From 47dde2be5732ec9dfa153813649e385e0227751e Mon Sep 17 00:00:00 2001
From: Philip Kaludercic <philipk@posteo.net>
Date: Tue, 15 Aug 2023 18:39:14 +0200
Subject: [PATCH] Add command to start Emacs with specific packages

* lisp/emacs-lisp/package.el (package-isolate): Add command.
* etc/NEWS: Announce it.
---
 etc/NEWS                   |  6 +++++
 lisp/emacs-lisp/package.el | 47 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/etc/NEWS b/etc/NEWS
index 57f04609679..c374695a571 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -505,6 +505,12 @@ project, that you can quickly select using 'project-switch-project'
 When non-nil, package specifications with side-effects for building
 software will be used when building a package.
 
+---
+*** New command to start Emacs only with specific packages
+The command 'package-isolate' is equivalent to starting Emacs with the
+-Q flag and loading specific packages (and their dependencies)
+manually.
+
 ** Flymake
 
 +++
diff --git a/lisp/emacs-lisp/package.el b/lisp/emacs-lisp/package.el
index b3062d2608b..838b2823466 100644
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -2625,6 +2625,53 @@ package-autoremove
                   removable))
         (message "Nothing to autoremove")))))
 
+(defun package-isolate (packages)
+  "Start an uncustomised Emacs and only load a set of PACKAGES."
+  (interactive
+   (cl-loop for p in (cl-loop for p in (package--alist) append (cdr p))
+	    unless (package-built-in-p p)
+	    collect (cons (package-desc-full-name p) p) into table
+	    finally return
+	    (list (cl-loop for c in (completing-read-multiple
+                                     "Isolate packages: " table
+                                     nil t)
+		           collect (alist-get c table nil nil #'string=)))))
+  (cl-assert (cl-every #'package-desc-p packages))
+  (let* ((name (concat "package-isolate-" (mapconcat #'package-desc-full-name
+                                                     packages ",")))
+         (tmp-init (make-temp-file name t))
+         (elpa (expand-file-name "elpa" tmp-init))
+         args)
+    (make-directory elpa)
+    (dolist (package packages)
+      ;; We need to recursively expand all the dependencies of the
+      ;; requested packages and all of them to `load-path'.
+      (dolist (package (named-let loop ((pkg-desc package))
+                         (let (deps)
+                           (dolist (req (package-desc-reqs pkg-desc))
+                             (setq deps (nconc
+                                         (catch 'found
+                                           (dolist (p (apply #'append (mapcar #'cdr (package--alist))))
+                                             (when (and (string= (car req) (package-desc-name p))
+                                                        (version-list-<= (cadr req) (package-desc-version p)))
+                                               (throw 'found (loop p)))))
+                                         deps)))
+                           (cons pkg-desc deps))))
+        (push (format "--directory=%s" (package-desc-dir package)) args)
+        (let* ((load-suffixes '(".el" ".elc"))
+               (autoload (locate-library (package--autoloads-file-name package))))
+          (push (format "--load=%s" autoload) args))))
+    (apply #'start-process (concat "*" name "*") nil
+           (append (list (expand-file-name invocation-name invocation-directory)
+                         "--quick" "--debug-init"
+                         (format "--eval=%S"
+                                 '(progn
+                                    (require 'warnings)
+                                    (add-to-list 'warning-suppress-log-types 'initialization)))
+                         "--init-directory" tmp-init)
+                   args))
+    (message "Started Emacs with the init directory: %s" tmp-init)))
+
 
 ;;;; Package description buffer.
 
-- 
2.39.2