react: example: Check all dependents to avoid partial write

exercises/react/example.rs

@@ -47,12 +47,14 @@ impl <'a, T: Copy + PartialEq> Reactor<'a, T> {
     }
 
     pub fn create_compute<F: Fn(&[T]) -> T + 'a>(&mut self, dependencies: &[CellID], compute_func: F) -> Result<CellID, &'static str> {
+        // Check all dependencies' validity before modifying any of them,
+        // so that we don't perform an incorrect partial write.
+        if !dependencies.iter().all(|&id| id < self.cells.len()) {
+            return Err("Nonexistent input");
+        }
         let new_id = self.cells.len();
         for &id in dependencies {
-            match self.cells.get_mut(id) {
-                Some(c) => c.dependents.push(new_id),
-                None => return Err("Nonexistent input"),
-            }
+            self.cells.get_mut(id).unwrap().dependents.push(new_id);
         }
         let inputs: Vec<_> = dependencies.iter().map(|&id| self.value(id).unwrap()).collect();
         let initial = compute_func(&inputs);

exercises/react/tests/react.rs

@@ -53,6 +53,19 @@ fn error_creating_compute_cell_if_input_doesnt_exist() {
     assert!(Reactor::new().create_compute(&vec![input], |_| 0).is_err());
 }
 
+#[test]
+#[ignore]
+fn do_not_break_cell_if_creating_compute_cell_with_valid_and_invalid_input() {
+    let mut dummy_reactor = Reactor::new();
+    let _ = dummy_reactor.create_input(1);
+    let dummy_cell = dummy_reactor.create_input(2);
+    let mut reactor = Reactor::new();
+    let input = reactor.create_input(1);
+    assert!(reactor.create_compute(&vec![input, dummy_cell], |_| 0).is_err());
+    assert!(reactor.set_value(input, 5).is_ok());
+    assert_eq!(reactor.value(input).unwrap(), 5);
+}
+
 #[test]
 #[ignore]
 fn compute_cells_update_value_when_dependencies_are_changed() {