Handle local changes correctly when receiving snapshot

Sources/Document/Change/ChangePack.swift

@@ -100,6 +100,13 @@ struct ChangePack {
         return self.snapshot
     }
 
+    /**
+     * `hasSnapshot` returns the whether this pack has snapshot or not.
+     */
+    func hasSnapshot() -> Bool {
+        self.snapshot != nil
+    }
+
     /**
      * `getMinSyncedTicket` returns the minimum synced ticket of this pack.
      */

Sources/Document/Document.swift

@@ -280,6 +280,14 @@ public actor Document {
         // 02. Update the checkpoint.
         self.checkpoint.forward(other: pack.getCheckpoint())
 
+        // NOTE(hackerwins): If the document has local changes, we need to apply
+        // them after applying the snapshot. We need to treat the local changes
+        // as remote changes because the application should apply the local
+        // changes to their own document.
+        if pack.hasSnapshot() {
+            try self.applyChanges(self.localChanges)
+        }
+
         // 03. Do Garbage collection.
         if let ticket = pack.getMinSyncedTicket() {
             self.garbageCollect(ticket)

Tests/Integration/ClientIntegrationTests.swift

@@ -778,4 +778,39 @@ final class ClientIntegrationTests: XCTestCase {
             try await c2.deactivate()
         }
     }
+
+    func test_should_handle_local_changes_correctly_when_receiving_snapshot() async throws {
+        try await withTwoClientsAndDocuments(self.description, detachDocuments: false) { c1, d1, c2, d2 in
+            try await d1.update { root, _ in
+                root.counter = JSONCounter(value: Int64(0))
+            }
+
+            try await c1.sync()
+            try await c2.sync()
+
+            // 01. c1 increases the counter for creating snapshot.
+            for _ in 0 ..< 500 {
+                try await d1.update { root, _ in
+                    (root.counter as? JSONCounter<Int64>)?.increase(value: 1)
+                }
+            }
+
+            try await c1.sync()
+
+            // 02. c2 receives the snapshot and increases the counter simultaneously.
+            Task {
+                try await c2.sync()
+            }
+            try await d2.update { root, _ in
+                (root.counter as? JSONCounter<Int64>)?.increase(value: 1)
+            }
+
+            try await c2.sync()
+            try await c1.sync()
+
+            let d1JSON = await d1.toSortedJSON()
+            let d2JSON = await d2.toSortedJSON()
+            XCTAssertEqual(d1JSON, d2JSON)
+        }
+    }
 }