bug#19296: [PATCH] Package archives now have priorities.

[Jorgen Schaefer]

When installing packages by name, only packages from archives with
the highest priority are considered, before versions are compared.

This solves the "MELPA problem", where MELPA assigns date-based
version numbers to packages which override all other archives.
Giving MELPA a lower priority means packages are installed from
MELPA only when the package is not available from other archives.

This can be overridden manually by the user.
---
 lisp/emacs-lisp/package.el     |  107 ++++++++++++++++++++++++++++++----------
 test/automated/package-test.el |   17 +++++++
 2 files changed, 98 insertions(+), 26 deletions(-)

diff --git a/lisp/emacs-lisp/package.el b/lisp/emacs-lisp/package.el
index 4e5c397..844e5ea 100644
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -228,6 +228,33 @@ a package can run arbitrary code."
   :group 'package
   :version "24.1")
 
+(defcustom package-archive-default-priority 500
+  "The default priority for archives.
+
+This is used if the archive is not found in
+`package-archive-priorities'."
+  :type 'integer
+  :risky t
+  :group 'package
+  :version "25.1")
+
+(defcustom package-archive-priorities nil
+  "An alist of priorities for packages.
+
+Each element has the form (ARCHIVE-ID . PRIORITY).
+
+When installing packages, the package with the highest version
+number from the archive with the highest priority is
+selected. When higher versions are available from archives with
+lower priorities, the user has to select those manually.
+
+Archives not in this list have the priority given in
+`package-archive-default-priority'."
+  :type 'integer
+  :risky t
+  :group 'package
+  :version "25.1")
+
 (defcustom package-pinned-packages nil
   "An alist of packages that are pinned to specific archives.
 This can be useful if you have multiple package archives enabled,
@@ -1063,23 +1090,32 @@ Also, add the originating archive to the `package-desc' 
structure."
                         ;; Older archive-contents files have only 4
                         ;; elements here.
                         (package--ac-desc-extras (cdr package)))))
-         (existing-packages (assq name package-archive-contents))
          (pinned-to-archive (assoc name package-pinned-packages)))
-    (cond
-     ;; Skip entirely if pinned to another archive.
-     ((and pinned-to-archive
-           (not (equal (cdr pinned-to-archive) archive)))
-      nil)
-     ((not existing-packages)
-      (push (list name pkg-desc) package-archive-contents))
-     (t
-      (while
-          (if (and (cdr existing-packages)
-                   (version-list-<
-                    version (package-desc-version (cadr existing-packages))))
-              (setq existing-packages (cdr existing-packages))
-            (push pkg-desc (cdr existing-packages))
-            nil))))))
+    ;; Skip entirely if pinned to another archive.
+    (when (not (and pinned-to-archive
+                    (not (equal (cdr pinned-to-archive) archive))))
+      (setq package-archive-contents
+            (package--add-to-alist pkg-desc package-archive-contents)))))
+
+(defun package--add-to-alist (pkg-desc alist)
+  "Add PKG-DESC to ALIST.
+
+Packages are grouped by name. The package descriptions are sorted
+by version number."
+  (let* ((name (package-desc-name pkg-desc))
+         (priority-version (package-desc-priority-version pkg-desc))
+         (existing-packages (assq name alist)))
+    (if (not existing-packages)
+        (cons (list name pkg-desc)
+              alist)
+      (while (if (and (cdr existing-packages)
+                      (version-list-< priority-version
+                                      (package-desc-priority-version
+                                       (cadr existing-packages))))
+                 (setq existing-packages (cdr existing-packages))
+               (push pkg-desc (cdr existing-packages))
+               nil))
+      alist)))
 
 (defun package-download-transaction (packages)
   "Download and install all the packages in PACKAGES.
@@ -1268,6 +1304,25 @@ The file can either be a tar file or an Emacs Lisp file."
   "Return the archive containing the package NAME."
   (cdr (assoc (package-desc-archive desc) package-archives)))
 
+(defun package-archive-priority (archive)
+  "Return the priority of ARCHIVE.
+
+The archive priorities are specified in
+`package-archive-priorities' and
+`package-archive-default-priority'."
+  (or (cdr (assoc archive package-archive-priorities))
+      package-archive-default-priority))
+
+(defun package-desc-priority-version (pkg-desc)
+  "Return the version PKG-DESC with the archive priority prepended.
+
+This allows for easy comparison of package versions from
+different archives if archive priorities are meant to be taken in
+consideration."
+  (cons (package-archive-priority
+         (package-desc-archive pkg-desc))
+        (package-desc-version pkg-desc)))
+
 (defun package--download-one-archive (archive file)
   "Retrieve an archive file FILE from ARCHIVE, and cache it.
 ARCHIVE should be a cons cell of the form (NAME . LOCATION),
@@ -1940,18 +1995,18 @@ If optional arg BUTTON is non-nil, describe its 
associated package."
       ;; ENTRY is (PKG-DESC [NAME VERSION STATUS DOC])
       (let ((pkg-desc (car entry))
            (status (aref (cadr entry) 2)))
-       (cond ((member status '("installed" "unsigned"))
-              (push pkg-desc installed))
-             ((member status '("available" "new"))
-              (push (cons (package-desc-name pkg-desc) pkg-desc)
-                     available)))))
+        (cond ((member status '("installed" "unsigned"))
+               (push pkg-desc installed))
+              ((member status '("available" "new"))
+               (setq available (package--add-to-alist pkg-desc available))))))
     ;; Loop through list of installed packages, finding upgrades.
     (dolist (pkg-desc installed)
-      (let ((avail-pkg (assq (package-desc-name pkg-desc) available)))
-       (and avail-pkg
-            (version-list-< (package-desc-version pkg-desc)
-                             (package-desc-version (cdr avail-pkg)))
-            (push avail-pkg upgrades))))
+      (let* ((name (package-desc-name pkg-desc))
+             (avail-pkg (cadr (assq name available))))
+        (and avail-pkg
+             (version-list-< (package-desc-priority-version pkg-desc)
+                             (package-desc-priority-version avail-pkg))
+             (push (cons name avail-pkg) upgrades))))
     upgrades))
 
 (defun package-menu-mark-upgrades ()
diff --git a/test/automated/package-test.el b/test/automated/package-test.el
index 6e7994a..2a337fb 100644
--- a/test/automated/package-test.el
+++ b/test/automated/package-test.el
@@ -230,6 +230,23 @@ Must called from within a `tar-mode' buffer."
     (package-refresh-contents)
     (package-install 'simple-single)))
 
+(ert-deftest package-test-install-prioritized ()
+  "Install a lower version from a higher-prioritized archive."
+  (with-package-test ()
+    (let* ((newer-version (expand-file-name "data/package/newer-versions"
+                                            package-test-file-dir))
+           (package-archives `(("older" . ,package-test-data-dir)
+                               ("newer" . ,newer-version)))
+           (package-archive-priorities '(("newer" . 100))))
+
+      (package-initialize)
+      (package-refresh-contents)
+      (package-install 'simple-single)
+
+      (let ((installed (cdr (assq 'simple-single package-alist))))
+        (should (version-list-= '(1 3)
+                                (package-desc-version installed)))))))
+
 (ert-deftest package-test-install-multifile ()
   "Check properties of the installed multi-file package."
   (with-package-test (:basedir "data/package" :install '(multi-file))
-- 
1.7.10.4