[wasm] EmccCompile: Fix incremental build, in case of only partial

.. rebuilds. Specifically, when a rebuild causes only the *linking* part
to be run again. In that case, we were correctly skipping over compiling
native files, but didn't add them to `@(FileWrites)`, which caused
msbuild's incremental clean logic to treat them as "orphaned" files,
and delete them!

src/mono/wasm/build/WasmApp.Native.targets

@@ -298,7 +298,8 @@
           Inputs="@(_BitcodeFile);$(_EmccDefaultFlagsRsp);$(_EmccCompileBitcodeRsp)"
           Outputs="@(_BitcodeFile->'%(ObjectFile)')"
           Condition="'$(_WasmShouldAOT)' == 'true' and @(_BitcodeFile->Count()) > 0"
-          DependsOnTargets="_WasmWriteRspForCompilingBitcode">
+          DependsOnTargets="_WasmWriteRspForCompilingBitcode"
+          Returns="@(FileWrites)">
 
     <ItemGroup>
       <_BitCodeFile Dependencies="%(_BitCodeFile.Dependencies);$(_EmccDefaultFlagsRsp);$(_EmccCompileBitcodeRsp)" />
@@ -364,7 +365,8 @@
   <Target Name="_WasmLinkDotNet"
           Inputs="@(_WasmLinkDependencies);$(_EmccDefaultFlagsRsp);$(_EmccLinkRsp)"
           Outputs="$(_WasmIntermediateOutputPath)dotnet.js;$(_WasmIntermediateOutputPath)dotnet.wasm"
-          DependsOnTargets="_WasmSelectRuntimeComponentsForLinking;_WasmCompileAssemblyBitCodeFilesForAOT;_WasmWriteRspFilesForLinking">
+          DependsOnTargets="_WasmSelectRuntimeComponentsForLinking;_WasmCompileAssemblyBitCodeFilesForAOT;_WasmWriteRspFilesForLinking"
+          Returns="@(FileWrites)" >
 
     <Message Text="Linking with emcc. This may take a while ..." Importance="High" />
     <Message Text="Running emcc with @(_EmccLinkStepArgs->'%(Identity)', ' ')" Importance="Low" />

src/tasks/WasmAppBuilder/EmccCompile.cs

@@ -95,6 +95,7 @@ private bool ExecuteActual()
                     if (!ShouldCompile(srcFile, objFile, depFiles, out string reason))
                     {
                         Log.LogMessage(MessageImportance.Low, $"Skipping {srcFile} because {reason}.");
+                        outputItems.Add(CreateOutputItemFor(srcFile, objFile));
                     }
                     else
                     {
@@ -107,7 +108,8 @@ private bool ExecuteActual()
                 if (_numCompiled == _totalFiles)
                 {
                     // nothing to do!
-                    return true;
+                    OutputFiles = outputItems.ToArray();
+                    return !Log.HasLoggedErrors;
                 }
 
                 if (_numCompiled > 0)
@@ -200,9 +202,7 @@ bool ProcessSourceFile(string srcFile, string objFile)
                     else
                         Log.LogMessage(MessageImportance.Low, $"Copied {tmpObjFile} to {objFile}");
 
-                    ITaskItem newItem = new TaskItem(objFile);
-                    newItem.SetMetadata("SourceFile", srcFile);
-                    outputItems.Add(newItem);
+                    outputItems.Add(CreateOutputItemFor(srcFile, objFile));
 
                     int count = Interlocked.Increment(ref _numCompiled);
                     Log.LogMessage(MessageImportance.High, $"[{count}/{_totalFiles}] {Path.GetFileName(srcFile)} -> {Path.GetFileName(objFile)} [took {elapsedSecs:F}s]");
@@ -219,6 +219,13 @@ bool ProcessSourceFile(string srcFile, string objFile)
                     File.Delete(tmpObjFile);
                 }
             }
+
+            ITaskItem CreateOutputItemFor(string srcFile, string objFile)
+            {
+                ITaskItem newItem = new TaskItem(objFile);
+                newItem.SetMetadata("SourceFile", srcFile);
+                return newItem;
+            }
         }
 
         private bool ShouldCompile(string srcFile, string objFile, string[] depFiles, out string reason)

src/tests/BuildWasmApps/Wasm.Build.Tests/BlazorWasmBuildPublishTests.cs

@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System;
 using System.IO;
 using System.Linq;
 using Xunit;

src/tests/BuildWasmApps/Wasm.Build.Tests/NativeRebuildTests/NativeRebuildTestsBase.cs

@@ -77,6 +77,7 @@ protected string Rebuild(bool nativeRelink, bool invariant, BuildArgs buildArgs,
             buildArgs = newBuildArgs;
 
             _testOutput.WriteLine($"{Environment.NewLine}Rebuilding with no changes ..{Environment.NewLine}");
+            Console.WriteLine($"{Environment.NewLine}Rebuilding with no changes ..{Environment.NewLine}");
             (_, string output) = BuildProject(buildArgs,
                                             id: id,
                                             dotnetWasmFromRuntimePack: false,
@@ -137,6 +138,18 @@ internal void CompareStat(IDictionary<string, FileStat> oldStat, IDictionary<str
                 throw new XunitException($"CompareStat failed:{Environment.NewLine}{msg}");
         }
 
+        internal IDictionary<string, (string fullPath, bool unchanged)> GetFilesTable(bool unchanged, params string[] baseDirs)
+        {
+            var dict = new Dictionary<string, (string fullPath, bool unchanged)>();
+            foreach (var baseDir in baseDirs)
+            {
+                foreach (var file in Directory.EnumerateFiles(baseDir, "*", new EnumerationOptions { RecurseSubdirectories = true }))
+                    dict[Path.GetFileName(file)] = (file, unchanged);
+            }
+
+            return dict;
+        }
+
         internal IDictionary<string, (string fullPath, bool unchanged)> GetFilesTable(BuildArgs buildArgs, BuildPaths paths, bool unchanged)
         {
             List<string> files = new()

src/tests/BuildWasmApps/Wasm.Build.Tests/NativeRebuildTests/NoopNativeRebuildTest.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.IO;
 using System.Linq;
 using Wasm.Build.Tests;
 using Xunit;
@@ -33,5 +34,60 @@ public void NoOpRebuildForNativeBuilds(BuildArgs buildArgs, bool nativeRelink, b
             CompareStat(originalStat, newStat, pathsDict.Values);
             RunAndTestWasmApp(buildArgs, buildDir: _projectDir, expectedExitCode: 42, host: host, id: id);
         }
+
+        [ConditionalTheory(typeof(BuildTestBase), nameof(IsUsingWorkloads))]
+        [InlineData("Debug")]
+        [InlineData("Release")]
+        public void BlazorNoopRebuild(string config)
+        {
+            string id = $"blz_rebuild_{config}";
+            string projectFile = CreateBlazorWasmTemplateProject(id);
+            AddItemsPropertiesToProject(projectFile, extraProperties: "<WasmBuildNative>true</WasmBuildNative>");
+
+            string objDir = Path.Combine(_projectDir!, "obj", config, "net6.0", "wasm");
+
+            BlazorBuild(id, config, NativeFilesType.Relinked);
+            File.Move(Path.Combine(s_buildEnv.LogRootPath, id, $"{id}-build.binlog"),
+                        Path.Combine(s_buildEnv.LogRootPath, id, $"{id}-build-first.binlog"));
+
+            var pathsDict = GetFilesTable(true, objDir);
+            pathsDict.Remove("runtime-icall-table.h");
+            var originalStat = StatFiles(pathsDict.Select(kvp => kvp.Value.fullPath));
+
+            // build again
+            BlazorBuild(id, config, NativeFilesType.Relinked);
+            var newStat = StatFiles(pathsDict.Select(kvp => kvp.Value.fullPath));
+
+            CompareStat(originalStat, newStat, pathsDict.Values);
+        }
+
+
+        [ConditionalTheory(typeof(BuildTestBase), nameof(IsUsingWorkloads))]
+        [InlineData("Debug")]
+        [InlineData("Release")]
+        public void BlazorOnlyLinkRebuild(string config)
+        {
+            string id = $"blz_relink_{config}";
+            string projectFile = CreateBlazorWasmTemplateProject(id);
+            AddItemsPropertiesToProject(projectFile, extraProperties: "<WasmBuildNative>true</WasmBuildNative>");
+
+            string objDir = Path.Combine(_projectDir!, "obj", config, "net6.0", "wasm");
+
+            BlazorBuild(id, config, NativeFilesType.Relinked);
+            File.Move(Path.Combine(s_buildEnv.LogRootPath, id, $"{id}-build.binlog"),
+                        Path.Combine(s_buildEnv.LogRootPath, id, $"{id}-build-first.binlog"));
+
+            var pathsDict = GetFilesTable(true, objDir);
+            pathsDict.Remove("runtime-icall-table.h");
+            pathsDict.UpdateTo(unchanged: false, "dotnet.wasm", "dotnet.js", "emcc-link.rsp");
+
+            var originalStat = StatFiles(pathsDict.Select(kvp => kvp.Value.fullPath));
+
+            // build again
+            BlazorBuild(id, config, NativeFilesType.Relinked, "-p:EmccLinkOptimizationFlag=-O1");
+            var newStat = StatFiles(pathsDict.Select(kvp => kvp.Value.fullPath));
+
+            CompareStat(originalStat, newStat, pathsDict.Values);
+        }
     }
 }