ProjectReferences Negotiate SetPlatform Metadata (#6655)

* GetPlatforms target added

* Add _GetProjectReferencePlatformProperties

* Add GetNearestPlatformTask v1

* Changes to GetNearestPlatformTask, Platform items
flow into TargetFramework logic.

documentation/ProjectReference-Protocol.md

@@ -58,6 +58,7 @@ If implementing a project with an “outer” (determine what properties to pass
     * `TargetFrameworks` indicating what TargetFrameworks are available in the project
     * `TargetFrameworkMonikers` and `TargetPlatformMonikers` indicating what framework / platform the `TargetFrameworks` map to.  This is to support implicitly setting the target platform version (for example inferring that `net5.0-windows` means the same as `net5.0-windows7.0`) as well as treating the `TargetFramework` values [as aliases](https://github.com/NuGet/Home/issues/5154)
     * Boolean metadata for `HasSingleTargetFramework` and `IsRidAgnostic`.
+    * `Platforms` indicating what platforms are available for the project to build as, and boolean metadata `IsVcxOrNativeProj` (used for [SetPlatform Negotiation](#setplatform-negotiation))
   * The `GetReferenceNearestTargetFrameworkTask` (provided by NuGet) is responsible for selecting the best matching `TargetFramework` of the referenced project
   * This target is _optional_. If not present, the reference will be built with no additional properties.
   * **New** in MSBuild 15.5.  (`TargetFrameworkMonikers` and `TargetPlatformMonikers` metadata is new in MSBuild 16.8)
@@ -84,7 +85,6 @@ If implementing a project with an “outer” (determine what properties to pass
   * As of 15.7, this is _optional_. If a project does not contain a `GetCopyToOutputDirectoryItems` target, projects that reference it will not copy any of its outputs to their own output folders, but the build can succeed.
 * `Clean` should delete all outputs of the project.
   * It is not called during a normal build, only during "Clean" and "Rebuild".
-
 ## Other protocol requirements
 
 As with all MSBuild logic, targets can be added to do other work with `ProjectReference`s.
@@ -119,4 +119,66 @@ These properties will then be gathered via the `GetTargetFrameworks` call.  They
 </AdditionalProjectProperties>
 ```
 
-The `NearestTargetFramework` metadata will be the target framework which was selected as the best one to use for the reference (via `GetReferenceNearestTargetFrameworkTask`).  This can be used to select which set of properties were used in the target framework that was active for the reference.
+The `NearestTargetFramework` metadata will be the target framework which was selected as the best one to use for the reference (via `GetReferenceNearestTargetFrameworkTask`).  This can be used to select which set of properties were used in the target framework that was active for the reference.
+
+## SetPlatform Negotiation
+As of version 17.0, MSBuild can now dynamically figure out what platform a `ProjectReference` should build as. This includes a new target and task to determine what the `SetPlatform` metadata should be, or whether to undefine the platform so the referenced project builds with its default platform.
+
+* `_GetProjectReferenceTargetFrameworkProperties` target performs the majority of the work for assigning `SetPlatform` metadata to project references.
+  * Calls the `GetCompatiblePlatform` task, which is responsible for negotiating between the current project's platform and the platforms of the referenced project to assign a `NearestPlatform` metadata to the item.
+  * Sets or undefines `SetPlatform` based on the `NearestPlatform` assignment from `GetCompatiblePlatform`
+  * This target explicitly runs after `_GetProjectReferenceTargetFrameworkProperties` because it needs to use the `IsVcxOrNativeProj` and `Platforms` properties returned by the `GetTargetFrameworks` call.
+
+Note: If a `ProjectReference` has `SetPlatform` metadata defined already, the negotiation logic is skipped over.
+### Impact on the build
+In addition to the above task and target, `.vcxproj` and `.nativeproj` projects will receive an extra MSBuild call to the `GetTargetFrameworks` target. Previously, TargetFramework negotiation skipped over these projects because they could not multi-target in the first place. Because SetPlatform negotiation needs information given from the `GetTargetFrameworks` target, it is required that the `_GetProjectReferenceTargetFrameworkProperties` target calls the MSBuild task on the ProjectReference.
+
+This means most projects will see an evaluation with no global properties defined, unless set by the user.
+
+### How To Opt In
+First, set the properties `EnableDynamicPlatformResolution` and `DisableTransitiveProjectReferences` to `true` for **every project** in your solution. The easiest way to do this is by creating a `Directory.Build.props` file and placing it at the root of your project directory:
+
+```xml
+<Project>
+  <PropertyGroup>
+    <EnableDynamicPlatformResolution>true</EnableDynamicPlatformResolution>
+    <DisableTransitiveProjectReferences>true</DisableTransitiveProjectReferences>
+  </PropertyGroup>
+</Project>
+```
+
+If only set in one project, the `SetPlatform` metadata will carry forward to every consecutive project reference.
+
+Next, every referenced project is required to define a `Platforms` property, where `Platforms` is a semicolon-delimited list of platforms that project could build as. For `.vcxproj` or `.nativeproj` projects, `Platforms` is constructed from the `ProjectConfiguration` items that already exist in the project. For managed SDK projects, the default is `AnyCPU`. Managed non-SDK projects need to define this manually.
+
+Lastly, a `PlatformLookupTable` may need to be defined for more complex scenarios. A `PlatformLookupTable` is a semicolon-delimited list of mappings between platforms. `<PlatformLookupTable>Win32=x86</PlatformLookupTable>`, for example. This means that when the current project is building as `Win32`, it will attempt to build the referenced project as x86. This property is **required** when a managed AnyCPU project references an unmanaged project because `AnyCPU` does not directly map to an architecture-specific platform. You can define the table in two ways:
+
+1. A standard property within the current project, in a Directory.Build.props/targets
+2. Metadata on the `ProjectReference` item. This option takes priority over the first to allow customizations per `ProjectReference`.
+
+### References between managed and unmanaged projects
+Some cases of `ProjectReference`s require a `$(PlatformLookupTable)` to correctly determine what a referenced project should build as. References between managed and unmanaged projects also get a default lookup table that can be opted out of by setting the property `UseDefaultPlatformLookupTables` to false. See the table below for details.
+
+Note: Defining a `PlatformLookupTable` overrides the default mapping.
+| Project Reference Type | `PlatformLookupTable` Required? | Notes |
+| :--  | :-: | :-: |
+| Unmanaged -> Unmanaged | No |  |
+| Managed -> Managed | No |  |
+| Unmanaged -> Managed | Optional | Uses default mapping: `Win32=x86` |
+| Managed -> Unmanaged | **Yes** when the project is AnyCPU | Uses default mapping: `x86=Win32` |
+
+Example:
+Project A: Managed, building as `AnyCPU`, has a `ProjectReference` on Project B.
+Project B: Unmanaged, has `$(Platforms)` constructed from its `Platform` metadata from its `ProjectConfiguration` items, defined as `x64;Win32`.
+
+Because `AnyCPU` does not map to anything architecture-specific, a custom mapping must be defined. Project A can either:
+1. Define `PlatformLookupTable` in its project or a Directory.Build.props as `AnyCPU=x64` or `AnyCPU=Win32`.
+2. Define `PlatformLookupTable` as metadata on the `ProjectReference` item, which would take priority over a lookup table defined elsewhere.
+     *  When only one mapping is valid, you could also directly define `SetPlatform` metadata as `Platform=foo` (for unmanaged) or `PlatformTarget=bar` (for managed). This would skip over most negotiation logic.
+
+Example of project A defining a lookup table directly on the `ProjectReference`:
+```xml
+<ItemGroup>
+  <ProjectReference Include="B.csproj" PlatformLookupTable="AnyCPU=Win32">
+</ItemGroup>
+```

ref/Microsoft.Build.Tasks.Core/net/Microsoft.Build.Tasks.Core.cs

@@ -613,6 +613,18 @@ public GetAssemblyIdentity() { }
         public Microsoft.Build.Framework.ITaskItem[] AssemblyFiles { get { throw null; } set { } }
         public override bool Execute() { throw null; }
     }
+    public partial class GetCompatiblePlatform : Microsoft.Build.Tasks.TaskExtension
+    {
+        public GetCompatiblePlatform() { }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public Microsoft.Build.Framework.ITaskItem[] AnnotatedProjects { get { throw null; } set { } }
+        [Microsoft.Build.Framework.OutputAttribute]
+        public Microsoft.Build.Framework.ITaskItem[] AssignedProjectsWithPlatform { get { throw null; } set { } }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public string CurrentProjectPlatform { get { throw null; } set { } }
+        public string PlatformLookupTable { get { throw null; } set { } }
+        public override bool Execute() { throw null; }
+    }
     public sealed partial class GetFileHash : Microsoft.Build.Tasks.TaskExtension
     {
         public GetFileHash() { }

ref/Microsoft.Build.Tasks.Core/netstandard/Microsoft.Build.Tasks.Core.cs

@@ -506,6 +506,18 @@ public GetAssemblyIdentity() { }
         public Microsoft.Build.Framework.ITaskItem[] AssemblyFiles { get { throw null; } set { } }
         public override bool Execute() { throw null; }
     }
+    public partial class GetCompatiblePlatform : Microsoft.Build.Tasks.TaskExtension
+    {
+        public GetCompatiblePlatform() { }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public Microsoft.Build.Framework.ITaskItem[] AnnotatedProjects { get { throw null; } set { } }
+        [Microsoft.Build.Framework.OutputAttribute]
+        public Microsoft.Build.Framework.ITaskItem[] AssignedProjectsWithPlatform { get { throw null; } set { } }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public string CurrentProjectPlatform { get { throw null; } set { } }
+        public string PlatformLookupTable { get { throw null; } set { } }
+        public override bool Execute() { throw null; }
+    }
     public sealed partial class GetFileHash : Microsoft.Build.Tasks.TaskExtension
     {
         public GetFileHash() { }

src/Tasks.UnitTests/GetCompatiblePlatform_Tests.cs

@@ -0,0 +1,203 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Microsoft.Build.UnitTests;
+using Microsoft.Build.Utilities;
+using Shouldly;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Microsoft.Build.Tasks.UnitTests
+{
+    sealed public class GetCompatiblePlatform_Tests
+    {
+        private readonly ITestOutputHelper _output;
+
+        public GetCompatiblePlatform_Tests(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        [Fact]
+        public void ResolvesViaPlatformLookupTable()
+        {
+            // PlatformLookupTable always takes priority. It is typically user-defined.
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", "x64;x86;AnyCPU");
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "win32",
+                PlatformLookupTable = "win32=x64",
+                AnnotatedProjects = new TaskItem[] { projectReference }
+            };
+
+            task.Execute().ShouldBeTrue();
+
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe("x64");
+        }
+
+        [Fact]
+        public void ResolvesViaProjectReferencesPlatformLookupTable()
+        {
+            // A ProjectReference's PlatformLookupTable takes priority over the current project's table.
+            // This allows overrides on a per-ProjectItem basis.
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", "x64;x86;AnyCPU");
+
+            // ProjectReference will be assigned x86 because its table takes priority
+            projectReference.SetMetadata("PlatformLookupTable", "win32=x86");
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "win32",
+                PlatformLookupTable = "win32=x64",
+                AnnotatedProjects = new TaskItem[] { projectReference }
+            };
+
+            task.Execute().ShouldBeTrue();
+
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe("x86");
+        }
+
+        [Fact]
+        public void ResolvesViaAnyCPUDefault()
+        {
+            // No valid mapping via the lookup table, should default to AnyCPU when the current project
+            // and ProjectReference platforms don't match.
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", "x64;AnyCPU");
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "x86",
+                PlatformLookupTable = "AnyCPU=x64", 
+                AnnotatedProjects = new TaskItem[] { projectReference }
+            };
+
+            task.Execute().ShouldBeTrue();
+
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe("AnyCPU");
+        }
+
+        [Fact]
+        public void ResolvesViaSamePlatform()
+        {
+            // No valid mapping via the lookup table. If the ProjectReference's platform
+            // matches the current project's platform, it takes priority over AnyCPU default.
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", "x86;x64;AnyCPU");
+            projectReference.SetMetadata("PlatformLookupTable", "x86=AnyCPU"); // matching platform takes priority over lookup tables
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "x86",
+                PlatformLookupTable = "x86=AnyCPU",
+                AnnotatedProjects = new TaskItem[] { projectReference }
+            };
+
+            task.Execute().ShouldBeTrue();
+
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe("x86");
+        }
+
+        [Fact]
+        public void FailsToResolve()
+        {
+            // No valid mapping via the lookup table, ProjectReference can't default to AnyCPU,
+            // it also can't match with current project, log a warning.
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", "x64");
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "x86",
+                PlatformLookupTable = "AnyCPU=x64",
+                AnnotatedProjects = new TaskItem[] { projectReference },
+            };
+
+            task.Execute().ShouldBeTrue();
+            // When the task logs a warning, it does not set NearestPlatform
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe(string.Empty);
+            ((MockEngine)task.BuildEngine).AssertLogContains("MSB3981");
+        }
+
+        [Fact]
+        public void WarnsWhenProjectReferenceHasNoPlatformOptions()
+        {
+            // Task should log a warning when a ProjectReference has no options to build as.
+            // It will continue and have no NearestPlatform metadata.
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", string.Empty);
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "x86",
+                PlatformLookupTable = "AnyCPU=x64",
+                AnnotatedProjects = new TaskItem[] { projectReference },
+            };
+
+            task.Execute().ShouldBeTrue();
+            // When the task logs a warning, it does not set NearestPlatform
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe(string.Empty);
+            ((MockEngine)task.BuildEngine).AssertLogContains("MSB3982");
+        }
+
+        /// <summary>
+        /// Invalid format on PlatformLookupTable results in an exception being thrown.
+        /// </summary>
+        [Fact]
+        public void WarnsOnInvalidFormatLookupTable()
+        {
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", "x64");
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "AnyCPU",
+                PlatformLookupTable = "AnyCPU=;A=B", // invalid format
+                AnnotatedProjects = new TaskItem[] { projectReference },
+            };
+
+            task.Execute().ShouldBeTrue();
+            // When the platformlookuptable is in an invalid format, it is discarded.
+            // There shouldn't have been a translation found from AnyCPU to anything.
+            // Meaning the projectreference would not have NearestPlatform set.
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe(string.Empty);
+            ((MockEngine)task.BuildEngine).AssertLogContains("MSB3983");
+        }
+
+        /// <summary>
+        /// Invalid format on PlatformLookupTable from the projectreference results in an exception being thrown.
+        /// </summary>
+        [Fact]
+        public void WarnsOnInvalidFormatProjectReferenceLookupTable()
+        {
+            TaskItem projectReference = new TaskItem("foo.bar");
+            projectReference.SetMetadata("Platforms", "x64;x86");
+            projectReference.SetMetadata("PlatformLookupTable", "x86=;b=d");
+
+            GetCompatiblePlatform task = new GetCompatiblePlatform()
+            {
+                BuildEngine = new MockEngine(_output),
+                CurrentProjectPlatform = "AnyCPU",
+                PlatformLookupTable = "AnyCPU=x86;A=B", // invalid format
+                AnnotatedProjects = new TaskItem[] { projectReference },
+            };
+
+            task.Execute().ShouldBeTrue();
+
+            // A ProjectReference PlatformLookupTable should take priority, but is thrown away when
+            // it has an invalid format. The current project's PLT should be the next priority.
+            task.AssignedProjectsWithPlatform[0].GetMetadata("NearestPlatform").ShouldBe("x86");
+            ((MockEngine)task.BuildEngine).AssertLogContains("MSB3983");
+        }
+    }
+}