* lisp/emacs-lisp/package.el: Filter by multiple keywords and cache keywords

(package-menu-filter): Accept a list of keywords.

(package--all-keywords): New variable to cache known keywords.
(package-all-keywords): Populate it if necessary.
(package-refresh-contents): Reset it.

lisp/emacs-lisp/package.el

@@ -1343,6 +1343,11 @@ it to the file."
 (defvar package--downloads-in-progress nil
   "List of in-progress asynchronous downloads.")
 
+(defvar package--all-keywords nil
+  "List of known keywords.
+Generated by `package-all-keywords'.  Reset to nil whenever the
+package archives are retrieved.")
+
 (declare-function epg-check-configuration "epg-config"
                   (config &optional minimum-version))
 (declare-function epg-configuration "epg-config" ())
@@ -1458,9 +1463,9 @@ and make them available for download.
 Optional argument ASYNC specifies whether to perform the
 downloads in the background."
   (interactive)
-  ;; FIXME: Do it asynchronously.
   (unless (file-exists-p package-user-dir)
     (make-directory package-user-dir t))
+  (setq package--all-keywords nil)
   (let ((default-keyring (expand-file-name "package-keyring.gpg"
                                            data-directory))
         (package--silence async))
@@ -2492,11 +2497,11 @@ KEYWORDS should be nil or a list of keywords."
 
 (defun package-all-keywords ()
   "Collect all package keywords"
-  (let (keywords)
+  (unless package--all-keywords
     (package--mapc (lambda (desc)
                      (let* ((desc-keywords (and desc (package-desc--keywords desc))))
-                       (setq keywords (append keywords desc-keywords)))))
-    keywords))
+                       (setq package--all-keywords (append desc-keywords package--all-keywords))))))
+  package--all-keywords)
 
 (defun package--mapc (function &optional packages)
   "Call FUNCTION for all known PACKAGES.
@@ -3005,9 +3010,17 @@ shown."
 (defun package-menu-filter (keyword)
   "Filter the *Packages* buffer.
 Show only those items that relate to the specified KEYWORD.
+KEYWORD can be a string or a list of strings.  If it is a list, a
+package will be displayed if it matches any of the keywords.
+Interactively, it is a list of strings separated by commas.
+
 To restore the full package list, type `q'."
-  (interactive (list (completing-read "Keyword: " (package-all-keywords))))
-  (package-show-package-list t (list keyword)))
+  (interactive
+   (list (completing-read-multiple
+          "Keywords (comma separated): " (package-all-keywords))))
+  (package-show-package-list t (if (stringp keyword)
+                                   (list keyword)
+                                 keyword)))
 
 (defun package-list-packages-no-fetch ()
   "Display a list of packages.