[dotnet] Prevent linking out code referenced by P/Invoke (dotnet#10182)

* [dotnet] Generate references.mm to prevent linking out code referenced by P/Invoke

* Address PR feedback: Namespaces.Foundation -> "Foundation."

* Address PR feedback: Simplify hasSymbols

* Reuse ListExportedSymbols step, respect App.SymbolMode

* Address PR feedback

* Workaround for tvOS

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -314,6 +314,14 @@
 		<ReadItemsFromFile File="$(_LinkerItemsDirectory)/_RegistrarFile.items" Condition="Exists('$(_LinkerItemsDirectory)/_RegistrarFile.items')">
 			<Output TaskParameter="Items" ItemName="_RegistrarFile" />
 		</ReadItemsFromFile>
+		<!-- Load _ReferencesFile -->
+		<ReadItemsFromFile File="$(_LinkerItemsDirectory)/_ReferencesFile.items" Condition="Exists('$(_LinkerItemsDirectory)/_ReferencesFile.items')">
+			<Output TaskParameter="Items" ItemName="_ReferencesFile" />
+		</ReadItemsFromFile>
+		<!-- Load _ReferencesLinkerFlags -->
+		<ReadItemsFromFile File="$(_LinkerItemsDirectory)/_ReferencesLinkerFlags.items" Condition="Exists('$(_LinkerItemsDirectory)/_ReferencesLinkerFlags.items')">
+			<Output TaskParameter="Items" ItemName="_ReferencesLinkerFlags" />
+		</ReadItemsFromFile>
 
 		<ItemGroup>
 			<_AssembliesToAOT Include="@(ResolvedFileToPublish)" Condition="'%(Extension)' == '.dll' Or '%(Extension)' == '.exe' " />
@@ -467,6 +475,9 @@
 			<_CompileNativeExecutableFile Include="@(_RegistrarFile)">
 				<OutputFile>$(_IntermediateNativeLibraryDir)%(Filename).o</OutputFile>
 			</_CompileNativeExecutableFile>
+			<_CompileNativeExecutableFile Include="@(_ReferencesFile)">
+				<OutputFile>$(_IntermediateNativeLibraryDir)%(Filename).o</OutputFile>
+			</_CompileNativeExecutableFile>
 			<_XamarinMainIncludeDirectory Include="$(_XamarinIncludeDirectory)" />
 		</ItemGroup>
 	</Target>
@@ -570,7 +581,7 @@
 			DylibRPath="$(_DylibRPath)"
 			EntitlementsInExecutable="$(_CompiledEntitlements)"
 			Frameworks="@(_NativeExecutableFrameworks);@(_BindingLibraryFrameworks)"
-			LinkerFlags="@(_BindingLibraryLinkerFlags);@(_MainLinkerFlags)"
+			LinkerFlags="@(_BindingLibraryLinkerFlags);@(_ReferencesLinkerFlags);@(_MainLinkerFlags)"
 			LinkWithLibraries="@(_XamarinMainLibraries);@(_BindingLibraryLinkWith);@(_MainLinkWith)"
 			MinimumOSVersion="$(_MinimumOSVersion)"
 			ObjectFiles="@(_NativeExecutableObjectFiles)"

tools/dotnet-linker/SetupStep.cs

@@ -85,10 +85,12 @@ protected override void TryProcess ()
 				post_sweep_substeps.Add (new RemoveAttributesStep ());
 			}
 
+			Steps.Add (new ListExportedSymbols (null));
 			Steps.Add (new LoadNonSkippedAssembliesStep ());
 			Steps.Add (new ExtractBindingLibrariesStep ());
 			Steps.Add (new RegistrarStep ());
 			Steps.Add (new GenerateMainStep ());
+			Steps.Add (new GenerateReferencesStep ());
 			Steps.Add (new GatherFrameworksStep ());
 
 			Configuration.Write ();

tools/dotnet-linker/Steps/GenerateReferencesStep.cs

@@ -0,0 +1,48 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+using Mono.Cecil;
+
+using Xamarin.Bundler;
+using Xamarin.Linker;
+
+namespace Xamarin {
+
+	public class GenerateReferencesStep : ConfigurationAwareStep {
+		protected override string Name { get; } = "Generate References";
+		protected override int ErrorCode { get; } = 2320;
+
+		protected override void TryEndProcess ()
+		{
+			base.TryEndProcess ();
+
+			var app = Configuration.Application;
+			var required_symbols = Configuration.DerivedLinkContext.RequiredSymbols;
+			var items = new List<MSBuildItem> ();
+
+			switch (app.SymbolMode) {
+			case SymbolMode.Ignore:
+				break;
+			case SymbolMode.Code:
+				string reference_m = Path.Combine (Configuration.CacheDirectory, "reference.m");
+				reference_m = Configuration.Target.GenerateReferencingSource (reference_m, required_symbols);
+				if (!string.IsNullOrEmpty (reference_m)) {
+					var item = new MSBuildItem { Include = reference_m };
+					items.Add (item);
+				}
+				Configuration.WriteOutputForMSBuild ("_ReferencesFile", items);
+				break;
+			case SymbolMode.Linker:
+				foreach (var symbol in required_symbols) {
+					var item = new MSBuildItem { Include = "-u" + symbol.Prefix + symbol.Name };
+					items.Add (item);
+				}
+				Configuration.WriteOutputForMSBuild ("_ReferencesLinkerFlags", items);
+				break;
+			default:
+				throw ErrorHelper.CreateError (99, Errors.MX0099, $"invalid symbol mode: {app.SymbolMode}");
+			}
+		}
+	}
+}

tools/dotnet-linker/dotnet-linker.csproj

@@ -146,6 +146,9 @@
     <Compile Include="..\linker\MonoTouch.Tuner\Extensions.cs">
       <Link>external\tools\linker\MonoTouch.Tuner\Extensions.cs</Link>
     </Compile>
+    <Compile Include="..\linker\MonoTouch.Tuner\ListExportedSymbols.cs">
+      <Link>external\tools\linker\MonoTouch.Tuner\ListExportedSymbols.cs</Link>
+    </Compile>
     <Compile Include="..\linker\MonoTouch.Tuner\PreserveSmartEnumConversionsSubStep.cs">
       <Link>external\tools\linker\MonoTouch.Tuner\PreserveSmartEnumConversionsSubStep.cs</Link>
     </Compile>

tools/linker/MonoTouch.Tuner/ListExportedSymbols.cs

@@ -9,8 +9,9 @@
 using Xamarin.Bundler;
 using Xamarin.Linker;
 using Xamarin.Tuner;
+using Xamarin.Utils;
 
-namespace MonoTouch.Tuner
+namespace Xamarin.Linker.Steps
 {
 	public class ListExportedSymbols : BaseStep
 	{
@@ -19,7 +20,11 @@ public class ListExportedSymbols : BaseStep
 
 		public DerivedLinkContext DerivedLinkContext {
 			get {
+#if NET
+				return LinkerConfiguration.GetInstance (Context).DerivedLinkContext;
+#else
 				return (DerivedLinkContext) Context;
+#endif
 			}
 		}
 
@@ -36,8 +41,10 @@ protected override void ProcessAssembly (AssemblyDefinition assembly)
 			if (Annotations.GetAction (assembly) == AssemblyAction.Delete)
 				return;
 
+#if !NET
 			if (skip_sdk_assemblies && Profile.IsSdkAssembly (assembly))
 				return;
+#endif
 
 			if (!assembly.MainModule.HasTypes)
 				return;
@@ -116,8 +123,14 @@ void ProcessMethod (MethodDefinition method)
 				case "__Internal":
 					DerivedLinkContext.RequiredSymbols.AddFunction (pinfo.EntryPoint).AddMember (method);
 					break;
-				case "System.Native":
+
 				case "System.Net.Security.Native":
+#if NET
+					if (DerivedLinkContext.App.Platform == ApplePlatform.TVOS)
+						break; // tvOS does not ship with System.Net.Security.Native due to https://github.com/dotnet/runtime/issues/45535
+					goto case "System.Native";
+#endif
+				case "System.Native":
 				case "System.Security.Cryptography.Native.Apple":
 					DerivedLinkContext.RequireMonoNative = true;
 					DerivedLinkContext.RequiredSymbols.AddFunction (pinfo.EntryPoint).AddMember (method);