From d0a4e1e3c2ff107327091d7ece191742f45e1429 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Fri, 20 Sep 2019 15:00:27 +0900
Subject: [PATCH 1/2] Force a checksum check before skipping FileStore copy op

---
 osu.Game/IO/FileStore.cs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/osu.Game/IO/FileStore.cs b/osu.Game/IO/FileStore.cs
index 370d6786f5..bf4e881ed0 100644
--- a/osu.Game/IO/FileStore.cs
+++ b/osu.Game/IO/FileStore.cs
@@ -50,7 +50,16 @@ namespace osu.Game.IO
                 string path = info.StoragePath;
 
                 // we may be re-adding a file to fix missing store entries.
-                if (!Storage.Exists(path))
+                bool requiresCopy = !Storage.Exists(path);
+
+                if (!requiresCopy)
+                {
+                    // even if the file already exists, check the existing checksum for safety.
+                    using (var stream = Storage.GetStream(path))
+                        requiresCopy |= stream.ComputeSHA2Hash() != hash;
+                }
+
+                if (requiresCopy)
                 {
                     data.Seek(0, SeekOrigin.Begin);
 

From f306fe27d82d790ec6df2764a04b6e7ab1903593 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Fri, 20 Sep 2019 15:05:48 +0900
Subject: [PATCH 2/2] Add test to cover corruption case

---
 .../Beatmaps/IO/ImportBeatmapTest.cs          | 42 +++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/osu.Game.Tests/Beatmaps/IO/ImportBeatmapTest.cs b/osu.Game.Tests/Beatmaps/IO/ImportBeatmapTest.cs
index 8b39946ab0..385ab4064d 100644
--- a/osu.Game.Tests/Beatmaps/IO/ImportBeatmapTest.cs
+++ b/osu.Game.Tests/Beatmaps/IO/ImportBeatmapTest.cs
@@ -90,6 +90,48 @@ namespace osu.Game.Tests.Beatmaps.IO
             }
         }
 
+        [Test]
+        public async Task TestImportCorruptThenImport()
+        {
+            //unfortunately for the time being we need to reference osu.Framework.Desktop for a game host here.
+            using (HeadlessGameHost host = new CleanRunHeadlessGameHost("TestImportThenImport"))
+            {
+                try
+                {
+                    var osu = loadOsu(host);
+
+                    var imported = await LoadOszIntoOsu(osu);
+
+                    var firstFile = imported.Files.First();
+
+                    var files = osu.Dependencies.Get<FileStore>();
+
+                    long originalLength;
+                    using (var stream = files.Storage.GetStream(firstFile.FileInfo.StoragePath))
+                        originalLength = stream.Length;
+
+                    using (var stream = files.Storage.GetStream(firstFile.FileInfo.StoragePath, FileAccess.Write, FileMode.Create))
+                        stream.WriteByte(0);
+
+                    var importedSecondTime = await LoadOszIntoOsu(osu);
+
+                    using (var stream = files.Storage.GetStream(firstFile.FileInfo.StoragePath))
+                        Assert.AreEqual(stream.Length, originalLength, "Corruption was not fixed on second import");
+
+                    // check the newly "imported" beatmap is actually just the restored previous import. since it matches hash.
+                    Assert.IsTrue(imported.ID == importedSecondTime.ID);
+                    Assert.IsTrue(imported.Beatmaps.First().ID == importedSecondTime.Beatmaps.First().ID);
+
+                    checkBeatmapSetCount(osu, 1);
+                    checkSingleReferencedFileCount(osu, 18);
+                }
+                finally
+                {
+                    host.Exit();
+                }
+            }
+        }
+
         [Test]
         public async Task TestRollbackOnFailure()
         {