Fix for schema migration involving 'NOT NULL' columns. (#365)

* Fix for schema migration involving 'NOT NULL' columns.

* wip

Author: mbrandonw

Sources/SQLiteData/CloudKit/SyncEngine.swift

@@ -2045,11 +2045,11 @@
               if data == nil {
                 reportIssue("Asset data not found on disk")
               }
-              return "\(quote: columnName) = \(data?.queryFragment ?? "NULL")"
+              return "\(quote: columnName) = \(data?.queryFragment ?? #""excluded".\#(quote: columnName)"#)"
             } else {
               return """
                 \(quote: columnName) = \
-                \(record.encryptedValues[columnName]?.queryFragment ?? "NULL")
+                \(record.encryptedValues[columnName]?.queryFragment ?? #""excluded".\#(quote: columnName)"#)
                 """
             }
           }

Tests/SQLiteDataTests/CloudKitTests/SchemaChangeTests.swift

@@ -196,6 +196,48 @@
         }
       }
 
+      /*
+       * Old schema creates record and synchronizes to iCloud.
+       * Schema is migrated to add a "NOT NULL" column.
+       * New sync engine is launched.
+       => Sync starts without emitting an error.
+       */
+      @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
+      @Test func addColumn_OldRecordsSyncToNewSchema() async throws {
+        let remindersList = RemindersList(id: 1, title: "Personal")
+        try await userDatabase.userWrite { db in
+          try db.seed {
+            remindersList
+          }
+        }
+        try await syncEngine.processPendingRecordZoneChanges(scope: .private)
+
+        syncEngine.stop()
+
+        try await userDatabase.userWrite { db in
+          try #sql(
+            """
+            ALTER TABLE "remindersLists" 
+            ADD COLUMN "position" INTEGER NOT NULL ON CONFLICT REPLACE DEFAULT 0
+            """
+          )
+          .execute(db)
+        }
+
+        // NB: Sync engine should start without emitting issue.
+        _ = try await SyncEngine(
+          container: syncEngine.container,
+          userDatabase: syncEngine.userDatabase,
+          tables: syncEngine.tables
+            .filter { $0.base != Reminder.self && $0.base != RemindersList.self }
+            + [
+              SynchronizedTable(for: ReminderWithPosition.self),
+              SynchronizedTable(for: RemindersListWithPosition.self),
+            ],
+          privateTables: syncEngine.privateTables
+        )
+      }
+
       /*
        * Test run from perspective of old device with old schema.
        * Old schema saves record in cloud database.