[Xamarin.Android.Build.Tasks] <ResolveAssemblies/> and ref assemblies (…

…#3053) (#3062)

Fixes: #3045

In VS 2019 16.1 Preview, a project using `Microsoft.Extensions.Http`
fails to build with:

	C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(2146,5): error MSB4018: The "LinkAssemblies" task failed unexpectedly.
	System.IO.FileNotFoundException: Could not load assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Perhaps it doesn't exist in the Mono for Android profile?
	    File name: 'System.Runtime.dll'
	    at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve(AssemblyNameReference reference, ReaderParameters parameters)
	    at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve(AssemblyNameReference reference)
	    at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
	    at Mono.Cecil.ModuleDefinition.Resolve(TypeReference type)
	    at Mono.Cecil.TypeReference.Resolve()
	    at Java.Interop.Tools.Cecil.TypeDefinitionRocks.<GetTypeAndBaseTypes>d__1.MoveNext()
	    at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
	    at Java.Interop.Tools.Cecil.TypeDefinitionRocks.IsSubclassOf(TypeDefinition type, String typeName)
	    at MonoDroid.Tuner.FixAbstractMethodsStep.ProcessAssembly(AssemblyDefinition assembly)
	    at Mono.Linker.Steps.BaseStep.Process(LinkContext context)
	    at Mono.Linker.Pipeline.Process(LinkContext context)
	    at MonoDroid.Tuner.Linker.Process(LinkerOptions options, ILogger logger, LinkContext& context)
	    at Xamarin.Android.Tasks.LinkAssemblies.Execute(DirectoryAssemblyResolver res)
	    at Xamarin.Android.Tasks.LinkAssemblies.Execute()
	    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
	    at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext() [C:\Temp\ResolveAssembliesSystemRuntime\AndroidApp1\AndroidApp1.csproj]

The problem being that the `<ResolveAssemblies/>` MSBuild task didn't
return `System.Runtime.dll` in `@(ResolvedAssemblies)`.

What further complicated matters, was that the same project was
working with latest xamarin-android/master (16.2)? I was only able to
reproduce the problem in a test after some work to create a project
that only has a `<PackageReference/>` to `Microsoft.Extensions.Http`
with no other references *at all*. I had to do some MSBuild trickery
to remove `Java.Interop` and `System.Runtime` references.

Looking at the dependency tree in the failing test,
`System.Runtime.CompilerServices.Unsafe.dll` should be reporting a
reference to `System.Runtime.dll`. Through debugging, I saw it only
had a reference to `netstandard.dll`?

Then I realized there were two assemblies:

	~\.nuget\packages\system.runtime.compilerservices.unsafe\4.5.1\ref\netstandard2.0\System.Runtime.CompilerServices.Unsafe.dll
	~\.nuget\packages\system.runtime.compilerservices.unsafe\4.5.1\lib\netstandard2.0\System.Runtime.CompilerServices.Unsafe.dll

The "ref" assembly, references `netstandard.dll` and the "lib"
assembly (that we should be using) references `System.Runtime.dll`.

Through further debugging, I found two bugs here:

  * We shouldn't call `resolver.AddSearchDirectory()` until *after*
    we've checked if the top-level assembly is a reference assembly.
    We don't want a reference assembly in the search paths!
  * `MetadataResolver` should key its cache on the resolved path to
    the assembly.  This allows `<ResolveAssemblies/>` to open two
    *different* instances of `System.Runtime.CompilerServices.Unsafe`.

src/Xamarin.Android.Build.Tasks/Tasks/ResolveAssemblies.cs

@@ -87,9 +87,6 @@ void Execute (MetadataResolver resolver)
 
 			try {
 				foreach (var assembly in Assemblies) {
-					var assembly_path = Path.GetDirectoryName (assembly.ItemSpec);
-					resolver.AddSearchDirectory (assembly_path);
-
 					// Add each user assembly and all referenced assemblies (recursive)
 					string resolved_assembly = resolver.Resolve (assembly.ItemSpec);
 					if (MonoAndroidHelper.IsReferenceAssembly (resolved_assembly)) {
@@ -102,6 +99,7 @@ void Execute (MetadataResolver resolver)
 						}
 					}
 					topAssemblyReferences.Add (resolved_assembly);
+					resolver.AddSearchDirectory (Path.GetDirectoryName (resolved_assembly));
 					var taskItem = new TaskItem (assembly) {
 						ItemSpec = Path.GetFullPath (resolved_assembly),
 					};

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -3403,6 +3403,35 @@ public void MissingOrgApacheHttpClient ([Values ("dx", "d8")] string dexTool)
 			}
 		}
 
+		//NOTE: Referencing only Microsoft.Extensions.Http, surfaced a bug in <ResolveAssemblies/>
+		[Test]
+		public void MicrosoftExtensionsHttp ()
+		{
+			// The goal is to create a project with only this <PackageReference/>
+			var proj = new XamarinAndroidApplicationProject {
+				PackageReferences = {
+					KnownPackages.Microsoft_Extensions_Http,
+				}
+			};
+			proj.References.Clear ();
+			proj.Sources.Clear ();
+			// We have to add a custom Target to remove Java.Interop and System.Runtime
+			proj.Imports.Add (new Import ("foo.targets") {
+				TextContent = () => @"<?xml version=""1.0"" encoding=""utf-16""?>
+<Project ToolsVersion=""4.0"" xmlns=""http://schemas.microsoft.com/developer/msbuild/2003"">
+	<Target Name=""_Foo"" BeforeTargets=""_ResolveAssemblies"">
+		<ItemGroup>
+			<_Remove Include=""@(_ReferencePath)"" Condition=""'%(FileName)' == 'Java.Interop' Or '%(FileName)' == 'System.Runtime'"" />
+			<_ReferencePath Remove=""@(_Remove)"" />
+		</ItemGroup>
+	</Target>
+</Project>"
+			});
+			using (var b = CreateApkBuilder (Path.Combine ("temp", TestName))) {
+				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+			}
+		}
+
 		[Test]
 		[TestCase ("armeabi;armeabi-v7a", TestName = "XA0115")]
 		[TestCase ("armeabi,armeabi-v7a", TestName = "XA0115Commas")]

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/KnownPackages.cs

@@ -700,6 +700,16 @@ public static class KnownPackages
 				}
 			},
 		};
+		public static Package Microsoft_Extensions_Http = new Package {
+			Id = "Microsoft.Extensions.Http",
+			Version = "2.2.0",
+			TargetFramework = "netstandard2.0",
+			References = {
+				new BuildItem.Reference ("Microsoft.Extensions.Http") {
+					MetadataValues = "HintPath=..\\packages\\Microsoft.Extensions.Http.2.2.0\\lib\\netstandard2.0\\Microsoft.Extensions.Http.dll"
+				}
+			},
+		};
 	}
 }
 

src/Xamarin.Android.Build.Tasks/Utilities/MetadataResolver.cs

@@ -16,10 +16,9 @@ public class MetadataResolver : IDisposable
 
 		public MetadataReader GetAssemblyReader (string assemblyName)
 		{
-			var key = Path.GetFileNameWithoutExtension (assemblyName);
-			if (!cache.TryGetValue (key, out PEReader reader)) {
-				var assemblyPath = Resolve (assemblyName);
-				cache.Add (key, reader = new PEReader (File.OpenRead (assemblyPath)));
+			var assemblyPath = Resolve (assemblyName);
+			if (!cache.TryGetValue (assemblyPath, out PEReader reader)) {
+				cache.Add (assemblyPath, reader = new PEReader (File.OpenRead (assemblyPath)));
 			}
 			return reader.GetMetadataReader ();
 		}