From 5a93080f67ba40ab82439b90b7fbc179441c5b6b Mon Sep 17 00:00:00 2001
From: Maxime Ollivier <maxo@meta.com>
Date: Tue, 6 Aug 2024 08:01:28 -0700
Subject: [PATCH] remove unsafe early exit on performDataSourceChange

Summary:
By making `-performUpdate` completely synchronous sometimes, it unearthed a bug with the early exit where multiple updates can happen at once. Specifically, the shortcut doesn't block updates from being started within `-didUpdateToObject`.

Lets test removing it, since it's happening in a pretty sensitive part of updates.

Differential Revision: D60773894

fbshipit-source-id: 721dffcb6d5353d12693fa873eb0f502bcdf4c02
---
 Source/IGListDiffKit/IGListExperiments.h          |  2 ++
 Source/IGListKit/IGListAdapterUpdater.m           |  4 +++-
 .../Internal/IGListUpdateTransactionBuilder.m     | 15 ++++++++-------
 3 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/Source/IGListDiffKit/IGListExperiments.h b/Source/IGListDiffKit/IGListExperiments.h
index 6b75ea151..d2c3d9b90 100644
--- a/Source/IGListDiffKit/IGListExperiments.h
+++ b/Source/IGListDiffKit/IGListExperiments.h
@@ -20,6 +20,8 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     IGListExperimentInvalidateLayoutForUpdates = 1 << 2,
     /// Throw NSInternalInconsistencyException during an update
     IGListExperimentThrowOnInconsistencyException = 1 << 3,
+    /// Remove the early exit so multiple updates can't happen at once
+    IGListExperimentRemoveDataSourceChangeEarlyExit = 1 << 4,
 };
 
 /**
diff --git a/Source/IGListKit/IGListAdapterUpdater.m b/Source/IGListKit/IGListAdapterUpdater.m
index 421569edb..6f8653c0b 100644
--- a/Source/IGListKit/IGListAdapterUpdater.m
+++ b/Source/IGListKit/IGListAdapterUpdater.m
@@ -208,7 +208,9 @@ - (void)performDataSourceChange:(IGListDataSourceChangeBlock)block {
     // Which means we need to cancel the current transaction, flatten the changes from
     // both the current transtion and builder, and execute that new transaction.
 
-    if (!self.transaction && ![self.transactionBuilder hasChanges]) {
+    if (!self.transaction 
+        && ![self.transactionBuilder hasChanges]
+        && !IGListExperimentEnabled(self.experiments, IGListExperimentRemoveDataSourceChangeEarlyExit)) {
         // If nothing is going on, lets take a shortcut.
         block();
         return;
diff --git a/Source/IGListKit/Internal/IGListUpdateTransactionBuilder.m b/Source/IGListKit/Internal/IGListUpdateTransactionBuilder.m
index f11289b4e..166c29c27 100644
--- a/Source/IGListKit/Internal/IGListUpdateTransactionBuilder.m
+++ b/Source/IGListKit/Internal/IGListUpdateTransactionBuilder.m
@@ -141,13 +141,12 @@ - (void)addChangesFromBuilder:(IGListUpdateTransactionBuilder *)builder {
 - (nullable id<IGListUpdateTransactable>)buildWithConfig:(IGListUpdateTransactationConfig)config
                                                 delegate:(nullable id<IGListAdapterUpdaterDelegate>)delegate
                                                  updater:(IGListAdapterUpdater *)updater {
-    IGListCollectionViewBlock collectionViewBlock = self.collectionViewBlock;
-    if (!collectionViewBlock) {
-        return nil;
-    }
-
     switch (self.mode) {
-        case IGListUpdateTransactionBuilderModeBatchUpdate:
+        case IGListUpdateTransactionBuilderModeBatchUpdate: {
+            IGListCollectionViewBlock collectionViewBlock = self.collectionViewBlock;
+            if (!collectionViewBlock) {
+                return nil;
+            }
             return [[IGListBatchUpdateTransaction alloc] initWithCollectionViewBlock:collectionViewBlock
                                                                              updater:updater
                                                                             delegate:delegate
@@ -157,9 +156,11 @@ - (void)addChangesFromBuilder:(IGListUpdateTransactionBuilder *)builder {
                                                                applySectionDataBlock:self.applySectionDataBlock
                                                                     itemUpdateBlocks:self.itemUpdateBlocks
                                                                     completionBlocks:self.completionBlocks];
+        }
         case IGListUpdateTransactionBuilderModeReload: {
             IGListReloadUpdateBlock reloadBlock = self.reloadBlock;
-            if (!reloadBlock) {
+            IGListCollectionViewBlock collectionViewBlock = self.collectionViewBlock;
+            if (!reloadBlock || !collectionViewBlock) {
                 return nil;
             }
             return [[IGListReloadTransaction alloc] initWithCollectionViewBlock:collectionViewBlock