[Mono.Android] Fix ServerCertificateCustomValidator (#8594)

Fixes: https://github.com/dotnet/runtime/issues/95506

In Release configuration the `X509ExtendedTrustManagerInvoker` class is trimmed and so 
the `trustManager is IX509TrustManager tm` pattern matching doesn't work. 

This PR addresses the problem in two ways:

    * an internal X509 trust manager is now required - it can't silently work with a null 
       internal trust manager anymore
    * `[DynamicDependency]` attribute to prevent trimming of the invoker classes for 
       the `IX509TrustManager` interface and for the `X509ExtendedTrustManager` 
       abstract class

src/Mono.Android/Xamarin.Android.Net/ServerCertificateCustomValidator.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Net.Http;
 using System.Net.Security;
 using System.Security.Cryptography.X509Certificates;
@@ -31,12 +32,12 @@ namespace Xamarin.Android.Net
 
 		private sealed class TrustManager : Java.Lang.Object, IX509TrustManager
 		{
-			private readonly IX509TrustManager? _internalTrustManager;
+			private readonly IX509TrustManager _internalTrustManager;
 			private readonly HttpRequestMessage _request;
 			private readonly Func<HttpRequestMessage, X509Certificate2?, X509Chain?, SslPolicyErrors, bool> _serverCertificateCustomValidationCallback;
 
 			public TrustManager (
-				IX509TrustManager? internalTrustManager,
+				IX509TrustManager internalTrustManager,
 				HttpRequestMessage request,
 				Func<HttpRequestMessage, X509Certificate2?, X509Chain?, SslPolicyErrors, bool> serverCertificateCustomValidationCallback)
 			{
@@ -50,7 +51,7 @@ namespace Xamarin.Android.Net
 				var sslPolicyErrors = SslPolicyErrors.None;
 
 				try {
-					_internalTrustManager?.CheckServerTrusted (javaChain, authType);
+					_internalTrustManager.CheckServerTrusted (javaChain, authType);
 				} catch (JavaCertificateException) {
 					sslPolicyErrors |= SslPolicyErrors.RemoteCertificateChainErrors;
 				}
@@ -158,33 +159,29 @@ namespace Xamarin.Android.Net
 			public bool Verify (string? hostname, ISSLSession? session) => true;
 		}
 
-		private static IX509TrustManager? FindX509TrustManager(ITrustManager[]? trustManagers)
+		[DynamicDependency(nameof(IX509TrustManager.CheckServerTrusted), typeof(IX509TrustManagerInvoker))]
+		[DynamicDependency(nameof(IX509TrustManager.CheckServerTrusted), typeof(X509ExtendedTrustManagerInvoker))]
+		private static IX509TrustManager FindX509TrustManager(ITrustManager[] trustManagers)
 		{
-			if (trustManagers is null)
-				return null;
-
 			foreach (var trustManager in trustManagers) {
 				if (trustManager is IX509TrustManager tm)
 					return tm;
 			}
 
-			return null;
+			throw new InvalidOperationException($"Could not find {nameof(IX509TrustManager)} in {nameof(ITrustManager)} array.");
 		}
 
-		private static ITrustManager[] ModifyTrustManagersArray (ITrustManager[] trustManagers, IX509TrustManager? original, IX509TrustManager replacement)
+		private static ITrustManager[] ModifyTrustManagersArray (ITrustManager[] trustManagers, IX509TrustManager original, IX509TrustManager replacement)
 		{
-			var modifiedTrustManagersCount = original is null ? trustManagers.Length + 1 : trustManagers.Length;
-			var modifiedTrustManagersArray = new ITrustManager [modifiedTrustManagersCount];
-
-			modifiedTrustManagersArray [0] = replacement;
-			int nextIndex = 1;
+			var modifiedTrustManagersArray = new ITrustManager [trustManagers.Length];
 
 			for (int i = 0; i < trustManagers.Length; i++) {
 				if (trustManagers [i] == original) {
-					continue;
+					modifiedTrustManagersArray [i] = replacement;
+				} else {
+					modifiedTrustManagersArray [i] = trustManagers [i];
 				}
 
-				modifiedTrustManagersArray [nextIndex++] = trustManagers [i];
 			}
 
 			return modifiedTrustManagersArray;

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

@@ -516,6 +516,40 @@ namespace UnnamedProject {
 			}
 		}
 
+		[Test]
+		public void PreserveIX509TrustManagerSubclasses ([Values(true, false)] bool hasServerCertificateCustomValidationCallback)
+		{
+			var proj = new XamarinAndroidApplicationProject { IsRelease = true };
+			proj.AddReferences ("System.Net.Http");
+			proj.MainActivity = proj.DefaultMainActivity.Replace (
+				"base.OnCreate (bundle);",
+				"base.OnCreate (bundle);\n" +
+				(hasServerCertificateCustomValidationCallback
+					? "var handler = new Xamarin.Android.Net.AndroidMessageHandler { ServerCertificateCustomValidationCallback = (message, certificate, chain, errors) => true };\n"
+					: "var handler = new Xamarin.Android.Net.AndroidMessageHandler();\n") +
+				"var client = new System.Net.Http.HttpClient (handler);\n" +
+				"client.GetAsync (\"https://microsoft.com\").GetAwaiter ().GetResult ();");
+
+			using (var b = CreateApkBuilder ()) {
+				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+				var assemblyPath = BuildTest.GetLinkedPath (b, true, "Mono.Android.dll");
+
+				using (var assembly = AssemblyDefinition.ReadAssembly (assemblyPath)) {
+					Assert.IsTrue (assembly != null);
+
+					var types = new[] { "Javax.Net.Ssl.X509ExtendedTrustManager", "Javax.Net.Ssl.IX509TrustManagerInvoker" };
+					foreach (var typeName in types) {
+						var td = assembly.MainModule.GetType (typeName);
+						if (hasServerCertificateCustomValidationCallback) {
+							Assert.IsNotNull (td, $"{typeName} shouldn't have been linked out");
+						} else {
+							Assert.IsNull (td, $"{typeName} should have been linked out");
+						}
+					}
+				}
+			}
+		}
+
 		[Test]
 		public void DoNotErrorOnPerArchJavaTypeDuplicates ([Values(true, false)] bool enableMarshalMethods)
 		{