Try to keep long running sessions from expiring

The function `ein:notebooklist-enable-keepalive` and
`ein:notebooklist-disable-keepalive`, respectively enable a timer that
ocassionally (on the order of hours) sends a query to the notebook server. The
goal is to keep cookies and sessions from expiring in long running notebooks.
We'll see if it works.

lisp/ein-cell-edit.el

@@ -138,7 +138,7 @@ original notebook cell, unless being called via
     (ein:remove-overlay)
     (when ein:src--allow-write-back
       (ein:edit-cell-save))
-    (ewoc-invalidate (oref ein:src--cell :ewoc)
+    (ewoc-invalidate (slot-value ein:src--cell 'ewoc)
                      (ein:cell-element-get cell :input))
     (kill-buffer edit-buffer)
     (switch-to-buffer-other-window (ein:worksheet--get-buffer ws))

lisp/ein-core.el

@@ -130,7 +130,11 @@ the source is in git repository."
 (defvar *running-ipython-version* (make-hash-table))
 
 (defun ein:get-ipython-major-version (vstr)
-  (string-to-number (car (split-string vstr "\\."))))
+  (if vstr
+      (string-to-number (car (split-string vstr "\\.")))
+    (if (>= ein:log-level (ein:log-level-name-to-int 'debug))
+        (throw 'error "Null value passed to ein:get-ipython-major-version.")
+      (ein:log 'warn "Null value passed to ein:get-ipython-major-version."))))
 
 ;; TODO: Use symbols instead of numbers for ipython version ('jupyter and 'legacy)?
 (defun ein:query-ipython-version (&optional url-or-port force)

lisp/ein-notebooklist.el

@@ -166,6 +166,34 @@ To suppress popup, you can pass a function `ein:do-nothing' as CALLBACK."
   ;(ein:notebooklist-get-buffer url-or-port)
   )
 
+(defcustom ein:notebooklist-keepalive-refresh-time 1
+  "When the notebook keepalive is enabled, the frequency, IN
+HOURS, with which to make calls to the jupyter content API to
+refresh the notebook connection."
+  :type 'float
+  :group 'ein)
+
+(defvar ein:notebooklist--keepalive-timer nil)
+
+;;;###autoload
+(defun ein:notebooklist-enable-keepalive (&optional url-or-port)
+  "Enable periodic calls to the notebook server t"
+  (interactive (list (ein:notebooklist-ask-url-or-port)))
+  (let ((success
+         (lambda (content)
+           (ein:log 'info "Refreshing notebooklist connection.")))
+        (refresh-time (* ein:notebooklist-keepalive-refresh-time 60 60)))
+    (setq ein:notebooklist--keepalive-timer
+          (run-at-time 0.1 refresh-time #'ein:content-query-contents "" url-or-port nil success))))
+
+;;;###autoload
+(defun ein:notebooklist-disable-keepalive ()
+  ""
+  (interactive)
+  (cancel-timer ein:notebooklist--keepalive-timer))
+
+
+
 (defun* ein:notebooklist-url-retrieve-callback (content)
   "Called via `ein:notebooklist-open'."
   (let ((url-or-port (ein:$content-url-or-port content))