Keep remote ReadItemsFromFile support

Some files are written remotely as part of other tasks (not through WriteItemsToFile), like the linker outputs (Illink).

Author: emaf

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -1018,7 +1018,7 @@
 			<_ExistingLinkerItemFiles Include="@(_LinkerItemFiles)" Condition="Exists('%(Identity)')" />
 		</ItemGroup>
 
-		<ReadItemsFromFile File="@(_ExistingLinkerItemFiles)" Condition="'@(_ExistingLinkerItemFiles)' != ''">
+		<ReadItemsFromFile SessionId="$(BuildSessionId)" File="@(_ExistingLinkerItemFiles)" Condition="'@(_ExistingLinkerItemFiles)' != ''">
 			<Output TaskParameter="Items" ItemName="_AllLinkerItems" />
 		</ReadItemsFromFile>
 

msbuild/Xamarin.MacDev.Tasks/Tasks/ReadItemsFromFile.cs

@@ -8,8 +8,10 @@
 using Microsoft.Build.Tasks;
 using System.Xml.Linq;
 
+using Xamarin.Messaging.Build.Client;
+
 namespace Xamarin.MacDev.Tasks {
-	public class ReadItemsFromFile : XamarinTask {
+	public class ReadItemsFromFile : XamarinTask, ITaskCallback {
 		static readonly XNamespace XmlNs = XNamespace.Get ("http://schemas.microsoft.com/developer/msbuild/2003");
 
 		static readonly XName ItemGroupElementName = XmlNs + "ItemGroup";
@@ -32,6 +34,9 @@ public class ReadItemsFromFile : XamarinTask {
 
 		public override bool Execute ()
 		{
+			if (ShouldExecuteRemotely ())
+				return new TaskRunner (SessionId, BuildEngine4).RunAsync (this).Result;
+
 			var result = new List<ITaskItem> ();
 			foreach (var file in File) {
 				// XDocument.Load(string) takes a string to a URI, not a file path, so with certain characters that becomes a problem.
@@ -68,5 +73,11 @@ ITaskItem CreateItemFromElement (XElement element, string sourceFile)
 
 			return item;
 		}
+
+		public bool ShouldCopyToBuildServer (ITaskItem item) => false;
+
+		public bool ShouldCreateOutputFile (ITaskItem item) => false;
+
+		public IEnumerable<ITaskItem> GetAdditionalItemsToBeCopied () => Enumerable.Empty<ITaskItem> ();
 	}
 }