jupyter-runtime-directory: Don't call shell commands at load time

* jupyter-env.el (jupyter-runtime-directory): Remove custom status.
New function definition.

* jupyter-kernel-manager.el (jupyter-write-connection-file):
Use new function.

* ob-jupyter.el (org-babel-jupyter--run-repl):
Remove setting of `jupyter-runtime-directory`.

* test/jupyter-test.el (jupyter-runtime-directory): New test.

jupyter-env.el

@@ -32,6 +32,14 @@
 (require 'jupyter-base)
 (eval-when-compile (require 'subr-x))
 
+(defvar jupyter-runtime-directory nil
+  "The Jupyter runtime directory.
+When a new kernel is started through `jupyter-start-kernel', this
+directory is where kernel connection files are written to.
+
+This variable should not be used. To obtain the runtime directory
+call the function `jupyter-runtime-directory'.")
+
 (defun jupyter-command (&rest args)
   "Run a Jupyter shell command synchronously, return its output.
 The shell command run is
@@ -44,12 +52,23 @@ return nil."
     (when (zerop (apply #'process-file "jupyter" nil t nil args))
       (string-trim-right (buffer-string)))))
 
-(defcustom jupyter-runtime-directory (jupyter-command "--runtime-dir")
-  "The Jupyter runtime directory.
-When a new kernel is started through `jupyter-start-kernel', this
-directory is where kernel connection files are written to."
-  :group 'jupyter
-  :type 'string)
+(defun jupyter-runtime-directory ()
+  "Return the runtime directory used by Jupyter.
+Create the directory if necessary. If `default-directory' is a
+remote directory, return the runtime directory on that remote.
+
+As a side effect, the variable `jupyter-runtime-directory' is set
+to the local runtime directory if it is nil."
+  (unless jupyter-runtime-directory
+    (setq jupyter-runtime-directory
+          (let ((default-directory user-emacs-directory))
+            (jupyter-command "--runtime-dir"))))
+  (let ((dir (if (file-remote-p default-directory)
+                 (concat (file-remote-p default-directory)
+                         (jupyter-command "--runtime-dir"))
+               jupyter-runtime-directory)))
+    (prog1 dir
+      (make-directory dir 'parents))))
 
 (defun jupyter-locate-python ()
   "Return the path to the python executable in use by Jupyter.

jupyter-kernel-manager.el

@@ -241,8 +241,7 @@ garbage collected. The file is also deleted when Emacs exits if
 it hasn't been already."
   (cl-check-type session jupyter-session)
   (cl-check-type obj jupyter-finalized-object)
-  (make-directory jupyter-runtime-directory 'parents)
-  (let* ((temporary-file-directory jupyter-runtime-directory)
+  (let* ((temporary-file-directory (jupyter-runtime-directory))
          (json-encoding-pretty-print t)
          (file (make-temp-file "emacs-kernel-" nil ".json"))
          (kill-hook (lambda () (when (and file (file-exists-p file))

ob-jupyter.el

@@ -206,9 +206,7 @@ variables in PARAMS."
   (let ((remote (file-remote-p session)))
     (when (and remote (zerop (length (file-local-name session))))
       (error "No remote session name"))
-    (let* ((default-directory (or remote default-directory))
-           (runtime-directory (jupyter-command "--runtime-dir"))
-           (jupyter-runtime-directory (concat remote runtime-directory)))
+    (let ((default-directory (or remote default-directory)))
       (jupyter-run-repl kernel nil nil 'jupyter-org-client))))
 
 (defun org-babel-jupyter-initiate-session-by-key (session params)

test/jupyter-test.el

@@ -683,6 +683,27 @@
                 (jupyter-stop-channels jupyter-current-client)))
           (jupyter-shutdown-kernel manager))))))
 
+;;; Environment
+
+(ert-deftest jupyter-runtime-directory ()
+  :tags '(env)
+  (let (dir-created jupyter-runtime-directory)
+    (cl-letf (((symbol-function #'jupyter-command)
+               (lambda (&rest _) "foo"))
+              ((symbol-function #'make-directory)
+               (lambda (&rest _)
+                 (setq dir-created t))))
+      (jupyter-runtime-directory)
+      (should dir-created)
+      (setq dir-created nil)
+      (should (equal jupyter-runtime-directory "foo"))
+      (let ((default-directory "/ssh:foo:~"))
+        (should (equal (jupyter-runtime-directory) "/ssh:foo:foo"))
+        (ert-info ("Variable definition is always local")
+          (setq jupyter-runtime-directory nil)
+          (jupyter-runtime-directory)
+          (should (equal jupyter-runtime-directory "foo")))))))
+
 ;;; Client
 
 ;; TODO: Different values of the session argument