core: add a basic timeout of 5s to fetch elpa archive

New file core-emacs-ext.el
This is a basic monkey-patch solution but it will do the job for now.
The timeout amount is not configurable for now.

Tested on 24.5 and 24.3.1

fixes syl20bnr#3284

core/core-emacs-ext.el

@@ -0,0 +1,49 @@
+;;; core-emacs-ext.el --- Spacemacs Core File
+;;
+;; Copyright (c) 2012-2014 Sylvain Benner
+;; Copyright (c) 2014-2015 Sylvain Benner & Contributors
+;;
+;; Author: Sylvain Benner <sylvain.benner@gmail.com>
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; License: GPLv3
+(require 'core-spacemacs-buffer)
+
+;; for some reason with-eval-after-load does not work here in 24.3
+;; maybe the backport is incorrect!
+(eval-after-load 'package
+  '(progn
+     (defun package-refresh-contents ()
+       "Download the ELPA archive description if needed.
+ This informs Emacs about the latest versions of all packages, and
+ makes them available for download.
+
+This redefinition adds a timeout of 5 seconds to contact each archive."
+       (interactive)
+       ;; the first part is not available before Emacs 24.4 so we just ignore
+       ;; it to be safe.
+       (unless (version< emacs-version "24.4")
+         ;; FIXME: Do it asynchronously.
+         (unless (file-exists-p package-user-dir)
+           (make-directory package-user-dir t))
+         (let ((default-keyring (expand-file-name "package-keyring.gpg"
+                                                  data-directory)))
+           (when (and package-check-signature (file-exists-p default-keyring))
+             (condition-case-unless-debug error
+                 (progn
+                   (epg-check-configuration (epg-configuration))
+                   (package-import-keyring default-keyring))
+               (error (message "Cannot import default keyring: %S" (cdr error)))))))
+       (dolist (archive package-archives)
+         (condition-case-unless-debug nil
+             (with-timeout (5 (spacemacs-buffer/warning
+                               "Cannot contact archive %s (reason: timeout)"
+                               (cdr archive)))
+               (package--download-one-archive archive "archive-contents"))
+           (error (message "Failed to download `%s' archive."
+                           (car archive)))))
+       (package-read-all-archive-contents))))
+
+(provide 'core-emacs-ext)

core/core-spacemacs.el

@@ -25,6 +25,7 @@
 (require 'core-toggle)
 (require 'core-micro-state)
 (require 'core-use-package-ext)
+(require 'core-emacs-ext)
 
 (defgroup spacemacs nil
   "Spacemacs customizations."