Prevent executing a source block already being executed

* ob-jupyter.el (org-babel-execute:jupyter): Do it.

* test/jupyter-test.el (org-babel-prevent-multiple-executions): New test.

Author: nnicandro

ob-jupyter.el

@@ -529,6 +529,11 @@ These parameters are handled internally."
   "Execute BODY according to PARAMS.
 BODY is the code to execute for the current Jupyter `:session' in
 the PARAMS alist."
+  (when org-babel-current-src-block-location
+    (save-excursion
+      (goto-char org-babel-current-src-block-location)
+      (when (jupyter-org-request-at-point)
+        (user-error "Source block currently being executed"))))
   (let* ((result-params (assq :result-params params))
          (async-p (jupyter-org-execute-async-p params)))
     (when (member "replace" result-params)

test/jupyter-test.el

@@ -3102,6 +3102,21 @@ raise Exception(\"This is an error\")
          (sleep-for 0.1))
        (funcall check-result ": 6")))))
 
+(ert-deftest org-babel-prevent-multiple-executions  ()
+  :tags '(org q)
+  (let ((src (format "\
+#+BEGIN_SRC jupyter-python :session %s :async yes
+1
+#+END_SRC
+"
+                    jupyter-org-test-session)))
+    (jupyter-org-test
+     (insert src)
+     (org-previous-block 1)
+     (org-babel-execute-src-block)
+     (should (jupyter-org-request-at-point))
+     (should-error (org-babel-execute-src-block)))))
+
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