Balance reload delete and insert calls

Summary:
In Instagram@073fc07 we deduped delete calls, but since we convert reloads into delete+insert when only deduping deletes, we end up with unbalanced delete+insert calls.

Unit test added reproduced a crash we see internally. Fix passes the test.

Not adding a changelog entry since this is a new regression fixed between releases. #trivial

Issue fixed: t17539856

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
Closes Instagram#687

Differential Revision: D4933545

Pulled By: rnystrom

fbshipit-source-id: d38a900a99b1aa796dd654ddedb42e3cb4ef4378

Source/IGListAdapterUpdater.m

@@ -280,12 +280,16 @@ - (IGListBatchUpdateData *)flushCollectionView:(UICollectionView *)collectionVie
     NSMutableArray<IGListMoveIndexPath *> *itemMoves = batchUpdates.itemMoves;
 
     NSSet<NSIndexPath *> *uniqueDeletes = [NSSet setWithArray:itemDeletes];
+    NSMutableSet<NSIndexPath *> *reloadDeletePaths = [NSMutableSet new];
+    NSMutableSet<NSIndexPath *> *reloadInsertPaths = [NSMutableSet new];
     for (IGListReloadIndexPath *reload in batchUpdates.itemReloads) {
         if (![uniqueDeletes containsObject:reload.fromIndexPath]) {
-            [itemDeletes addObject:reload.fromIndexPath];
-            [itemInserts addObject:reload.toIndexPath];
+            [reloadDeletePaths addObject:reload.fromIndexPath];
+            [reloadInsertPaths addObject:reload.toIndexPath];
         }
     }
+    [itemDeletes addObjectsFromArray:[reloadDeletePaths allObjects]];
+    [itemInserts addObjectsFromArray:[reloadInsertPaths allObjects]];
 
     IGListBatchUpdateData *updateData = [[IGListBatchUpdateData alloc] initWithInsertSections:inserts
                                                                                deleteSections:deletes

Tests/IGListAdapterE2ETests.m

@@ -1460,5 +1460,24 @@ - (void)FIXME_test_whenDeletingItemsTwice_withDataUpdatedTwice_thatAllUpdatesApp
     [self waitForExpectationsWithTimeout:30 handler:nil];
 }
 
+- (void)test_whenReloadingSameItemTwice_thatDeletesAndInsertsAreBalanced {
+    [self setupWithObjects:@[
+                             genTestObject(@1, @4),
+                             ]];
+
+    IGTestObject *object = self.dataSource.objects[0];
+    IGListSectionController *sectionController = [self.adapter sectionControllerForObject:object];
+
+    XCTestExpectation *expectation = genExpectation;
+    [sectionController.collectionContext performBatchAnimated:YES updates:^(id<IGListBatchContext> batchContext) {
+        [batchContext reloadInSectionController:sectionController atIndexes:[NSIndexSet indexSetWithIndex:0]];
+        [batchContext reloadInSectionController:sectionController atIndexes:[NSIndexSet indexSetWithIndex:0]];
+    } completion:^(BOOL finished2) {
+        XCTAssertEqual([self.collectionView numberOfSections], 1);
+        XCTAssertEqual([self.collectionView numberOfItemsInSection:0], 4);
+        [expectation fulfill];
+    }];
+    [self waitForExpectationsWithTimeout:30 handler:nil];
+}
 
 @end