[FIX] observer: protect observed state against changes

src/observer.ts

@@ -18,6 +18,9 @@ const ModifiedArrayProto = Object.create(ArrayProto);
 for (let method of methodsToPatch) {
   const initialMethod = ArrayProto[method];
   ModifiedArrayProto[method] = function(...args) {
+    if (!this.__observer__.allowMutations) {
+      throw new Error(`Array cannot be changed here")`);
+    }
     this.__observer__.notifyChange();
     this.__owl__.rev++;
     let parent = this;
@@ -122,12 +125,12 @@ export class Observer {
         return value;
       },
       set(newVal) {
-        if (!self.allowMutations) {
-          throw new Error(
-            `State cannot be changed outside a mutation! (key: "${key}", val: "${newVal}")`
-          );
-        }
         if (newVal !== value) {
+          if (!self.allowMutations) {
+            throw new Error(
+              `Observed state cannot be changed here! (key: "${key}", val: "${newVal}")`
+            );
+          }
           self.unobserve(value);
           value = newVal;
           self.observe(newVal, obj);

tests/observer.test.ts

@@ -279,4 +279,26 @@ describe("observer", () => {
     await nextMicroTick();
     expect(observer.notifyCB).toBeCalledTimes(4);
   });
+
+  test("throw error when state is mutated in object if allowMutation=false", async () => {
+    const observer = new Observer();
+    observer.allowMutations = false;
+    const obj: any = { a: 1 };
+    observer.observe(obj);
+
+    expect(() => {
+      obj.a = 2;
+    }).toThrow('Observed state cannot be changed here! (key: "a", val: "2")');
+  });
+
+  test("throw error when state is mutated in array if allowMutation=false", async () => {
+    const observer = new Observer();
+    observer.allowMutations = false;
+    const obj: any = { a: [1] };
+    observer.observe(obj);
+
+    expect(() => {
+      obj.a.push(2);
+    }).toThrow("Array cannot be changed here");
+  });
 });