[linker] Add metadata to linked assemblies indicating the RID for which they were created

src/Microsoft.Android.Sdk.ILLink/PreserveLists/System.Private.CoreLib.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<linker>
+	<assembly fullname="System.Private.CoreLib">
+		<type fullname="System.Reflection.AssemblyMetadataAttribute" />
+	</assembly>
+</linker>

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/AddRidMetadataAttributeStep.cs

@@ -0,0 +1,100 @@
+using System;
+
+using Mono.Cecil;
+using Mono.Linker;
+using Mono.Linker.Steps;
+
+#if ILLINK
+using Microsoft.Android.Sdk.ILLink;
+#endif
+
+namespace MonoDroid.Tuner;
+
+public class AddRidMetadataAttributeStep : BaseStep
+{
+	protected override void ProcessAssembly (AssemblyDefinition assembly)
+	{
+		if (!Annotations.HasAction (assembly)) {
+				return;
+		}
+
+		var action = Annotations.GetAction (assembly);
+		if (action == AssemblyAction.Skip || action == AssemblyAction.Delete) {
+			return;
+		}
+
+		string? rid = null;
+#if ILLINK
+		if (!Context.TryGetCustomData ("XARuntimeIdentifier", out rid)) {
+			throw new InvalidOperationException ("Missing XARuntimeIdentifier custom data");
+		}
+#endif
+		if (String.IsNullOrEmpty (rid)) {
+			throw new InvalidOperationException ("RID must have a non-empty value");
+		}
+
+		AssemblyDefinition corlib = GetCorlib ();
+		MethodDefinition assemblyMetadataAttributeCtor = FindAssemblyMetadataAttributeCtor (corlib);
+		TypeDefinition systemString = GetSystemString (corlib);
+
+		var attr = new CustomAttribute (assembly.MainModule.ImportReference (assemblyMetadataAttributeCtor));
+		attr.ConstructorArguments.Add (new CustomAttributeArgument (systemString, "XamarinAndroidAbi")); // key
+
+		// TODO: figure out how to get the RID...
+		attr.ConstructorArguments.Add (new CustomAttributeArgument (systemString, rid)); // value
+
+		assembly.CustomAttributes.Add (attr);
+
+		if (action == AssemblyAction.Copy) {
+			Annotations.SetAction (assembly, AssemblyAction.Save);
+		}
+	}
+
+	TypeDefinition GetSystemString (AssemblyDefinition asm) => FindType (asm, "System.String", required: true);
+
+	AssemblyDefinition GetCorlib ()
+	{
+		const string ImportAssembly = "System.Private.CoreLib";
+		AssemblyDefinition? asm = Context.Resolve (AssemblyNameReference.Parse (ImportAssembly));
+		if (asm == null) {
+			throw new InvalidOperationException ($"Unable to import assembly '{ImportAssembly}'");
+		}
+
+		return asm;
+	}
+
+	MethodDefinition FindAssemblyMetadataAttributeCtor (AssemblyDefinition asm)
+	{
+		const string AttributeType = "System.Reflection.AssemblyMetadataAttribute";
+
+		TypeDefinition assemblyMetadataAttribute = FindType (asm, AttributeType, required: true);
+		foreach (MethodDefinition md in assemblyMetadataAttribute!.Methods) {
+			if (!md.IsConstructor) {
+				continue;
+			}
+
+			return md;
+		}
+
+		throw new InvalidOperationException ($"Unable to find the {AttributeType} type constructor");
+	}
+
+	TypeDefinition? FindType (AssemblyDefinition asm, string typeName, bool required)
+	{
+		foreach (ModuleDefinition md in asm.Modules) {
+			foreach (TypeDefinition et in md.Types) {
+				if (String.Compare (typeName, et.FullName, StringComparison.Ordinal) != 0) {
+					continue;
+				}
+
+				return et;
+			}
+		}
+
+		if (required) {
+			throw new InvalidOperationException ($"Internal error: required type '{typeName}' in assembly {asm} not found");
+		}
+
+		return null;
+	}
+}

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets

@@ -40,6 +40,7 @@ This file contains the .NET 5-specific targets to customize ILLink
         https://github.com/dotnet/sdk/blob/a5393731b5b7b225692fff121f747fbbc9e8b140/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.ILLink.targets#L147
         -->
       <_TrimmerCustomData Include="XATargetFrameworkDirectories" Value="$(_XATargetFrameworkDirectories)" />
+      <_TrimmerCustomData Include="XARuntimeIdentifier" Value="$(RuntimeIdentifier)"  Condition=" '$(_AndroidAddRuntimeIdentifierToAssemblies)' == 'true' " />
       <_TrimmerCustomData
           Condition=" '$(_ProguardProjectConfiguration)' != '' "
           Include="ProguardConfiguration"
@@ -94,6 +95,12 @@ This file contains the .NET 5-specific targets to customize ILLink
           BeforeStep="MarkStep"
           Type="MonoDroid.Tuner.FixLegacyResourceDesignerStep"
       />
+      <_TrimmerCustomSteps
+          Condition=" '$(_AndroidAddRuntimeIdentifierToAssemblies)' == 'true' "
+          Include="$(_AndroidLinkerCustomStepAssembly)"
+          AfterStep="CleanStep"
+          Type="MonoDroid.Tuner.AddRidMetadataAttributeStep"
+      />
       <_PreserveLists Include="$(MSBuildThisFileDirectory)..\PreserveLists\*.xml" />
       <TrimmerRootDescriptor
           Condition=" '@(ResolvedFileToPublish->Count())' != '0' and '%(Filename)' != '' "