emacs: Asynchronous retrieval of GPG keys

Rather than blocking emacs while gpg does its' thing, by default run
key retrieval asynchronously, possibly updating the display of the
message on successful completion.

emacs/notmuch-crypto.el

@@ -43,6 +43,11 @@ mode."
   :package-version '(notmuch . "0.25")
   :group 'notmuch-crypto)
 
+(defcustom notmuch-crypto-get-keys-asynchronously t
+  "Retrieve gpg keys asynchronously."
+  :type 'boolean
+  :group 'notmuch-crypto)
+
 (defface notmuch-crypto-part-header
   '((((class color)
       (background dark))
@@ -114,7 +119,7 @@ mode."
       (let ((keyid (concat "0x" (plist-get sigstatus :keyid))))
 	(setq label (concat "Unknown key ID " keyid " or unsupported algorithm"))
 	(setq button-action 'notmuch-crypto-sigstatus-error-callback)
-	(setq help-msg (concat "Click to retrieve key ID " keyid " from keyserver and redisplay."))))
+	(setq help-msg (concat "Click to retrieve key ID " keyid " from keyserver."))))
      ((string= status "bad")
       (let ((keyid (concat "0x" (plist-get sigstatus :keyid))))
 	(setq label (concat "Bad signature (claimed key ID " keyid ")"))
@@ -148,19 +153,77 @@ mode."
 	(call-process epg-gpg-program nil t t "--batch" "--no-tty" "--list-keys" fingerprint))
       (recenter -1))))
 
+(defun notmuch-crypto--async-key-sentinel (process event)
+  "When the user asks for a GPG key to be retrieved
+asynchronously, handle completion of that task.
+
+If the retrieval is successful, the thread where the retrieval
+was initiated is still displayed and the cursor has not moved,
+redisplay the thread."
+  (let ((status (process-status process))
+	(exit-status (process-exit-status process))
+	(keyid (process-get process :gpg-key-id)))
+    (when (memq status '(exit signal))
+      (message "Getting the GPG key %s asynchronously...%s."
+	       keyid
+	       (if (= exit-status 0)
+		   "completed"
+		 "failed"))
+      ;; If the original buffer is still alive and point didn't move
+      ;; (i.e. the user didn't move on or away), refresh the buffer to
+      ;; show the updated signature status.
+      (let ((show-buffer (process-get process :notmuch-show-buffer))
+	    (show-point (process-get process :notmuch-show-point)))
+	(when (and (bufferp show-buffer)
+		   (buffer-live-p show-buffer)
+		   (= show-point
+		      (with-current-buffer show-buffer
+			(point))))
+	  (with-current-buffer show-buffer
+	    (notmuch-show-refresh-view)))))))
+
+(defun notmuch-crypto--set-button-label (button label)
+  "Set the text displayed in BUTTON to LABEL."
+  (save-excursion
+    (let ((inhibit-read-only t))
+      ;; This knows rather too much about how we typically format
+      ;; buttons.
+      (goto-char (button-start button))
+      (forward-char 2)
+      (delete-region (point) (- (button-end button) 2))
+      (insert label))))
+
 (defun notmuch-crypto-sigstatus-error-callback (button)
   (let* ((sigstatus (button-get button :notmuch-sigstatus))
 	 (keyid (concat "0x" (plist-get sigstatus :keyid)))
-	 (buffer (get-buffer-create "*notmuch-crypto-gpg-out*"))
+	 (buffer (get-buffer-create "*notmuch-crypto-gpg-out*")))
-	 (window (display-buffer buffer t nil)))
+    (if notmuch-crypto-get-keys-asynchronously
+	(progn
+	  (notmuch-crypto--set-button-label
+	   button (format "Retrieving key %s asynchronously..." keyid))
+	  (let ((p (make-process :name "notmuch GPG key retrieval"
+				 :buffer buffer
+				 :command (list epg-gpg-program "--recv-keys" keyid)
+				 :connection-type 'pipe
+				 :sentinel #'notmuch-crypto--async-key-sentinel
+				 ;; Create the process stopped so that
+				 ;; we have time to store the key id,
+				 ;; etc. on it.
+				 :stop t)))
+	    (process-put p :gpg-key-id keyid)
+	    (process-put p :notmuch-show-buffer (current-buffer))
+	    (process-put p :notmuch-show-point (point))
+	    (message "Getting the GPG key %s asynchronously..." keyid)
+	    (continue-process p)))
+      (let ((window (display-buffer buffer t nil)))
 	(with-selected-window window
 	  (with-current-buffer buffer
 	    (goto-char (point-max))
-	(call-process epg-gpg-program nil t t "--batch" "--no-tty" "--recv-keys" keyid)
+	    (call-process epg-gpg-program nil t t "--recv-keys" keyid)
 	    (insert "\n")
-	(call-process epg-gpg-program nil t t "--batch" "--no-tty" "--list-keys" keyid))
+	    (call-process epg-gpg-program nil t t "--list-keys" keyid))
 	  (recenter -1))
-    (notmuch-show-refresh-view)))
+	(notmuch-show-refresh-view)))))
 
 (defun notmuch-crypto-insert-encstatus-button (encstatus)
   (let* ((status (plist-get encstatus :status))