether a package has been _directly_ installed
into `package-user-dir' before marking it eligible for auto-removal.

[copyright assignment already done]

>From 5a4234b3c1289960fd760256aa6399efbd04bfc3 Mon Sep 17 00:00:00 2001
From: Yuri D'Elia <address@hidden>
Date: Tue, 17 Jul 2018 12:59:35 +0200
Subject: [PATCH] Do not consider external packages to be removable

Packages which are not directly user-installed shouldn't be autoremoved,
since they can be setup through a different path (via
`package-directory-list') where we have no authority over.
---
 lisp/emacs-lisp/package.el | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/lisp/emacs-lisp/package.el b/lisp/emacs-lisp/package.el
index 36233522..534f155c 100644
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -1754,6 +1754,15 @@ if it is still empty."
       (indirect indirect-deps)
       (t        (delete-dups (append direct-deps indirect-deps))))))
 
+(defun package--user-installed-p (pkg)
+  "Check if the package is a user-installed package.
+PKG is a package name.
+Checks whether the package was installed into `package-user-dir' where
+we assume to have control over."
+  (let* ((pkg-desc (cadr (assq pkg package-alist)))
+         (dir (package-desc-dir pkg-desc)))
+    (file-in-directory-p dir package-user-dir)))
+
 (defun package--removable-packages ()
   "Return a list of names of packages no longer needed.
 These are packages which are neither contained in
@@ -1763,7 +1772,9 @@ These are packages which are neither contained in
                          ;; `p' and its dependencies are needed.
                          append (cons p (package--get-deps p)))))
     (cl-loop for p in (mapcar #'car package-alist)
-             unless (memq p needed)
+             unless (or (memq p needed)
+                        ;; do not auto-remove external packages
+                        (not (package--user-installed-p p)))
              collect p)))
 
 (defun package--used-elsewhere-p (pkg-desc &optional pkg-list all)
-- 
2.18.0