Trigger rebuild when the CLI changed

- Stamp each project with the CLI version it was last compiled with
- Rebuild those projects with a local version file that does not match the one of the current CLI that is building it

src/Microsoft.DotNet.Cli.Utils/DotnetFiles.cs

@@ -0,0 +1,16 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.IO;
+using System.Reflection;
+
+namespace Microsoft.DotNet.Cli.Utils
+{
+    public static class DotnetFiles
+    {
+        /// <summary>
+        /// The CLI ships with a .version file that stores the commit information and CLI version
+        /// </summary>
+        public static string VersionFile => Path.GetFullPath(Path.Combine(typeof(DotnetFiles).GetTypeInfo().Assembly.Location, "..", ".version"));
+    }
+}

src/Microsoft.DotNet.Compiler.Common/ProjectContextExtensions.cs

@@ -31,6 +31,11 @@ namespace Microsoft.DotNet.Cli.Compiler.Common
             return baseOption;
         }
 
+        public static string GetSDKVersionFile(this ProjectContext context, string configuration, string buildBasePath, string outputPath)
+        {
+            var intermediatePath = context.GetOutputPaths(configuration, buildBasePath, outputPath).IntermediateOutputDirectoryPath;
+            return Path.Combine(intermediatePath, ".SDKVersion");
+        }
 
         // used in incremental compilation for the key file
         public static CommonCompilerOptions ResolveCompilationOptions(this ProjectContext context, string configuration)

src/dotnet/Program.cs

@@ -159,8 +159,7 @@ namespace Microsoft.DotNet.Cli
         
         private static string GetCommitSha()
         {
-            // The CLI ships with a .version file that stores the commit information
-            var versionFile = Path.GetFullPath(Path.Combine(AppContext.BaseDirectory, ".version"));
+            var versionFile = DotnetFiles.VersionFile;
             
             if (File.Exists(versionFile))
             {

src/dotnet/commands/dotnet-build/CompileContext.cs

@@ -53,17 +53,23 @@ namespace Microsoft.DotNet.Tools.Build
 
         private bool CompileRootProject(bool incremental)
         {
-            if (incremental && !NeedsRebuilding(_rootProject, _rootProjectDependencies))
+            try
             {
-                // todo: what if the previous build had errors / warnings and nothing changed? Need to propagate them in case of incremental
-                return true;
+                if (incremental && !NeedsRebuilding(_rootProject, _rootProjectDependencies))
+                {
+                    return true;
+                }
+
+                var success = InvokeCompileOnRootProject();
+
+                PrintSummary(success);
+
+                return success;
+            }
+            finally
+            {
+                StampProjectWithSDKVersion(_rootProject);
             }
-
-            var success = InvokeCompileOnRootProject();
-
-            PrintSummary(success);
-
-            return success;
         }
 
         private bool CompileDependencies(bool incremental)
@@ -75,28 +81,37 @@ namespace Microsoft.DotNet.Tools.Build
 
             foreach (var dependency in Sort(_rootProjectDependencies.ProjectDependenciesWithSources))
             {
-                if (incremental && !DependencyNeedsRebuilding(dependency))
-                {
-                    continue;
-                }
+                var dependencyProjectContext = ProjectContext.Create(dependency.Path, dependency.Framework, new[] { _rootProject.RuntimeIdentifier });
 
-                if (!InvokeCompileOnDependency(dependency))
+                try
                 {
-                    return false;
+                    if (incremental && !NeedsRebuilding(dependencyProjectContext, new ProjectDependenciesFacade(dependencyProjectContext, _args.ConfigValue)))
+                    {
+                        continue;
+                    }
+
+                    if (!InvokeCompileOnDependency(dependency))
+                    {
+                        return false;
+                    }
+                }
+                finally
+                {
+                    StampProjectWithSDKVersion(dependencyProjectContext);
                 }
             }
 
             return true;
         }
 
-        private bool DependencyNeedsRebuilding(ProjectDescription dependency)
-        {
-            var dependencyProjectContext = ProjectContext.Create(dependency.Path, dependency.Framework, new[] { _rootProject.RuntimeIdentifier });
-            return NeedsRebuilding(dependencyProjectContext, new ProjectDependenciesFacade(dependencyProjectContext, _args.ConfigValue));
-        }
-
         private bool NeedsRebuilding(ProjectContext project, ProjectDependenciesFacade dependencies)
         {
+            if (CLIChangedSinceLastCompilation(project))
+            {
+                Reporter.Output.WriteLine($"Project {project.GetDisplayName()} will be compiled because the CLI changed");
+                return true;
+            }
+            
             var compilerIO = GetCompileIO(project, dependencies);
 
             // rebuild if empty inputs / outputs
@@ -174,6 +189,48 @@ namespace Microsoft.DotNet.Tools.Build
 
             return true;
         }
+        
+        private bool CLIChangedSinceLastCompilation(ProjectContext project)
+        {
+            var currentVersionFile = DotnetFiles.VersionFile;
+            var versionFileFromLastCompile = project.GetSDKVersionFile(_args.ConfigValue, _args.BuildBasePathValue, _args.OutputValue);
+
+            if (!File.Exists(currentVersionFile))
+            {
+                // this CLI does not have a version file; cannot tell if CLI changed
+                return false;
+            }
+
+            if (!File.Exists(versionFileFromLastCompile))
+            {
+                // this is the first compilation; cannot tell if CLI changed
+                return false;
+            }
+
+            var versionsAreEqual = string.Equals(File.ReadAllText(currentVersionFile), File.ReadAllText(versionFileFromLastCompile), StringComparison.OrdinalIgnoreCase);
+
+            return !versionsAreEqual;
+        }
+
+        private void StampProjectWithSDKVersion(ProjectContext project)
+        {
+            if (File.Exists(DotnetFiles.VersionFile))
+            {
+                var projectVersionFile = project.GetSDKVersionFile(_args.ConfigValue, _args.BuildBasePathValue,_args.OutputValue);
+                var parentDirectory = Path.GetDirectoryName(projectVersionFile);
+
+                if (!Directory.Exists(parentDirectory))
+                {
+                    Directory.CreateDirectory(parentDirectory);
+                }
+
+                File.Copy(DotnetFiles.VersionFile, projectVersionFile, true);
+            }
+            else
+            {
+                Reporter.Verbose.WriteLine($"Project {project.GetDisplayName()} was not stamped with a CLI version because the version file does not exist: {DotnetFiles.VersionFile}");
+            }
+        }
 
         private void PrintSummary(bool success)
         {

test/dotnet-build.Tests/IncrementalTestBase.cs

@@ -124,5 +124,12 @@ namespace Microsoft.DotNet.Tools.Builder.Tests
 
             return executablePath;
         }
+
+        protected string GetIntermediaryOutputPath()
+        {
+            var executablePath = Path.Combine(TestProjectRoot, "obj", "Debug", "netstandardapp1.5");
+
+            return executablePath;
+        }
     }
 }

test/dotnet-build.Tests/IncrementalTests.cs

@@ -4,6 +4,7 @@
 using System;
 using System.IO;
 using System.Linq;
+using FluentAssertions;
 using Microsoft.DotNet.Cli.Utils;
 using Microsoft.DotNet.Tools.Test.Utilities;
 using Xunit;
@@ -79,6 +80,40 @@ namespace Microsoft.DotNet.Tools.Builder.Tests
             Assert.Contains("does not have a lock file", buildResult.StdErr);
         }
 
+        [Fact]
+        public void TestModifiedVersionFile()
+        {
+            CreateTestInstance();
+            BuildProject().Should().HaveCompiledProject(MainProject);
+
+            //change version file
+            var versionFile = Path.Combine(GetIntermediaryOutputPath(), ".SDKVersion");
+            File.Exists(versionFile).Should().BeTrue();
+            File.AppendAllText(versionFile, "text");
+
+            //assert rebuilt
+            BuildProject().Should().HaveCompiledProject(MainProject);
+        }
+
+        [Fact]
+        public void TestNoVersionFile()
+        {
+            CreateTestInstance();
+            BuildProject().Should().HaveCompiledProject(MainProject);
+
+            //delete version file
+            var versionFile = Path.Combine(GetIntermediaryOutputPath(), ".SDKVersion");
+            File.Exists(versionFile).Should().BeTrue();
+            File.Delete(versionFile);
+            File.Exists(versionFile).Should().BeFalse();
+
+            //assert build skipped due to no version file
+            BuildProject().Should().HaveSkippedProjectCompilation(MainProject);
+
+            //the version file should have been regenerated during the build, even if compilation got skipped
+            File.Exists(versionFile).Should().BeTrue();
+        }
+
         [Fact]
         public void TestRebuildChangedLockFile()
         {