Fix VoiceOver infinite recursion crash when delegate set to collectionView (Issue 1658)

Summary:
#1658

This fix addresses a crash that occurs when VoiceOver is enabled and `scrollViewDelegate` or `collectionViewDelegate` is set to the adapter's own `UICollectionView`.

## The Problem

When VoiceOver is enabled, accessibility queries trigger delegate method calls on the collection view. If the collection view is also set as its own scroll view delegate (via `IGListAdapter`'s proxy), these delegate calls get forwarded back to the collection view, creating infinite recursion:

1. VoiceOver queries accessibility on collectionView
2. `collectionView.delegate` (the proxy) forwards to `scrollViewTarget`
3. `scrollViewTarget` IS the collectionView → back to step 1
4. Stack overflow crash

Example crash-triggering code:
```
adapter.collectionView = collectionView
adapter.scrollViewDelegate = collectionView  // Crash with VoiceOver!
```

## The Fix

Here I've added validation in three places to prevent this configuration:

1. `setScrollViewDelegate:` - Rejects if `delegate == collectionView`
2. `setCollectionViewDelegate:` - Rejects if `delegate == collectionView`
3. `setCollectionView:` - Clears delegates if they equal the new collectionView

## Why This Is Safe (No Negative Side Effects)

**Setting UICollectionView as its own delegate provides no useful functionality:**
- `UICollectionView` already handles scroll events internally (it's a `UIScrollView` subclass)
- The `collectionView` doesn't implement custom `UIScrollViewDelegate` methods
- This configuration ONLY results in crashes when VoiceOver is enabled

**Behavior change is strictly beneficial:**
- Before: Works without VoiceOver, crashes WITH VoiceOver
- After: Silently rejects invalid config, works with VoiceOver

**Developer feedback is preserved:**
- `IGFailAssert` fires in `DEBUG` builds to alert developers to fix their code
- Production gracefully handles the situation instead of crashing

**No breaking changes for valid usage:**
- Normal delegate patterns (separate delegate objects) are unaffected
- Only the invalid self-referential pattern is blocked

See: #1658

Reviewed By: dinhvh

Differential Revision: D100641752

fbshipit-source-id: a1885c4ede6ca0555f3f94dd0c72b328a5af62bc

Author: camroth

CHANGELOG.md

@@ -5,6 +5,10 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 5.3.0 (Upcoming Release)
 -----
 
+### Fixes
+
+- An infinite recursion crash when VoiceOver is enabled and `scrollViewDelegate` or `collectionViewDelegate` is set to the adapter's own `UICollectionView`. [Cameron Roth](https://github.com/camroth) [(#1658)](https://github.com/Instagram/IGListKit/issues/1658)
+
 5.2.0
 -----
 

Source/IGListKit/IGListAdapter.m

@@ -112,6 +112,20 @@ - (void)setCollectionView:(UICollectionView *)collectionView {
         _registeredSupplementaryViewIdentifiers = [NSMutableSet new];
         _registeredSupplementaryViewNibNames = [NSMutableSet new];
 
+        // Clear the scrollViewDelegate or collectionViewDelegate if they were pointing to the new collectionView,
+        // as this would cause infinite recursion when VoiceOver is enabled.
+        // See: https://github.com/Instagram/IGListKit/issues/1658
+        if (_scrollViewDelegate == (id<UIScrollViewDelegate>)collectionView) {
+            IGFailAssert(@"scrollViewDelegate was set to the UICollectionView which causes infinite recursion "
+                         @"when VoiceOver is enabled. Clearing the scrollViewDelegate.");
+            _scrollViewDelegate = nil;
+        }
+        if (_collectionViewDelegate == (id<UICollectionViewDelegate>)collectionView) {
+            IGFailAssert(@"collectionViewDelegate was set to the UICollectionView which causes infinite recursion "
+                         @"when VoiceOver is enabled. Clearing the collectionViewDelegate.");
+            _collectionViewDelegate = nil;
+        }
+
         // We can't just swap out the collectionView, because we might have on-going or pending updates.
         // `_updater` can take care of that by wrapping the change in `performDataSourceChange`.
         [_updater performDataSourceChange:^{
@@ -169,6 +183,14 @@ - (void)setCollectionViewDelegate:(id<UICollectionViewDelegate>)collectionViewDe
            @"UICollectionViewDelegateFlowLayout conformance is automatically handled by IGListAdapter.");
 
     if (_collectionViewDelegate != collectionViewDelegate) {
+        // Setting the collectionViewDelegate to the collectionView itself will cause infinite recursion
+        // when VoiceOver is enabled, as accessibility queries trigger delegate methods that get
+        // forwarded back to the collectionView. See: https://github.com/Instagram/IGListKit/issues/1658
+        if (collectionViewDelegate == (id<UICollectionViewDelegate>)_collectionView) {
+            IGFailAssert(@"Setting collectionViewDelegate to the adapter's UICollectionView is not allowed. "
+                         @"This causes infinite recursion when VoiceOver is enabled.");
+            return;
+        }
         _collectionViewDelegate = collectionViewDelegate;
         [self _createProxyAndUpdateCollectionViewDelegate];
     }
@@ -178,6 +200,14 @@ - (void)setScrollViewDelegate:(id<UIScrollViewDelegate>)scrollViewDelegate {
     IGAssertMainThread();
 
     if (_scrollViewDelegate != scrollViewDelegate) {
+        // Setting the scrollViewDelegate to the collectionView itself will cause infinite recursion
+        // when VoiceOver is enabled, as accessibility queries trigger delegate methods that get
+        // forwarded back to the collectionView. See: https://github.com/Instagram/IGListKit/issues/1658
+        if (scrollViewDelegate == (id<UIScrollViewDelegate>)_collectionView) {
+            IGFailAssert(@"Setting scrollViewDelegate to the adapter's UICollectionView is not allowed. "
+                         @"This causes infinite recursion when VoiceOver is enabled.");
+            return;
+        }
         _scrollViewDelegate = scrollViewDelegate;
         [self _createProxyAndUpdateCollectionViewDelegate];
     }

Tests/IGListAdapterTests.m

@@ -2382,4 +2382,58 @@ - (void)test_whenSettingSupplementaryView_thatViewForSupplementaryElementExists
     XCTAssertNil([self.adapter viewForSupplementaryElementOfKind:UICollectionElementKindSectionHeader atIndex:1 sectionController:controller]);
 }
 
+#pragma mark - Recursive delegation prevention (Issue 1658)
+
+- (void)test_whenSettingScrollViewDelegateToCollectionView_thatScrollViewDelegateIsNotSet {
+    // Issue 1658: Setting scrollViewDelegate to collectionView causes infinite recursion when VoiceOver is enabled
+    // The adapter should reject this and not update the scrollViewDelegate
+    self.adapter.scrollViewDelegate = (id<UIScrollViewDelegate>)self.collectionView;
+    XCTAssertNil(self.adapter.scrollViewDelegate);
+}
+
+- (void)test_whenSettingCollectionViewDelegateToCollectionView_thatCollectionViewDelegateIsNotSet {
+    // Issue 1658: Setting collectionViewDelegate to collectionView causes infinite recursion when VoiceOver is enabled
+    // The adapter should reject this and not update the collectionViewDelegate
+    self.adapter.collectionViewDelegate = (id<UICollectionViewDelegate>)self.collectionView;
+    XCTAssertNil(self.adapter.collectionViewDelegate);
+}
+
+- (void)test_whenSettingCollectionViewAfterScrollViewDelegateWasSet_thatScrollViewDelegateIsCleared {
+    // If scrollViewDelegate is set to a collectionView before that collectionView is assigned to the adapter,
+    // the adapter should detect this and clear the scrollViewDelegate to prevent recursion
+    UICollectionView *newCollectionView = [[UICollectionView alloc] initWithFrame:CGRectZero
+                                                             collectionViewLayout:[UICollectionViewFlowLayout new]];
+
+    // First, create a new adapter with no collection view
+    IGListAdapter *newAdapter = [[IGListAdapter alloc] initWithUpdater:[IGListReloadDataUpdater new] viewController:nil];
+
+    // Set the scrollViewDelegate to the collection view we're about to set
+    // Note: This is done via the ivar since the setter would reject it if collectionView was already set
+    [newAdapter setValue:newCollectionView forKey:@"_scrollViewDelegate"];
+
+    // Now set the collection view - this should detect the issue and clear the scrollViewDelegate
+    newAdapter.collectionView = newCollectionView;
+
+    XCTAssertNil(newAdapter.scrollViewDelegate);
+}
+
+- (void)test_whenSettingCollectionViewAfterCollectionViewDelegateWasSet_thatCollectionViewDelegateIsCleared {
+    // If collectionViewDelegate is set to a collectionView before that collectionView is assigned to the adapter,
+    // the adapter should detect this and clear the collectionViewDelegate to prevent recursion
+    UICollectionView *newCollectionView = [[UICollectionView alloc] initWithFrame:CGRectZero
+                                                             collectionViewLayout:[UICollectionViewFlowLayout new]];
+
+    // First, create a new adapter with no collection view
+    IGListAdapter *newAdapter = [[IGListAdapter alloc] initWithUpdater:[IGListReloadDataUpdater new] viewController:nil];
+
+    // Set the collectionViewDelegate to the collection view we're about to set
+    // Note: This is done via the ivar since the setter would reject it if collectionView was already set
+    [newAdapter setValue:newCollectionView forKey:@"_collectionViewDelegate"];
+
+    // Now set the collection view - this should detect the issue and clear the collectionViewDelegate
+    newAdapter.collectionView = newCollectionView;
+
+    XCTAssertNil(newAdapter.collectionViewDelegate);
+}
+
 @end