guard againt simultaneous execution

src/cmd.js

@@ -216,11 +216,15 @@ const external_imports_loaded = (
       }
     }
   } else {
-    return eval_modules_finished(state, result, node, toplevel)
+    return eval_modules_finished(state, state, result, node, toplevel)
   }
 }
 
-const eval_modules_finished = (state, result, node, toplevel) => {
+const eval_modules_finished = (state, prev_state, result, node, toplevel) => {
+  if(state.calltree_changed_token != prev_state.calltree_changed_token) {
+    // code was modified after prev vesion of code was executed, discard
+    return state
+  }
   const next = apply_eval_result(state, result)
 
   let active_calltree_node

src/effects.js

@@ -182,7 +182,12 @@ export const render_common_side_effects = async (prev, next, command, ui) => {
   ) {
     const s = next.eval_modules_state
     s.promise.then(result => {
-      exec('eval_modules_finished', result, s.node, s.toplevel)
+      exec('eval_modules_finished', 
+        next, /* becomes prev_state */
+        result, 
+        s.node, 
+        s.toplevel
+      )
     })
   }
 

test/self_hosted_test.js

@@ -85,7 +85,7 @@ const loaded = i
 assert_equal(loaded.eval_modules_state != null, true)
 const s = loaded.eval_modules_state
 const result = await s.promise
-const state = COMMANDS.eval_modules_finished(loaded , result, s.node, s.toplevel)
+const state = COMMANDS.eval_modules_finished(loaded, loaded, result, s.node, s.toplevel)
 const root = root_calltree_node(state)
 const run = root.children[0]
 

test/test.js

@@ -2876,6 +2876,7 @@ const y = x()`
     const result = await after_edit.eval_modules_state.promise
     const after_edit_finished = COMMANDS.eval_modules_finished(
       after_edit, 
+      after_edit,
       result, 
       after_edit.eval_modules_state.node, 
       after_edit.eval_modules_state.toplevel

test/utils.js

@@ -71,6 +71,7 @@ export const test_initial_state_async = async code => {
   const result = await s.eval_modules_state.promise
   return COMMANDS.eval_modules_finished(
     s, 
+    s,
     result, 
     s.eval_modules_state.node, 
     s.eval_modules_state.toplevel