Add jupyter-read-connection

* jupyter-base.el (jupyter-read-connection): New function.

* jupyter-kernel-process-manager.el
(jupyter-start-kernel) [jupyter-command-kernel]: Use it.

jupyter-base.el

@@ -529,6 +529,23 @@ Note only SSH tunnels are currently supported."
                      (jupyter-make-ssh-tunnel lport maybe-rport server remoteip)))
          else collect maybe-rport)))))
 
+(defun jupyter-read-connection (conn-file)
+  "Return the connection information in CONN-FILE.
+Return a property list representation of the JSON in CONN-FILE, a
+Jupyter connection file.
+
+If CONN-FILE is a remote file, possibly create an SSH tunnel
+between the localhost and the kernel on the remote host where
+CONN-FILE lives.  The returned connection info. will reflect
+these changes.
+
+See `jupyter-tunnel-connection' for more details on creating
+tunnels.  For more information on connection files, see
+https://jupyter-client.readthedocs.io/en/stable/kernels.html#connection-files"
+  (if (file-remote-p conn-file)
+      (jupyter-tunnel-connection conn-file)
+    (jupyter-read-plist conn-file)))
+
 ;;; Helper functions
 
 (defun jupyter-canonicalize-language-string (str)

jupyter-kernel-process-manager.el

@@ -151,12 +151,13 @@ argument of the process."
     :wait-form (and (process-live-p (oref kernel process))
                     (goto-char (point-min))
                     (re-search-forward "Connection file: \\(.+\\)\n" nil t))
-    (let* ((conn-file (match-string 1))
-           (remote (file-remote-p default-directory))
-           (conn-info (if remote (jupyter-tunnel-connection
-                                  (concat remote conn-file))
-                        (jupyter-read-plist conn-file))))
-      (oset kernel session (jupyter-session
+    (oset kernel session
+          (let ((conn-info (jupyter-read-connection
+                            (concat
+                             (save-match-data
+                               (file-remote-p default-directory))
+                             (match-string 1)))))
+            (jupyter-session
              :conn-info conn-info
              :key (plist-get conn-info :key))))))