Fix watch so it doesn't throw errors

watch.el

@@ -19,6 +19,8 @@
   "Seconds between polls.  This is the watch rate, not the frame rate.")
 
 (defvar clj-watch--timer nil)
+(defvar clj-watch--connection nil
+  "The CIDER connection buffer to poll, captured when watching starts.")
 
 (defun clj-watch--paint (text)
   "Replace the watch buffer's contents with TEXT."
@@ -36,17 +38,24 @@
 
 (defun clj-watch--tick ()
   "Poll the snapshot once, asynchronously."
-  (if (not (get-buffer clj-watch-buffer))
-      (clj-watch-stop)
+  (cond
+   ((not (get-buffer clj-watch-buffer)) (clj-watch-stop))
+   ((not (buffer-live-p clj-watch--connection))
+    (clj-watch--paint "error:\nCIDER connection lost")
+    (clj-watch-stop))
+   (t
     ;; Async, not `cider-nrepl-sync-request': a sync request on a timer blocks
-    ;; Emacs's UI thread every tick.
+    ;; Emacs's UI thread every tick. Pass the connection explicitly -- the
+    ;; timer fires with no reliable "current buffer" to resolve one from.
     (cider-nrepl-request:eval
      "(watch/render)"
      (lambda (response)
        (nrepl-dbind-response response (value err)
          (cond
           (err   (clj-watch--paint (format "error:\n%s" err)))
-          (value (clj-watch--paint (car (read-from-string value))))))))))
+          (value (clj-watch--paint (car (read-from-string value)))))))
+     nil nil nil nil
+     clj-watch--connection))))
 
 (define-derived-mode clj-watch-mode special-mode "clj-watch"
   "Major mode for the pinned watch buffer."
@@ -56,7 +65,7 @@
 (defun clj-watch ()
   "Open the pinned watch buffer and start polling."
   (interactive)
-  (cider-current-repl nil 'ensure)
+  (setq clj-watch--connection (cider-current-repl nil 'ensure))
   (with-current-buffer (get-buffer-create clj-watch-buffer)
     (unless (eq major-mode 'clj-watch-mode)
       (clj-watch-mode)))