Fix snippet undo

* yasnippet.el (yas--snippet-revive): Add revived snippet to
yas--active-snippets.
* yasnippet-debug.el (yas-debug-snippets): Print yas--active-snippets
length.
* yasnippet-tests.el (undo-revive-and-do-again): New test.

yasnippet-debug.el

@@ -254,6 +254,8 @@ buffer-locally, otherwise install it globally.  If HOOK is
             (setq yas-debug-target-snippets
                   (cl-delete-if-not #'yas--snippet-p yas-debug-target-snippets)))
           (let ((yas-debug-recently-live-indicators nil))
+            (printf "(length yas--snippets-snippets) => %d\n"
+                    (length yas--active-snippets))
             (dolist (snippet (or yas-debug-target-snippets
                                  (yas-active-snippets)))
               (printf "snippet %d\n" (yas--snippet-id snippet))

yasnippet-tests.el

@@ -346,6 +346,31 @@ attention to case differences."
         (ert-simulate-command '(undo))  ; Redo (re-expand snippet).
         (should (string-match-p "\\`one,and done" (buffer-string)))))))
 
+(ert-deftest undo-revive-and-do-again ()
+  "Check undo-revived snippet is properly ended."
+  ;; See https://github.com/joaotavora/yasnippet/issues/1006.
+  (yas-with-snippet-dirs '((".emacs.d/snippets"
+                            ("emacs-lisp-mode" ("x" . "${1:one},and done"))))
+    (with-temp-buffer
+      (emacs-lisp-mode)
+      (yas-reload-all)
+      (yas-minor-mode 1)
+      (yas-expand-snippet "x$0")
+      (setq buffer-undo-list nil)
+      (ert-simulate-command '(yas-expand))
+      (push nil buffer-undo-list)
+      (ert-simulate-command '(yas-next-field)) ; $1 -> exit snippet.
+      (should (string-match-p "\\`one,and done" (buffer-string)))
+      (push nil buffer-undo-list)
+      (ert-simulate-command '(undo))    ; Revive snippet.
+      (yas-mock-insert "abc")
+      (ert-simulate-command '(yas-next-field)) ; $1 -> exit snippet again.
+      (should (string-match-p "\\`abc,and done" (buffer-string)))
+      ;; We should have exited snippet and cleaned up any overlays.
+      (should-not (cl-some (lambda (o) (overlay-get o 'yas--snippet))
+                           (overlays-in (point-min) (point-max)))))))
+
+
 (defun yas-test-expand-and-undo (mode snippet-entry initial-contents)
   (yas-with-snippet-dirs
    `((".emacs.d/snippets" (,(symbol-name mode) ,snippet-entry)))

yasnippet.el

@@ -4149,6 +4149,7 @@ After revival, push the `yas--take-care-of-redo' in the
   (when (yas--maybe-move-to-active-field snippet)
     (setf (yas--snippet-control-overlay snippet) (yas--make-control-overlay snippet beg end))
     (overlay-put (yas--snippet-control-overlay snippet) 'yas--snippet snippet)
+    (push snippet yas--active-snippets)
     (when (listp buffer-undo-list)
       (push `(apply yas--take-care-of-redo ,snippet)
             buffer-undo-list))))