[Mono.Android] Improve the GetHttpMessageHandler function

src/Mono.Android/Android.Runtime/AndroidEnvironment.cs

@@ -344,18 +344,60 @@ static IWebProxy GetDefaultProxy ()
 #if !MONOANDROID1_0
 		[DynamicDependency (DynamicallyAccessedMemberTypes.PublicParameterlessConstructor, typeof (Xamarin.Android.Net.AndroidMessageHandler))]
 #endif
-		static object? GetHttpMessageHandler ()
+		static object GetHttpMessageHandler ()
 		{
-			var envvar = Environment.GetEnvironmentVariable ("XA_HTTP_CLIENT_HANDLER_TYPE")?.Trim ();
+			var handlerTypeName = Environment.GetEnvironmentVariable ("XA_HTTP_CLIENT_HANDLER_TYPE")?.Trim ();
 			Type? handlerType = null;
-			if (!String.IsNullOrEmpty (envvar))
-				handlerType = Type.GetType (envvar, false);
-			else
-				return null;
-			if (handlerType == null)
-				return null;
+			if (!String.IsNullOrEmpty (handlerTypeName))
+				handlerType = Type.GetType (handlerTypeName, throwOnError: false);
+
 			// We don't do any type checking or casting here to avoid dependency on System.Net.Http in Mono.Android.dll
-			return Activator.CreateInstance (handlerType);
+			if (handlerType is null || !IsAcceptableHttpMessageHandlerType (handlerType)) {
+				handlerType = GetFallbackHttpMessageHandlerType ();
+			}
+
+			return Activator.CreateInstance (handlerType)
+				?? throw new InvalidOperationException ($"Could not create an instance of HTTP message handler type {handlerType.AssemblyQualifiedName}");
+		}
+
+		static bool IsAcceptableHttpMessageHandlerType (Type handlerType)
+		{
+#if !MONOANDROID1_0
+			if (Extends (handlerType, "System.Net.Http.HttpClientHandler")) {
+				// It's not possible to construct HttpClientHandler in this method because it would cause infinite recursion
+				// as HttpClientHandler's constructor calls the GetHttpMessageHandler function
+				Logger.Log (LogLevel.Warn, "MonoAndroid", $"The type {handlerType.AssemblyQualifiedName} cannot be used as the native HTTP handler because it is derived from System.Net.Htt.HttpClientHandler. Use a type that extends System.Net.Http.HttpMessageHandler instead.");
+				return false;
+			}
+#endif
+			if (!Extends (handlerType, "System.Net.Http.HttpMessageHandler")) {
+				Logger.Log (LogLevel.Warn, "MonoAndroid", $"The type {handlerType.AssemblyQualifiedName} set as the default HTTP handler is invalid. Use a type that extends System.Net.Http.HttpMessageHandler.");
+				return false;
+			}
+
+			return true;
+		}
+
+		static bool Extends (Type handlerType, string typeName, string assemblyName = "System.Net.Http")
+		{
+			while (handlerType != null) {
+				if (handlerType.FullName == typeName && handlerType.Assembly.GetName ().Name == assemblyName) {
+					return true;
+				}
+
+				handlerType = handlerType.BaseType;
+			}
+
+			return false;
+		}
+
+		static Type GetFallbackHttpMessageHandlerType (string typeName = "Xamarin.Android.Net.AndroidMessageHandler")
+		{
+			var handlerType = Type.GetType (typeName, throwOnError: false)
+				?? throw new InvalidOperationException ($"The {typeName} was not found. The type was probably linked away.");
+
+			Logger.Log (LogLevel.Info, "MonoAndroid", $"Using {typeName} as the native HTTP message handler.");
+			return handlerType;
 		}
 
 		internal static bool VSAndroidDesignerIsEnabled { get; } = InitializeVSAndroidDesignerIsEnabled ();

tests/Mono.Android-Tests/Android.Runtime/AndroidEnvironmentTest.cs

@@ -0,0 +1,82 @@
+using System;
+using System.Reflection;
+using Android.Runtime;
+using NUnit.Framework;
+
+namespace Android.RuntimeTests {
+	[TestFixture]
+	public class AndroidEnvironmentTest {
+		const string EnvironmentVariable = "XA_HTTP_CLIENT_HANDLER_TYPE";
+
+		static object envLock = new object ();
+
+		private string? _originalValue = null;
+
+		[TestFixtureSetUp]
+		public void SetUp ()
+		{
+			_originalValue = Environment.GetEnvironmentVariable (EnvironmentVariable);
+		}
+
+		[TestFixtureTearDown]
+		public void TearDown ()
+		{
+			Environment.SetEnvironmentVariable (EnvironmentVariable, _originalValue);
+		}
+
+		[Test]
+		[TestCase (null)]
+		[TestCase ("Xamarin.Android.Net.AndroidHttpResponseMessage")] // does not extend HttpMessageHandler
+#if NET
+		// instantiating AndroidClientHandler or HttpClientHandler (or any other type extending HttpClientHandler)
+		// would cause infinite recursion in the .NET build and so it is replaced with AndroidMessageHandler
+		[TestCase ("System.Net.Http.HttpClientHandler, System.Net.Http")]
+		[TestCase ("Xamarin.Android.Net.AndroidClientHandler")]
+#endif
+		public void GetHttpMessageHandler_FallbackToAndroidMessageHandler (string? typeName)
+		{
+			var handler = GetHttpMessageHandler (typeName);
+
+			Assert.IsNotNull (handler, "GetHttpMessageHandler returned null");
+			Assert.IsNotNull ("Xamarin.Android.Net.AndroidMessageHandler", handler.GetType ().FullName);
+		}
+
+		[Test]
+		[TestCase ("System.Net.Http.HttpClientHandler")] // the type name doesn't contain the name of the assembly so the type won't be found
+		[TestCase ("Some.Nonexistent.Type")]
+		public void GetHttpMessageHandler_FallbackForInaccessibleTypes (string typeName)
+		{
+			var handler = GetHttpMessageHandler (typeName);
+
+			Assert.IsNotNull (handler, "GetHttpMessageHandler returned null");
+			Assert.IsNotNull ("Xamarin.Android.Net.AndroidMessageHandler", handler.GetType ().FullName);
+		}
+
+		[Test]
+		[TestCase ("Xamarin.Android.Net.AndroidMessageHandler")]
+#if !NET
+		[TestCase ("Xamarin.Android.Net.AndroidClientHandler")]
+		[TestCase ("System.Net.Http.HttpClientHandler, System.Net.Http")]
+#endif
+		public void GetHttpMessageHandler_OverridesDefaultValue (string typeName)
+		{
+			var handler = GetHttpMessageHandler (typeName);
+
+			Assert.IsNotNull (handler, "GetHttpMessageHandler returned null");
+
+			// type's FullName doesn't contain the assembly name
+			var indexOfComma = typeName.IndexOf(',');
+			var expectedTypeName = indexOfComma > 0 ? typeName.Substring(0, indexOfComma) : typeName;
+			Assert.AreEqual (expectedTypeName, handler.GetType ().FullName);
+		}
+
+		private static object? GetHttpMessageHandler (string? typeName)
+		{
+			var method = typeof (AndroidEnvironment).GetMethod ("GetHttpMessageHandler", BindingFlags.Static | BindingFlags.NonPublic);
+			lock (envLock) {
+				Environment.SetEnvironmentVariable (EnvironmentVariable, typeName);
+				return method.Invoke (null, null);
+			}
+		}
+	}
+}

tests/Mono.Android-Tests/Mono.Android-Test.Shared.projitems

@@ -19,6 +19,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Android.Content.Res\AssetManagerTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Content\IntentTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Graphics\NinePatchTests.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Android.Runtime\AndroidEnvironmentTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Runtime\CharSequenceTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Runtime\InputStreamInvokerTest.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Runtime\JavaCollectionTest.cs" />