fix: Drop current folder if we have too many changes (#1762)

Author: valentinperignon

MailCore/Cache/MailboxManager/MailboxManager+Thread.swift

@@ -87,7 +87,14 @@ public extension MailboxManager {
         let newCursor: String
 
         if let previousCursor {
-            newCursor = try await getMessagesDelta(signature: previousCursor, folder: folder)
+            do {
+                newCursor = try await getMessagesDelta(signature: previousCursor, folder: folder)
+            } catch ErrorDomain.tooManyDiffs {
+                try await resetFolder(folder)
+
+                // fetch folder as if we had no cursor
+                newCursor = try await fetchOldMessagesUids(folder: folder)
+            }
         } else {
             newCursor = try await fetchOldMessagesUids(folder: folder)
         }
@@ -161,6 +168,9 @@ public extension MailboxManager {
                 folderId: folder.remoteId,
                 signature: signature
             )
+
+            try messagesDelta.ensureValidDelta()
+
             await handleDelta(messagesDelta: messagesDelta, folder: folder)
 
             return messagesDelta.cursor
@@ -170,6 +180,9 @@ public extension MailboxManager {
                 folderId: folder.remoteId,
                 signature: signature
             )
+
+            try messagesDelta.ensureValidDelta()
+
             await handleDelta(messagesDelta: messagesDelta, folder: folder)
 
             return messagesDelta.cursor
@@ -591,6 +604,21 @@ public extension MailboxManager {
         writableRealm.delete(orphanMessages)
     }
 
+    private func resetFolder(_ folder: Folder) async throws {
+        try writeTransaction { realm in
+            guard let liveFolder = folder.fresh(using: realm) else { return }
+
+            liveFolder.remainingOldMessagesToFetch = Constants.messageQuantityLimit
+            liveFolder.oldMessagesUidsToFetch.removeAll()
+            liveFolder.newMessagesUidsToFetch.removeAll()
+            realm.delete(liveFolder.threads)
+            realm.delete(liveFolder.messages)
+            liveFolder.lastUpdate = nil
+            liveFolder.cursor = nil
+            liveFolder.unreadCount = 0
+        }
+    }
+
     private func removeDuplicatedThreads(
         messageIds: MutableSet<String>,
         threadsToUpdate: inout Set<Thread>,

MailCore/Cache/MailboxManager/MailboxManager.swift

@@ -42,6 +42,7 @@ public final class MailboxManager: ObservableObject, MailboxManageable {
     enum ErrorDomain: Error {
         case missingFolder
         case missingDraft
+        case tooManyDiffs
     }
 
     public final class MailboxManagerConstants {

MailCore/Models/MessagesDelta.swift

@@ -35,6 +35,29 @@ public struct MessagesDelta<Flags: DeltaFlags>: Decodable, Sendable {
     }
 }
 
+public extension MessagesDelta {
+    func ensureValidDelta() throws {
+        let deletedCount = deletedShortUids.count
+        let addedCount = addedShortUids.count
+        let updatedCount = updated.count
+
+        guard deletedCount < Constants.maxChangesCount,
+              addedCount < Constants.maxChangesCount,
+              updatedCount < Constants.maxChangesCount else {
+            SentrySDK.capture(message: "tooManyDiffs") { scope in
+                scope.setExtras([
+                    "cursor": cursor,
+                    "deletedCount": deletedCount,
+                    "addedCount": addedCount,
+                    "updatedCount": updatedCount
+                ])
+            }
+
+            throw MailboxManager.ErrorDomain.tooManyDiffs
+        }
+    }
+}
+
 public protocol DeltaFlags: Decodable, Sendable {
     var shortUid: String { get }
 }

MailCore/Utils/Constants.swift

@@ -161,6 +161,7 @@ public enum Constants {
 
     public static let messageQuantityLimit = 500
     public static let numberOfOldUidsToFetch = 10000
+    public static let maxChangesCount = 10000
     public static let oldPageSize = 50
     public static let newPageSize = 200
     public static let contactSuggestionLimit = 5