Fix missing LastWriteTime update in CoW SDK causing excess recopying in incremental builds

Directory.Packages.props

@@ -8,10 +8,12 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="CopyOnWrite" Version="0.3.7" Condition=" '$(TargetFramework)' != 'net46' " />
+    <PackageVersion Include="Microsoft.Build.Framework" Version="$(MicrosoftBuildPackageVersion)" />
     <PackageVersion Include="Microsoft.Build.Tasks.Core" Version="$(MicrosoftBuildPackageVersion)" />
     <PackageVersion Include="Microsoft.Win32.Registry" Version="5.0.0" />
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageVersion Include="System.CodeDom" Version="7.0.0" />
+    <PackageVersion Include="System.Collections.Immutable" Version="7.0.0" />
     <PackageVersion Include="System.Threading.Tasks.Dataflow" Version="7.0.0" Condition=" '$(TargetFramework)' != 'net46' " />
     <PackageVersion Include="System.Threading.Tasks.Dataflow" Version="4.11.1" Condition=" '$(TargetFramework)' == 'net46' " />
   </ItemGroup>

src/CopyOnWrite.UnitTests/CopyUpToDateTests.cs

@@ -0,0 +1,131 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+//
+// Licensed under the MIT license.
+
+using Microsoft.Build.Framework;
+using Microsoft.Build.Tasks;
+using Microsoft.Build.UnitTests.Common;
+using Microsoft.Build.Utilities;
+using System;
+using System.IO;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Microsoft.Build.CopyOnWrite.UnitTests;
+
+// These tests rely on Microsoft.Build.Framework which has only a net472 and current-framework target.
+// Don't compile these tests for .NET versions in between as Microsoft.Build.Framework.dll will not be
+// propagated to the output dir.
+#if !NET6_0 && !NET7_0
+
+public class CopyUpToDateTests : MSBuildSdkTestBase
+{
+    // If the developer has specified this environment variable (also used in the base CoW library unit tests),
+    // also run tests under ReFS locally even if the current drive volume is NTFS.
+    private static readonly string? ReFsTestDriveRoot = Environment.GetEnvironmentVariable("CoW_Test_ReFS_Drive");
+
+    private readonly ITestOutputHelper _output;
+
+    public CopyUpToDateTests(ITestOutputHelper output)
+    {
+        _output = output;
+    }
+
+    [Theory]
+    [InlineData(false, false)]
+    [InlineData(false, true)]
+    [InlineData(true, false)]
+    [InlineData(true, true)]
+    public void CopyFileMultipleTimes(bool skipUnchangedFiles, bool runUnderReFsIfAvailable)
+    {
+        if (runUnderReFsIfAvailable && ReFsTestDriveRoot is null)
+        {
+            return;
+        }
+
+        DisposableTempDirectory tempDir;
+        if (runUnderReFsIfAvailable)
+        {
+            _output.WriteLine($"Running ReFS test under {ReFsTestDriveRoot}");
+            tempDir = new DisposableTempDirectory(basePath: Path.Combine(ReFsTestDriveRoot!, "CoWTests"));
+        }
+        else
+        {
+            tempDir = new DisposableTempDirectory();
+        }
+
+        using (tempDir)
+        {
+            string file1Path = Path.Combine(tempDir.Path, "file1.txt");
+            File.WriteAllBytes(file1Path, new byte[] { 0x00, 0x01, 0x02 });
+            var originalFileInfo = new FileInfo(file1Path);
+            string destDir = Path.Combine(tempDir.Path, "dest");
+
+            var engine = new MockEngine(logToConsole: true);
+
+            Copy copy = CreateFreshCopyTask();
+
+            copy.Execute();
+            engine.MockLogger.AssertNoErrors();
+            engine.MockLogger.AssertNoWarnings();
+            var file1Info = new FileInfo(file1Path);
+            AssertExistenceAndEqualTimes();
+
+            // Update the original file with different contents at the same size, and reset its last write time to the original.
+            // This allows determining a rewritten file for skipUnchangedFiles=false.
+            File.WriteAllBytes(file1Path, new byte[] { 0x04, 0x05, 0x06 });
+            File.SetLastWriteTimeUtc(file1Path, originalFileInfo.LastWriteTimeUtc);
+
+            string destFilePath = Path.Combine(destDir, "file1.txt");
+
+            for (int i = 0; i < 2; i++)
+            {
+                copy = CreateFreshCopyTask();
+                copy.Execute();
+                engine.MockLogger.AssertNoErrors();
+                engine.MockLogger.AssertNoWarnings();
+
+                file1Info = new FileInfo(file1Path);
+                AssertExistenceAndEqualTimes();
+
+                byte[] contents = File.ReadAllBytes(destFilePath);
+                Assert.Equal(3, contents.Length);
+                if (skipUnchangedFiles)
+                {
+                    Assert.Equal(0x00, contents[0]);
+                    Assert.Equal(0x01, contents[1]);
+                    Assert.Equal(0x02, contents[2]);
+                }
+                else
+                {
+                    Assert.Equal(0x04, contents[0]);
+                    Assert.Equal(0x05, contents[1]);
+                    Assert.Equal(0x06, contents[2]);
+                }
+            }
+
+            void AssertExistenceAndEqualTimes()
+            {
+                Assert.True(file1Info.Exists);
+
+                // Copy or clone should uphold file time propagation semantics to ensure up-to-date checks work in the future.
+                Assert.Equal(originalFileInfo.CreationTimeUtc, file1Info.CreationTimeUtc);
+                Assert.Equal(originalFileInfo.LastWriteTimeUtc, file1Info.LastWriteTimeUtc);
+            }
+
+            Copy CreateFreshCopyTask()
+            {
+                return new Copy
+                {
+                    BuildEngine = engine,
+                    DestinationFolder = new TaskItem(destDir),
+                    RetryDelayMilliseconds = 1,
+                    SourceFiles = new ITaskItem[] { new TaskItem(file1Path) },
+                    SkipUnchangedFiles = skipUnchangedFiles,
+                };
+            }
+        }
+    }
+}
+
+#endif

src/CopyOnWrite.UnitTests/Microsoft.Build.CopyOnWrite.UnitTests.csproj

@@ -5,12 +5,14 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="CopyOnWrite" />
-    <PackageReference Include="Microsoft.Build.Tasks.Core" />
+    <PackageReference Include="Microsoft.Build.Framework" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="Microsoft.Win32.Registry" />
     <PackageReference Include="MSBuild.ProjectCreation" />
     <PackageReference Include="Newtonsoft.Json" />
+    <PackageReference Include="Shouldly" />
     <PackageReference Include="System.CodeDom" />
+    <PackageReference Include="System.Collections.Immutable" />
     <PackageReference Include="xunit" />
     <PackageReference Include="xunit.runner.visualstudio" />
   </ItemGroup>