emacs-lisp/package.el (package-install): Mark dependencies as selected.

In particular, when given a package-desc object which is already
installed, the package is not downloaded again.

lisp/ChangeLog

@@ -2,6 +2,11 @@
 
 	* emacs-lisp/package.el (package-delete): Remove package from
 	`package-selected-packages' even if it can't be deleted.
+	(package-installed-p): Accept package-desc objects.
+	(package-install): Can be used to mark dependencies as
+	selected. When given a package-desc object which is already
+	installed, the package is not downloaded again, but it is marked
+	as selected (if it wasn't already).
 
 2015-02-03  Artur Malabarba  <bruce.connor.am@gmail.com>
 

lisp/emacs-lisp/package.el

@@ -996,15 +996,22 @@ GnuPG keyring is located under \"gnupg\" in `package-user-dir'."
 
 (defun package-installed-p (package &optional min-version)
   "Return true if PACKAGE, of MIN-VERSION or newer, is installed.
-MIN-VERSION should be a version list."
+If PACKAGE is a symbol, it is the package name and MIN-VERSION
+should be a version list.
+
+If PACKAGE is a package-desc object, MIN-VERSION is ignored."
   (unless package--initialized (error "package.el is not yet initialized!"))
-  (or
-   (let ((pkg-descs (cdr (assq package package-alist))))
-     (and pkg-descs
-          (version-list-<= min-version
-                           (package-desc-version (car pkg-descs)))))
-   ;; Also check built-in packages.
-   (package-built-in-p package min-version)))
+  (if (package-desc-p package)
+      (let ((dir (package-desc-dir pkg-desc)))
+        (and (stringp dir)
+             (file-exists-p dir)))
+    (or
+     (let ((pkg-descs (cdr (assq package package-alist))))
+       (and pkg-descs
+            (version-list-<= min-version
+                             (package-desc-version (car pkg-descs)))))
+     ;; Also check built-in packages.
+     (package-built-in-p package min-version))))
 
 (defun package-compute-transaction (packages requirements &optional seen)
   "Return a list of packages to be installed, including PACKAGES.
@@ -1219,7 +1226,10 @@ PKG can be a package-desc or the package name of one the available packages
 in an archive in `package-archives'.  Interactively, prompt for its name.
 
 If called interactively or if MARK-SELECTED is non-nil, add PKG
-to `package-selected-packages'."
+to `package-selected-packages'.
+
+if PKG is a package-desc and it is already installed, don't try
+to install it but still mark it as selected."
   (interactive
    (progn
      ;; Initialize the package system to get the list of package
@@ -1243,10 +1253,13 @@ to `package-selected-packages'."
     (when (and mark-selected (not (package--user-selected-p name)))
       (customize-save-variable 'package-selected-packages
                                (cons name package-selected-packages))))
-  (package-download-transaction
-   (if (package-desc-p pkg)
-       (package-compute-transaction (list pkg)
-                                    (package-desc-reqs pkg))
+  (if (package-desc-p pkg)
+      (if (package-installed-p pkg)
+          (message "`%s' is already installed" (package-desc-full-name pkg))
+        (package-download-transaction
+         (package-compute-transaction (list pkg)
+                                      (package-desc-reqs pkg))))
+    (package-download-transaction
      (package-compute-transaction ()
                                   (list (list pkg))))))