Handle hardlinks with file-based app csc optimization (#51135)

Fixes a bug which I discovered while using file-based apps in roslyn repo with hardlinks in build opted-in (via `ROSLYNUSEHARDLINKS` env var).

The `File.Copy` resulted in "Access denied" exception. I was puzzled at first, but after some investigation discovered that's because the MSBuild task hardlinks `obj/app.dll` with `bin/app.dll` and when we then try to `File.Copy("obj/app.dll", "bin/app.dll")` it fails with that exception because it tries to write to a file which it has locked for reading (because both files are actually the same file via the hardlink).

MSBuild's Copy task doesn't have this problem because it by default checks the files have the same size and timestamp and if so, the copy is skipped. I have implemented the same behavior to fix the issue.

Author: jjonescz

src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs

@@ -110,9 +110,18 @@ public int Execute(out bool fallbackToNormalBuild)
         if (BuildResultFile != null &&
             CSharpCommandLineParser.Default.Parse(CscArguments, BaseDirectory, sdkDirectory: null) is { OutputFileName: { } outputFileName } parsedArgs)
         {
-            var objFile = parsedArgs.GetOutputFilePath(outputFileName);
-            Reporter.Verbose.WriteLine($"Copying '{objFile}' to '{BuildResultFile}'.");
-            File.Copy(objFile, BuildResultFile, overwrite: true);
+            var objFile = new FileInfo(parsedArgs.GetOutputFilePath(outputFileName));
+            var binFile = new FileInfo(BuildResultFile);
+
+            if (HaveMatchingSizeAndTimeStamp(objFile, binFile))
+            {
+                Reporter.Verbose.WriteLine($"Skipping copy of '{objFile}' to '{BuildResultFile}' because the files have matching size and timestamp.");
+            }
+            else
+            {
+                Reporter.Verbose.WriteLine($"Copying '{objFile}' to '{BuildResultFile}'.");
+                File.Copy(objFile.FullName, binFile.FullName, overwrite: true);
+            }
         }
 
         return exitCode;
@@ -153,6 +162,27 @@ static int ProcessBuildResponse(BuildResponse response, out bool fallbackToNorma
                     return 1;
             }
         }
+
+        // Inspired by MSBuild: https://github.com/dotnet/msbuild/blob/a7a4d5af02be5aa6dc93a492d6d03056dc811388/src/Tasks/Copy.cs#L208
+        static bool HaveMatchingSizeAndTimeStamp(FileInfo sourceFile, FileInfo destinationFile)
+        {
+            if (!destinationFile.Exists)
+            {
+                return false;
+            }
+
+            if (sourceFile.LastWriteTimeUtc != destinationFile.LastWriteTimeUtc)
+            {
+                return false;
+            }
+
+            if (sourceFile.Length != destinationFile.Length)
+            {
+                return false;
+            }
+
+            return true;
+        }
     }
 
     private void PrepareAuxiliaryFiles(out string rspPath)

test/dotnet.Tests/CommandTests/Run/RunFileTests.cs

@@ -3258,6 +3258,37 @@ Release config
             """);
     }
 
+    /// <summary>
+    /// See <see cref="CscOnly_AfterMSBuild"/>.
+    /// If hard links are enabled, the <c>bin/app.dll</c> and <c>obj/app.dll</c> files are going to be the same,
+    /// so our "copy obj to bin" logic must account for that.
+    /// </summary>
+    [Fact]
+    public void CscOnly_AfterMSBuild_HardLinks()
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory(baseDirectory: OutOfTreeBaseDirectory);
+        var programPath = Path.Join(testInstance.Path, "Program.cs");
+
+        var code = $"""
+            #:property CreateHardLinksForCopyFilesToOutputDirectoryIfPossible=true
+            #:property CreateSymbolicLinksForCopyFilesToOutputDirectoryIfPossible=true
+            {s_program}
+            """;
+
+        File.WriteAllText(programPath, code);
+
+        // Remove artifacts from possible previous runs of this test.
+        var artifactsDir = VirtualProjectBuildingCommand.GetArtifactsPath(programPath);
+        if (Directory.Exists(artifactsDir)) Directory.Delete(artifactsDir, recursive: true);
+
+        Build(testInstance, BuildLevel.All);
+
+        code = code.Replace("Hello", "Hi");
+        File.WriteAllText(programPath, code);
+
+        Build(testInstance, BuildLevel.Csc, expectedOutput: "Hi from Program");
+    }
+
     /// <summary>
     /// See <see cref="CscOnly_AfterMSBuild"/>.
     /// This optimization currently does not support <c>#:project</c> references and hence is disabled if those are present.