From c0097ba75290b61b5d35978ef0a2333bf97d9f93 Mon Sep 17 00:00:00 2001
From: Shadowfacts <me@shadowfacts.net>
Date: Sun, 15 Aug 2021 18:44:23 -0400
Subject: [PATCH] Fix potential race condition with
 DiffableTimelineLikeTableViewController

---
 .../TimelineTableViewController.swift         | 14 ++++----
 ...fableTimelineLikeTableViewController.swift | 32 +++++++++++--------
 2 files changed, 25 insertions(+), 21 deletions(-)

diff --git a/Tusker/Screens/Timeline/TimelineTableViewController.swift b/Tusker/Screens/Timeline/TimelineTableViewController.swift
index 254bd8459d..b5423d0b7c 100644
--- a/Tusker/Screens/Timeline/TimelineTableViewController.swift
+++ b/Tusker/Screens/Timeline/TimelineTableViewController.swift
@@ -168,7 +168,7 @@ class TimelineTableViewController: DiffableTimelineLikeTableViewController<Timel
         }
     }
     
-    override func loadOlderItems(currentSnapshot: Snapshot, completion: @escaping (LoadResult) -> Void) {
+    override func loadOlderItems(currentSnapshot: @escaping () -> Snapshot, completion: @escaping (LoadResult) -> Void) {
         guard let older = older else {
             completion(.failure(.noOlder))
             return
@@ -176,12 +176,12 @@ class TimelineTableViewController: DiffableTimelineLikeTableViewController<Timel
         
         if #available(iOS 15.0, *),
            Preferences.shared.disableInfiniteScrolling && !didConfirmLoadMore {
-            guard !currentSnapshot.itemIdentifiers(inSection: .footer).contains(.confirmLoadMore) else {
+            var snapshot = currentSnapshot()
+            guard !snapshot.itemIdentifiers(inSection: .footer).contains(.confirmLoadMore) else {
                 // todo: need something more accurate than "success"/"failure"
-                completion(.success(currentSnapshot))
+                completion(.success(snapshot))
                 return
             }
-            var snapshot = currentSnapshot
             snapshot.appendItems([.confirmLoadMore], toSection: .footer)
             self.dataSource.apply(snapshot)
             completion(.success(snapshot))
@@ -199,7 +199,7 @@ class TimelineTableViewController: DiffableTimelineLikeTableViewController<Timel
                 self.older = pagination?.older
                 
                 self.mastodonController.persistentContainer.addAll(statuses: statuses) {
-                    var snapshot = currentSnapshot
+                    var snapshot = currentSnapshot()
                     snapshot.appendItems(statuses.map { .status(id: $0.id, state: .unknown) }, toSection: .statuses)
                     snapshot.deleteItems([.confirmLoadMore])
                     completion(.success(snapshot))
@@ -208,7 +208,7 @@ class TimelineTableViewController: DiffableTimelineLikeTableViewController<Timel
         }
     }
     
-    override func loadNewerItems(currentSnapshot: Snapshot, completion: @escaping (LoadResult) -> Void) {
+    override func loadNewerItems(currentSnapshot: @escaping () -> Snapshot, completion: @escaping (LoadResult) -> Void) {
         guard let newer = newer else {
             completion(.failure(.noNewer))
             return
@@ -228,7 +228,7 @@ class TimelineTableViewController: DiffableTimelineLikeTableViewController<Timel
                 }
                 
                 self.mastodonController.persistentContainer.addAll(statuses: statuses) {
-                    var snapshot = currentSnapshot
+                    var snapshot = currentSnapshot()
                     let newIdentifiers = statuses.map { Item.status(id: $0.id, state: .unknown) }
                     if let first = snapshot.itemIdentifiers(inSection: .statuses).first {
                         snapshot.insertItems(newIdentifiers, beforeItem: first)
diff --git a/Tusker/Screens/Utilities/DiffableTimelineLikeTableViewController.swift b/Tusker/Screens/Utilities/DiffableTimelineLikeTableViewController.swift
index 2a0eca1b53..050b509e74 100644
--- a/Tusker/Screens/Utilities/DiffableTimelineLikeTableViewController.swift
+++ b/Tusker/Screens/Utilities/DiffableTimelineLikeTableViewController.swift
@@ -146,7 +146,7 @@ class DiffableTimelineLikeTableViewController<Section: Hashable & CaseIterable,
         
         state = .loadingOlder
         
-        loadOlderItems(currentSnapshot: dataSource.snapshot()) { result in
+        loadOlderItems(currentSnapshot: dataSource.snapshot) { result in
             DispatchQueue.main.async {
                 self.state = .loaded
                 
@@ -212,18 +212,22 @@ class DiffableTimelineLikeTableViewController<Section: Hashable & CaseIterable,
         
         state = .loadingNewer
         
-        let snapshot = dataSource.snapshot()
-        
-        var item: Item? = nil
-        for section in timelineContentSections() {
-            if snapshot.indexOfSection(section) != nil,
-               let first = snapshot.itemIdentifiers(inSection: section).first {
-                item = first
-                break
+        var firstItem: Item? = nil
+        let currentSnapshot: () -> Snapshot = {
+            let snapshot = self.dataSource.snapshot()
+            
+            for section in self.timelineContentSections() {
+                if snapshot.indexOfSection(section) != nil,
+                   let first = snapshot.itemIdentifiers(inSection: section).first {
+                    firstItem = first
+                    break
+                }
             }
+            
+            return snapshot
         }
         
-        loadNewerItems(currentSnapshot: snapshot) { result in
+        loadNewerItems(currentSnapshot: currentSnapshot) { result in
             DispatchQueue.main.async {
                 self.refreshControl?.endRefreshing()
                 self.state = .loaded
@@ -231,8 +235,8 @@ class DiffableTimelineLikeTableViewController<Section: Hashable & CaseIterable,
                 switch result {
                 case let .success(snapshot):
                     self.dataSource.apply(snapshot, animatingDifferences: false)
-                    if let item = item,
-                       let indexPath = self.dataSource.indexPath(for: item) {
+                    if let firstItem = firstItem,
+                       let indexPath = self.dataSource.indexPath(for: firstItem) {
                         // maintain the current position in the list (don't scroll to top)
                         self.tableView.scrollToRow(at: indexPath, at: .top, animated: false)
                     }
@@ -265,11 +269,11 @@ class DiffableTimelineLikeTableViewController<Section: Hashable & CaseIterable,
         fatalError("loadInitialItems(completion:) must be implemented by subclasses")
     }
     
-    func loadOlderItems(currentSnapshot: Snapshot, completion: @escaping (LoadResult) -> Void) {
+    func loadOlderItems(currentSnapshot: @escaping () -> Snapshot, completion: @escaping (LoadResult) -> Void) {
         fatalError("loadOlderItesm(completion:) must be implemented by subclasses")
     }
     
-    func loadNewerItems(currentSnapshot: Snapshot, completion: @escaping (LoadResult) -> Void) {
+    func loadNewerItems(currentSnapshot: @escaping () -> Snapshot, completion: @escaping (LoadResult) -> Void) {
         fatalError("loadNewerItems(completion:) must be implemented by subclasses")
     }