Make elpa directory more flexible

- Rename variable to dotspacemacs-elpa-subdirectory
- Allow arbitrary form to be evaluated

core/core-configuration-layer.el

@@ -46,10 +46,20 @@
       configuration-layer-private-directory))
   "Spacemacs default directory for private layers.")
 
-(defun configuration-layer/emacs-version-dirname ()
-  "Directory name for current emacs version.
-Example output: \"24.5\"."
-  (format "%d%s%d" emacs-major-version version-separator emacs-minor-version))
+(defun configuration-layer/elpa-directory (root)
+  "Evaluate the correct package subdirectory of ROOT. This is
+done according to the value of `dotspacemacs-elpa-subdirectory'.
+If it is nil, then ROOT is returned. Otherwise a subdirectory of
+ROOT is returned."
+  (if (not dotspacemacs-elpa-subdirectory)
+      root
+    (let ((subdir (if (eq 'emacs-version dotspacemacs-elpa-subdirectory)
+                      (format "%d%s%d"
+                              emacs-major-version
+                              version-separator
+                              emacs-minor-version)
+                    (eval dotspacemacs-elpa-subdirectory))))
+      (file-name-as-directory (expand-file-name subdir root)))))
 
 (defvar configuration-layer-rollback-directory
   (concat spacemacs-cache-directory ".rollback/")
@@ -249,15 +259,10 @@ cache folder.")
   "Initialize `package.el'."
   (setq configuration-layer--refresh-package-timeout dotspacemacs-elpa-timeout)
   (unless package--initialized
-    (when dotspacemacs-enable-multiple-emacs-version
-      (setq configuration-layer-rollback-directory
-            (file-name-as-directory
-             (expand-file-name (configuration-layer/emacs-version-dirname)
-                               configuration-layer-rollback-directory)))
-      (setq package-user-dir
-            (file-name-as-directory
-             (expand-file-name (configuration-layer/emacs-version-dirname)
-                               package-user-dir))))
+    (setq configuration-layer-rollback-directory
+          (configuration-layer/elpa-directory configuration-layer-rollback-directory))
+    (setq package-user-dir
+          (configuration-layer/elpa-directory package-user-dir))
     (setq package-archives (configuration-layer//resolve-package-archives
                             configuration-layer--elpa-archives))
     ;; optimization, no need to activate all the packages so early

core/core-dotspacemacs.el

@@ -63,10 +63,10 @@ environment, otherwise it is strongly recommended to let it set to t.")
 (defvar dotspacemacs-elpa-timeout 5
   "Maximum allowed time in seconds to contact an ELPA repository.")
 
-(defvar dotspacemacs-enable-multiple-emacs-version nil
-  "If non nil, different emacs versions have different package directories.
-e.g. for Emacs 24.5, packages are stored in elpa/24.5/. Rollback
-directories are also separated.")
+(defvar dotspacemacs-elpa-subdirectory nil
+  "If non-nil, a form that evaluates to a package directory. For
+example, to use different package directories for different Emacs
+versions, set this to `emacs-version'.")
 
 (defvar dotspacemacs-configuration-layer-path '()
   "List of additional paths where to look for configuration layers.

core/templates/.spacemacs.template

@@ -91,10 +91,10 @@ values."
    ;; new versions works via git commands, thus it calls GitHub services
    ;; whenever you start Emacs. (default nil)
    dotspacemacs-check-for-update nil
-   ;; If non nil, different emacs versions have different package directories.
-   ;; e.g. for Emacs 24.5, packages are stored in elpa/24.5/. Rollback
-   ;; directories are also separated.
-   dotspacemacs-enable-multiple-emacs-version nil
+   ;; If non-nil, a form that evaluates to a package directory. For example, to
+   ;; use different package directories for different Emacs versions, set this
+   ;; to `emacs-version'.
+   dotspacemacs-elpa-subdirectory nil
    ;; One of `vim', `emacs' or `hybrid'.
    ;; `hybrid' is like `vim' except that `insert state' is replaced by the
    ;; `hybrid state' with `emacs' key bindings. The value can also be a list