assert monotonic order for migration

SignalMessaging/environment/migrations/OWS110SortIdMigration.swift

@@ -46,8 +46,27 @@ class OWS110SortIdMigration: OWSDatabaseMigration {
 
             let totalCount: UInt = legacySorting.numberOfItemsInAllGroups()
             var completedCount: UInt = 0
+
+            var seenGroups: Set<String> = Set()
             legacySorting.enumerateGroups { group, _ in
                 autoreleasepool {
+                    // Sanity Check #1
+                    // Make sure our enumeration is monotonically increasing.
+                    // Note: sortIds increase monotonically WRT timestampForLegacySorting, but only WRT the interaction's thread.
+                    //
+                    // e.g. When we migrate the **next** thread, we start with that thread's oldest interaction. So it's possible and expected
+                    // that thread2's oldest interaction will have a smaller timestampForLegacySorting, but a greater sortId then an interaction
+                    // in thread1. That's OK because we only sort messages with respect to the thread they belong in. We don't have any sort of
+                    // "global sort" of messages across all threads.
+                    var previousTimestampForLegacySorting: UInt64 = 0
+
+                    // Sanity Check #2
+                    // Ensure we only process a DB View's group (i.e. threadId) once.
+                    guard !seenGroups.contains(group) else {
+                        owsFail("unexpectedly seeing a repeated group: \(group)")
+                    }
+                    seenGroups.insert(group)
+
                     legacySorting.enumerateKeysAndObjects(inGroup: group) { (_, _, object, _, _) in
                         autoreleasepool {
                             guard let interaction = object as? TSInteraction else {
@@ -55,10 +74,14 @@ class OWS110SortIdMigration: OWSDatabaseMigration {
                                 return
                             }
 
+                            guard interaction.timestampForLegacySorting() >= previousTimestampForLegacySorting else {
+                                owsFail("unexpected object ordering previousTimestampForLegacySorting: \(previousTimestampForLegacySorting) interaction.timestampForLegacySorting: \(interaction.timestampForLegacySorting)")
+                            }
+                            previousTimestampForLegacySorting = interaction.timestampForLegacySorting()
+
                             interaction.saveNextSortId(transaction: transaction)
 
                             completedCount += 1
-
                             if completedCount % 100 == 0 {
                                 // Legit usage of legacy sorting for migration to new sorting
                                 Logger.info("thread: \(interaction.uniqueThreadId), timestampForLegacySorting:\(interaction.timestampForLegacySorting()), sortId: \(interaction.sortId) totalCount: \(totalCount), completedcount: \(completedCount)")