[illink] refactor code sharing between ILLink and MSBuild tasks

src/Microsoft.Android.Sdk.ILLink/Microsoft.Android.Sdk.ILLink.csproj

@@ -12,6 +12,7 @@
     <ProjectReference Include="..\Xamarin.Android.Build.Tasks\Xamarin.Android.Build.Tasks.csproj" ReferenceOutputAssembly="False" />
 
     <!--Include shared linker sources-->
+    <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\External\Linker\BaseMarkHandler.cs" Link="External\BaseMarkHandler.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\AddKeepAlivesStep.cs" Link="MonoDroid.Tuner\AddKeepAlivesStep.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\AndroidLinkConfiguration.cs" Link="MonoDroid.Tuner\AndroidLinkConfiguration.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\Extensions.cs" Link="MonoDroid.Tuner\Extensions.cs" />

src/Xamarin.Android.Build.Tasks/Linker/External/Linker.Steps/BaseStep.cs

@@ -46,7 +46,7 @@ public Tracer Tracer {
 			get { return _context.Tracer; }
 		}
 
-		public MarkingHelpers MarkingHelpers => _context.MarkingHelpers;
+		public void Initialize (LinkContext context) => _context = context;
 
 		public void Process (LinkContext context)
 		{
@@ -79,5 +79,19 @@ protected virtual void EndProcess ()
 		protected virtual void ProcessAssembly (AssemblyDefinition assembly)
 		{
 		}
+
+		public virtual void LogMessage (string message)
+		{
+			Context.LogMessage (message);
+		}
+
+		public virtual void LogError (int code, string message)
+		{
+#if ILLINK
+			Context.LogMessage (MessageContainer.CreateCustomErrorMessage (message, code, origin: new MessageOrigin ()));
+#else   // !ILLINK
+			Context.LogError ($"XA{code}", message);
+#endif  // !ILLINK
+		}
 	}
 }

src/Xamarin.Android.Build.Tasks/Linker/External/Linker.Steps/IMarkHandler.cs

@@ -0,0 +1,20 @@
+// 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.
+
+namespace Mono.Linker.Steps
+{
+
+	/// <summary>
+	/// This API supports the product infrastructure and is not intended to be used directly from your code.
+	/// Extensibility point for custom logic that run during MarkStep, for marked members.
+	/// </summary>
+	public interface IMarkHandler
+	{
+		/// <summary>
+		/// Initialize is called at the beginning of MarkStep. This should be
+		/// used to perform global setup, and register callbacks through the
+		/// MarkContext.Register* methods) to be called when pieces of IL are marked.
+		/// </summary>
+		void Initialize (LinkContext context, MarkContext markContext);
+	}
+}

src/Xamarin.Android.Build.Tasks/Linker/External/Linker.Steps/MarkContext.cs

@@ -0,0 +1,30 @@
+// 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;
+using Mono.Cecil;
+
+namespace Mono.Linker.Steps
+{
+	/// <summary>
+	/// Context which can be used to register actions to call during MarkStep
+	/// when various members are marked.
+	/// </summary>
+	public abstract class MarkContext
+	{
+		/// <summary>
+		/// Register a callback that will be invoked once for each marked assembly.
+		/// </summary>
+		public abstract void RegisterMarkAssemblyAction (Action<AssemblyDefinition> action);
+
+		/// <summary>
+		/// Register a callback that will be invoked once for each marked type.
+		/// </summary>
+		public abstract void RegisterMarkTypeAction (Action<TypeDefinition> action);
+
+		/// <summary>
+		/// Register a callback that will be invoked once for each marked method.
+		/// </summary>
+		public abstract void RegisterMarkMethodAction (Action<MethodDefinition> action);
+	}
+}

src/Xamarin.Android.Build.Tasks/Linker/External/Linker/Annotations.cs

@@ -59,7 +59,6 @@ public partial class AnnotationStore {
 		protected readonly HashSet<CustomAttribute> marked_attributes = new HashSet<CustomAttribute> ();
 		readonly HashSet<TypeDefinition> marked_types_with_cctor = new HashSet<TypeDefinition> ();
 		protected readonly HashSet<TypeDefinition> marked_instantiated = new HashSet<TypeDefinition> ();
-		protected readonly HashSet<MethodDefinition> indirectly_called = new HashSet<MethodDefinition>();
 
 
 		public AnnotationStore (LinkContext context) => this.context = context;
@@ -167,24 +166,6 @@ public bool IsMarked (CustomAttribute attribute)
 			return marked_attributes.Contains (attribute);
 		}
 
-		public void MarkIndirectlyCalledMethod (MethodDefinition method)
-		{
-			if (!context.AddReflectionAnnotations)
-				return;
-
-			indirectly_called.Add (method);
-		}
-
-		public bool HasMarkedAnyIndirectlyCalledMethods ()
-		{
-			return indirectly_called.Count != 0;
-		}
-
-		public bool IsIndirectlyCalled (MethodDefinition method)
-		{
-			return indirectly_called.Contains (method);
-		}
-
 		public void MarkInstantiated (TypeDefinition type)
 		{
 			marked_instantiated.Add (type);

src/Microsoft.Android.Sdk.ILLink/BaseMarkHandler.cs → src/Xamarin.Android.Build.Tasks/Linker/External/Linker/BaseMarkHandler.cs

@@ -1,4 +1,3 @@
-using Microsoft.Android.Sdk.ILLink;
 using Mono.Cecil;
 using Mono.Linker.Steps;