Extract last source package release from local VCS data

* lisp/emacs-lisp/package-vc.el (package-vc-archive-spec-alist):
Unmention :release-rev
(package-vc-desc->spec): Fall back on other archives if a
specification is missing.
(package-vc-main-file): Add new function, copying the behaviour of
elpa-admin.el.
(package-vc-generate-description-file): Use 'package-vc-main-file'.
(package-vc-unpack): Handle special value ':last-release'.
(package-vc-release-rev): Add new function using 'last-change'.
(package-vc-install): Pass ':last-release' as REV instead of a
release.
* lisp/vc/vc-git.el (vc-git-last-change): Add Git 'last-change'
implementation.
* lisp/vc/vc.el (vc-default-last-change): Add default 'last-change'
implementation.

This attempts to replicate the behaviour of elpa-admin.el's
"elpaa--get-last-release-commit".

2 files changed, 35 insertions, 0 deletions

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 6137ce75ce4..cd62effd08d 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1632,6 +1632,23 @@ This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
 		    (expand-file-name fname (vc-git-root default-directory))))
 	  revision)))))
 
+(defun vc-git-last-change (from to)
+  (vc-buffer-sync)
+  (let ((file (file-relative-name
+               (buffer-file-name)
+               (vc-git-root (buffer-file-name))))
+        (start (line-number-at-pos from t))
+        (end (line-number-at-pos to t)))
+    (with-temp-buffer
+      (when (vc-git--out-ok
+             "blame" "--porcelain"
+             (format "-L%d,%d" start end)
+             file)
+        (goto-char (point-min))
+        (save-match-data
+          (when (looking-at "\\`\\([[:alnum:]]+\\)[[:space:]]+")
+            (match-string 1)))))))
+
 ;;; TAG/BRANCH SYSTEM
 
 (declare-function vc-read-revision "vc"
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 38209ef39ed..c8d28c144b5 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -448,6 +448,11 @@
 ;; - mergebase (rev1 &optional rev2)
 ;;
 ;;   Return the common ancestor between REV1 and REV2 revisions.
+;;
+;; - last-change (from to)
+;;
+;;   Return the most recent revision that made a change between FROM
+;;   and TO.
 
 ;; TAG/BRANCH SYSTEM
 ;;
@@ -3584,6 +3589,19 @@ it indicates a specific revision to check out."
                            remote directory rev)))
             (throw 'ok res)))))))
 
+(declare-function log-view-current-tag "log-view" (&optional pos))
+(defun vc-default-last-change (_backend from to)
+  "Default `last-change' implementation.
+FROM and TO are used as region markers"
+  (save-window-excursion
+    (let* ((buf (window-buffer (vc-region-history from to)))
+           (proc (get-buffer-process buf)))
+      (cl-assert (processp proc))
+      (while (accept-process-output proc))
+      (with-current-buffer buf
+        (prog1 (log-view-current-tag)
+          (kill-buffer))))))
+
 
 
 ;; These things should probably be generally available