Add the jupyter-use-zmq custom variable

jupyter-base.el

@@ -63,6 +63,13 @@ messages consider this variable."
                       (const "error")
                       (const "stream"))))
 
+(defcustom jupyter-use-zmq (and (locate-library "zmq") t)
+  "Whether or not ZMQ can be used to communicate with kernels.
+If ZMQ is not available for use, kernels can only be launched
+from a backing notebook server."
+  :group 'jupyter
+  :type 'boolean)
+
 (defconst jupyter-root (file-name-directory load-file-name)
   "Root directory containing emacs-jupyter.")
 

jupyter-repl.el

@@ -2087,7 +2087,11 @@ command on the host."
   (or client-class (setq client-class 'jupyter-repl-client))
   (jupyter-error-if-not-client-class-p client-class 'jupyter-repl-client)
   (jupyter-bootstrap-repl
+   (let ((spec (jupyter-kernelspec-name
+                (jupyter-guess-kernelspec kernel-name))))
      (jupyter-client
+      (if jupyter-use-zmq
+          (jupyter-kernel :spec spec)
         (jupyter-kernel
          :server (let ((server jupyter--run-repl-server))
                    (if (and server
@@ -2098,9 +2102,8 @@ command on the host."
                             (url (format "http://localhost:%s" port)))
                        (setq jupyter--run-repl-server
                              (jupyter-server :url url)))))
-     :spec (jupyter-kernelspec-name
-            (jupyter-guess-kernelspec kernel-name)))
-    client-class)
+         :spec spec))
+      client-class))
    repl-name associate-buffer display))
 
 ;;;###autoload

test/test-helper.el

@@ -39,6 +39,8 @@
 
 (declare-function jupyter-servers "jupyter-server")
 
+(setq jupyter-use-zmq nil)
+
 ;; Increase timeouts when testing for consistency. I think what is going on is
 ;; that communication with subprocesses gets slowed down when many processes
 ;; are being open and closed? The kernel processes are cached so they are