Review the :checksum patch, implement M-x el-get-checksum.

README.asciidoc

@@ -331,6 +331,13 @@ M-x el-get-make-recipes::
     recipe files: one file for each entry in +el-get-sources+ that is not
     just a +symbol+ and that is not found anywhere in +el-get-recipe-path+.
 
+M-x el-get-checksum::
+
+    Will prompt for the name of an installed package, with complement, then
+    compute its checksum if the package type supports that feature.  The
+    checksum is added to the kill-ring so that you're ready to yank it into
+    your +el-get-sources+ :checksum property if you want to.
+
 M-x el-get-emacswiki-refresh::
 
     Will launch a subprocess that connects to EmacsWiki and fetch from there

el-get-custom.el

@@ -283,7 +283,7 @@ definition provided by `el-get' recipes locally.
     Checksum calculation is currently supported by these methods
     with the following meaning:
 
-      * `http', `ftp' and `emacswiki' with the SHA1 of the downloaded file,
+      * `http', `ftp' and `emacswiki' with the SHA1 of the downloaded file
 "
 
   :type

el-get.el

@@ -40,6 +40,7 @@
 ;;   - implement :builtin property (useful for dealing with package.el)
 ;;   - fix recipes :build commands, must be either lists of strings or expr
 ;;   - add support for el-get-reload and do that at update time
+;;   - implement :checksum property for http kinds of files
 ;;
 ;;  3.1 - 2011-09-15 - Get a fix
 ;;
@@ -487,20 +488,18 @@ PACKAGE may be either a string or the corresponding symbol."
 	 (compute-checksum (el-get-method type :compute-checksum)))
 
     ;; check the checksum of the package here, as early as possible
-    (and checksum
-	 (not compute-checksum)
-	 (error
-	  "Checksum verification of package %s is not supported with method %s."
-	  package type))
+    (when (and checksum (not compute-checksum))
+      (error
+       "Checksum verification of package %s is not supported with method %s."
+       package type))
     (when compute-checksum
       (let ((computed (funcall compute-checksum package)))
 	(if checksum
-	    (when (not (equal computed checksum))
+	    (unless (equal computed checksum)
 	      (error "Checksum verification failed. Required: %s, actual: %s."
 		     checksum computed))
-	  (el-get-verbose-message
-	   "el-get: Checksum value for unchecked pakage %s is %s."
-	   package computed))))
+	  (el-get-verbose-message "el-get: pakage %s checksum is %s."
+				  package computed))))
 
     ;; post-install is the right place to run install-hook
     (run-hook-with-args hooks package)
@@ -675,6 +674,23 @@ entry which is not a symbol and is not already a known recipe."
       (el-get-write-recipe r dir)))
   (dired dir))
 
+	 (compute-checksum (el-get-method type :compute-checksum)))
+
+;;;###autoload
+(defun el-get-checksum (package)
+  "Compute the checksum of the given package, and put it in the kill-ring"
+  (interactive
+   (list (el-get-read-package-with-status "Checksum" "installed")))
+  (let* ((type             (el-get-package-type package))
+	 (checksum         (plist-get (el-get-package-def package) :checksum))
+	 (compute-checksum (el-get-method type :compute-checksum)))
+    (when (and checksum (not compute-checksum))
+      (error "package method %s does not support checksums" type))
+    (when compute-checksum
+      (let ((checksum (funcall compute-checksum package)))
+	(message "Checksum for package %s is: %s" package checksum)
+	(kill-new checksum)))))
+
 
 ;;
 ;; User Interface, Non Interactive part

methods/el-get-http.el

@@ -20,6 +20,9 @@
   :group 'el-get
   :type 'hook)
 
+(defvar el-get-http-checksums (make-hash-table)
+  "Hash table for storing downloaded SHA1 checksums.")
+
 (defun el-get-filename-from-url (url)
   "return a suitable filename from given url
 
@@ -27,9 +30,6 @@ Test url: http://repo.or.cz/w/ShellArchive.git?a=blob_plain;hb=HEAD;f=ack.el"
   (replace-regexp-in-string "[^a-zA-Z0-9-_\.\+]" "_"
 			    (file-name-nondirectory url)))
 
-(defvar el-get-http-checksums (make-hash-table)
-  "Hash table for storing downloaded SHA1 checksums.")
-
 (defun el-get-http-retrieve-callback (status package post-install-fun &optional dest sources)
   "Callback function for `url-retrieve', store the emacs lisp file for the package."
   (let* ((pdir   (el-get-package-directory package))
@@ -52,16 +52,21 @@ Test url: http://repo.or.cz/w/ShellArchive.git?a=blob_plain;hb=HEAD;f=ack.el"
     (kill-buffer))
   (funcall post-install-fun package))
 
+(defun el-get-http-dest-filename (package &optional url)
+  "Return where to store the file at given URL for given PACKAGE"
+  (let* ((pdir   (el-get-package-directory package))
+	 (url    (or url (plist-get (el-get-package-def package) :url)))
+	 (fname  (or (plist-get (el-get-package-def package) :localname)
+		     (el-get-filename-from-url url))))
+    (expand-file-name fname pdir)))
+
 (defun el-get-http-install (package url post-install-fun &optional dest)
   "Dowload a single-file PACKAGE over HTTP and store it in DEST.
 
 Should dest be omitted (nil), the url content will get written
 into the package :localname option or its `file-name-nondirectory' part."
   (let* ((pdir   (el-get-package-directory package))
-	 (fname  (or (plist-get (el-get-package-def package) :localname)
-		     (el-get-filename-from-url url)))
-	 (dest   (or dest
-		     (concat (file-name-as-directory pdir) fname))))
+	 (dest   (or dest (el-get-http-dest-filename package url))))
     (unless (file-directory-p pdir)
       (make-directory pdir))
 
@@ -75,7 +80,15 @@ into the package :localname option or its `file-name-nondirectory' part."
 
 (defun el-get-http-compute-checksum (package)
   "Look up download time SHA1 of PACKAGE."
-  (gethash package el-get-http-checksums "not installed in this session"))
+  (let ((checksum (gethash package el-get-http-checksums)))
+    (unless checksum
+      ;; compute the checksum
+      (setq checksum
+	    (with-temp-buffer
+	      (insert-file-contents-literally (el-get-http-dest-filename package))
+	      (sha1 (current-buffer))))
+      (puthash package checksum el-get-http-checksums))
+    checksum))
 
 (el-get-register-method
  :http #'el-get-http-install #'el-get-http-install #'el-get-rmdir