From f90f8205e4ec1ffd20477b223e84ef74a2649e81 Mon Sep 17 00:00:00 2001
From: Alex Kanunnikov <lifeart92@gmail.com>
Date: Fri, 10 May 2024 11:42:30 +0300
Subject: [PATCH] effect guards

---
 src/tests/integration/effect-test.gts | 21 +++++++++++++++++++++
 src/utils/vm.ts                       | 18 +++++++++++++++++-
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/src/tests/integration/effect-test.gts b/src/tests/integration/effect-test.gts
index 3adb64a3..7e155936 100644
--- a/src/tests/integration/effect-test.gts
+++ b/src/tests/integration/effect-test.gts
@@ -134,4 +134,25 @@ module('Integration | Internal | effect', function () {
     );
     assert.equal(executionsCount, 3, `executions count not changed`);
   });
+  test('check loop guard', async function (assert) {
+    const source = cell(1);
+    const derived = cell(1);
+    let executionsCount = 0;
+    const destructor = effect(() => {
+      derived.update(source.value);
+      source.value = Math.random();
+      executionsCount++;
+      console.log('effect executed');
+    });
+    assert.notEqual(derived.value, source.value);
+    assert.equal(executionsCount, 1, `effect executed once`);
+    source.update(2);
+    await rerender();
+    assert.notEqual(derived.value, source.value);
+    assert.equal(executionsCount, 2, `effect executed second time`);
+    await rerender();
+    assert.equal(executionsCount, 2, `effect executed second time`);
+
+    destructor();
+  });
 });
diff --git a/src/utils/vm.ts b/src/utils/vm.ts
index 3e9cda8f..3d02b1d4 100644
--- a/src/utils/vm.ts
+++ b/src/utils/vm.ts
@@ -8,6 +8,7 @@ import {
   formula,
   opsFor,
   inNewTrackingFrame,
+  tagsToRevalidate,
 } from './reactive';
 import { isFn } from './shared';
 
@@ -34,7 +35,22 @@ export function effect(cb: () => void): () => void {
   const tag = formula(() => {
     runEffectDestructor(destructor);
     destructor = undefined;
-    return sourceTag.value;
+    if (import.meta.env.DEV) {
+      const value = sourceTag.value;
+      try {
+        return value;
+      } finally {
+        if (import.meta.env.DEV) {
+          for (const tag of tagsToRevalidate) {
+            if (sourceTag.relatedCells?.has(tag)) {
+              throw new Error(`effect mutating its source: ${tag._debugName} ${JSON.stringify(tag._value)}`)
+            }
+          }    
+        }       
+      }
+    } else {
+      return sourceTag.value;
+    }   
   }, 'effect');
   const destroyOpcode = opcodeFor(tag, (value: unknown) => {
     if (IS_DEV_MODE) {