Add more edge case testing

osu.Game.Tests/NonVisual/CustomDataDirectoryTest.cs

@@ -123,6 +123,56 @@ namespace osu.Game.Tests.NonVisual
             }
         }
 
+        [Test]
+        public void TestMigrationBetweenTwoTargets()
+        {
+            using (HeadlessGameHost host = new CleanRunHeadlessGameHost(nameof(TestMigrationBetweenTwoTargets)))
+            {
+                try
+                {
+                    var osu = loadOsu(host);
+                    var storage = osu.Dependencies.Get<Storage>();
+
+                    string customPath2 = $"{customPath}-2";
+
+                    const string database_filename = "client.db";
+
+                    Assert.DoesNotThrow(() => (storage as OsuStorage)?.Migrate(customPath));
+                    Assert.That(File.Exists(Path.Combine(customPath, database_filename)));
+
+                    Assert.DoesNotThrow(() => (storage as OsuStorage)?.Migrate(customPath2));
+                    Assert.That(File.Exists(Path.Combine(customPath2, database_filename)));
+
+                    Assert.DoesNotThrow(() => (storage as OsuStorage)?.Migrate(customPath));
+                    Assert.That(File.Exists(Path.Combine(customPath, database_filename)));
+                }
+                finally
+                {
+                    host.Exit();
+                }
+            }
+        }
+
+        [Test]
+        public void TestMigrationToSameTargetFails()
+        {
+            using (HeadlessGameHost host = new CleanRunHeadlessGameHost(nameof(TestMigrationToSameTargetFails)))
+            {
+                try
+                {
+                    var osu = loadOsu(host);
+                    var storage = osu.Dependencies.Get<Storage>();
+
+                    Assert.DoesNotThrow(() => (storage as OsuStorage)?.Migrate(customPath));
+                    Assert.Throws<InvalidOperationException>(() => (storage as OsuStorage)?.Migrate(customPath));
+                }
+                finally
+                {
+                    host.Exit();
+                }
+            }
+        }
+
         private OsuGameBase loadOsu(GameHost host)
         {
             var osu = new OsuGameBase();