keep pointer to self.collectionView.dataSource in IGListBatchUpdateTr…

…ansaction

Summary:
## Issue
We're seeing crashes like this one:

```
Invalid batch updates detected: the number of sections and/or items returned by the data source before and after performing the batch updates are inconsistent with the updates.
Data source before updates = { 6 sections with item counts: [1, 0, 6, 8, 5, 7] }
Data source after updates = { 1 section with item counts: [0] }
Updates = [
]
```

I'm thinking the `IGListAdapter` gets deallocated during an update.
1. `1 section with item counts: [0]` is the default when the `self.collectionView.dataSource` is `nil`. I don't really see a way we'd return a single section with zero cells.
2. We're not seeing any of the additional crash information set by the `IGListAdapterUpdaterDelegate`, which would happen if `IGListAdapter` gets deallocated (it holds the strong pointer to the delegate).

## Fix

Lets keep a strong pointer within the scope of the function to make sure it sticks around. Theoretically, there could be a situation where the section-controller holds a strong pointer to the list-adapter in an update block, which gets released after it runs. This seems pretty unlikely, so lets test this fix to see if that's it.

Reviewed By: candance

Differential Revision: D52747014

fbshipit-source-id: 545aaa3deb90af85a011e716ac870659da42106f

Source/IGListDiffKit/IGListExperiments.h

@@ -19,7 +19,9 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     /// Test invalidating layout when cell reloads/updates in IGListBindingSectionController.
     IGListExperimentInvalidateLayoutForUpdates = 1 << 2,
     /// Throw NSInternalInconsistencyException during an update
-    IGListExperimentThrowOnInconsistencyException = 1 << 3
+    IGListExperimentThrowOnInconsistencyException = 1 << 3,
+    /// Test keeping a strong pointer to the collectionView.dataSource during a batch update to avoid a crash
+    IGListExperimentKeepPointerToCollectionViewDataSource = 1 << 4
 };
 
 /**

Source/IGListKit/Internal/IGListBatchUpdateTransaction.m

@@ -136,7 +136,11 @@ - (void)_didDiff:(IGListIndexSetResult *)diffResult onBackground:(BOOL)onBackgro
     [self.delegate listAdapterUpdater:self.updater didDiffWithResults:diffResult onBackgroundThread:onBackground];
 
     @try {
-        if (self.collectionView.dataSource == nil) {
+        // Experiment with keeping a pointer to the self.collectionView.dataSource, because we're seeing a crash where it could be missing.
+        const BOOL keepDataSource = IGListExperimentEnabled(self.config.experiments, IGListExperimentKeepPointerToCollectionViewDataSource);
+        id<UICollectionViewDataSource> const collectionViewDataSource = keepDataSource ? self.collectionView.dataSource : nil;
+
+        if (self.collectionView.dataSource == nil || (keepDataSource && collectionViewDataSource == nil)) {
             // If the data source is nil, we should not call any collection view update.
             [self _bail];
         } else if (diffResult.changeCount > 100 && self.config.allowsReloadingOnTooManyUpdates) {