[2020-02] Allow users to switch to MonoWebRequestHandler on Android via UI (#18785)

* Bring back old HttpHandler for Android
Co-authored-by: Egor Bogatov <egorbo@gmail.com>

7 files changed, 123 insertions, 17 deletions

diff --git a/mcs/class/System.Net.Http/HttpClientHandler.SocketsHandler.Android.cs b/mcs/class/System.Net.Http/HttpClientHandler.SocketsHandler.Android.cs
new file mode 100644
index 00000000000..c483c28f03b
--- /dev/null
+++ b/mcs/class/System.Net.Http/HttpClientHandler.SocketsHandler.Android.cs
@@ -0,0 +1,20 @@
+namespace System.Net.Http
+{
+	partial class HttpClientHandler : HttpMessageHandler
+	{
+		static IMonoHttpClientHandler CreateDefaultHandler ()
+		{
+			string envvar = Environment.GetEnvironmentVariable ("XA_HTTP_CLIENT_HANDLER_TYPE")?.Trim ();
+			if (envvar?.StartsWith("System.Net.Http.MonoWebRequestHandler", StringComparison.InvariantCulture) == true)
+				return new MonoWebRequestHandler ();
+			// Ignore other types of handlers here (e.g. AndroidHttpHandler) to keep the old behavior
+			// and always create SocketsHttpHandler for code like this if MonoWebRequestHandler was not specified:
+			//
+			//    var handler = new HttpClientHandler { Credentials = ... };
+			//    var httpClient = new HttpClient (handler);
+			//
+			// AndroidHttpHandler is used only when we use the parameterless ctor of HttpClient
+			return new SocketsHttpHandler (); 
+		}
+	}
+}
diff --git a/mcs/class/System.Net.Http/System.Net.Http.csproj b/mcs/class/System.Net.Http/System.Net.Http.csproj
index ccd76ef08cf..554e2d19150 100644
--- a/mcs/class/System.Net.Http/System.Net.Http.csproj
+++ b/mcs/class/System.Net.Http/System.Net.Http.csproj
@@ -1328,7 +1328,7 @@
         <Compile Include="..\..\..\external\corefx\src\System.Net.Http\src\System\Net\Http\StreamContent.cs" />
         <Compile Include="..\..\..\external\corefx\src\System.Net.Http\src\System\Net\Http\StreamToStreamCopy.cs" />
         <Compile Include="..\..\..\external\corefx\src\System.Net.Http\src\System\Net\Http\StringContent.cs" />
-        <Compile Include="HttpClientHandler.SocketsHandler.cs" />
+        <Compile Include="HttpClientHandler.SocketsHandler.Android.cs" />
         <Compile Include="HttpClientHandler.cs" />
         <Compile Include="HttpRequestMessage.Mono.cs" />
         <Compile Include="IMonoHttpClientHandler.cs" />
diff --git a/mcs/class/System.Net.Http/System.Net.Http/HttpClient.android.cs b/mcs/class/System.Net.Http/System.Net.Http/HttpClient.android.cs
index 1ef1af81b77..9b899bcffd8 100644
--- a/mcs/class/System.Net.Http/System.Net.Http/HttpClient.android.cs
+++ b/mcs/class/System.Net.Http/System.Net.Http/HttpClient.android.cs
@@ -8,22 +8,31 @@ namespace System.Net.Http {
 
 		static HttpMessageHandler CreateDefaultHandler ()
 		{
-			Type type = Type.GetType("Android.Runtime.AndroidEnvironment, Mono.Android");
-			if (type == null)
-				return GetFallback ("Invalid Mono.Android assembly? Cannot find Android.Runtime.AndroidEnvironment");
-
-			MethodInfo method = type.GetMethod ("GetHttpMessageHandler", BindingFlags.Static | BindingFlags.NonPublic);
-			if (method == null)
-				return GetFallback ("Your Xamarin.Android version does not support obtaining of the custom HttpClientHandler");
-
-			object ret = method.Invoke (null, null);
-			if (ret == null)
-				return GetFallback ("Xamarin.Android returned no custom HttpClientHandler");
-
-			var handler = ret as HttpMessageHandler;
-			if (handler == null)
-				return GetFallback ($"{ret?.GetType()} is not a valid HttpMessageHandler");
-			return handler;
+			string envvar = Environment.GetEnvironmentVariable ("XA_HTTP_CLIENT_HANDLER_TYPE")?.Trim ();
+
+			if (string.IsNullOrEmpty (envvar))
+				return GetFallback ($"XA_HTTP_CLIENT_HANDLER_TYPE is empty");
+
+			if (envvar?.StartsWith("System.Net.Http.MonoWebRequestHandler", StringComparison.InvariantCulture) == true)
+				return new HttpClientHandler (new MonoWebRequestHandler ());
+
+			Type handlerType = Type.GetType (envvar, false);
+			if (handlerType == null && !envvar.Contains (", "))
+			{
+				// if assembly was not specified - look for it in Mono.Android too 
+				// (e.g. AndroidHttpHandler is there)
+				handlerType = Type.GetType (envvar + ", Mono.Android", false);
+			}
+
+			if (handlerType == null)
+				return GetFallback ($"'{envvar}' type was not found");
+
+			object handlerObj = Activator.CreateInstance (handlerType);
+
+			if (handlerObj is HttpMessageHandler msgHandler)
+				return msgHandler;
+
+			return GetFallback ($"{handlerObj?.GetType ()} is not a valid HttpMessageHandler or MonoWebRequestHandler");
 		}
 
 		static HttpMessageHandler GetFallback (string message)
diff --git a/mcs/class/System.Net.Http/UnitTests/HttpClientHandlerTests.Android.cs b/mcs/class/System.Net.Http/UnitTests/HttpClientHandlerTests.Android.cs
new file mode 100644
index 00000000000..00c1cda6405
--- /dev/null
+++ b/mcs/class/System.Net.Http/UnitTests/HttpClientHandlerTests.Android.cs
@@ -0,0 +1,74 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Text;
+using System.Net.Http;
+using System.Reflection;
+
+using Xunit;
+using Xunit.Abstractions;
+
+namespace System.Net.Http.Tests
+{
+	public class HttpClientHandlerTestsAndroid
+	{
+		static Type GetInnerHandlerType (HttpClient httpClient)
+		{
+			BindingFlags bflasgs = BindingFlags.Instance | BindingFlags.NonPublic;
+			FieldInfo handlerField = typeof (HttpMessageInvoker).GetField("_handler", bflasgs);
+			Assert.NotNull (handlerField);
+			object handler = handlerField.GetValue (httpClient);
+			FieldInfo innerHandlerField = handler.GetType ().GetField ("_delegatingHandler", bflasgs);
+			Assert.NotNull (handlerField);
+			object innerHandler = innerHandlerField.GetValue (handler);
+			return innerHandler.GetType ();
+		}
+
+		[Fact]
+		public void TestEnvVarSwitchForInnerHttpHandler ()
+		{
+			const string xaHandlerKey = "XA_HTTP_CLIENT_HANDLER_TYPE";
+			var prevHandler = Environment.GetEnvironmentVariable (xaHandlerKey);
+
+			// ""
+			Environment.SetEnvironmentVariable (xaHandlerKey, "");
+			var httpClient1 = new HttpClient ();
+			Assert.Equal ("SocketsHttpHandler", GetInnerHandlerType (httpClient1).Name);
+
+			var handler2 = new HttpClientHandler ();
+			var httpClient2 = new HttpClient (handler2);
+			Assert.Equal ("SocketsHttpHandler", GetInnerHandlerType (httpClient2).Name);
+
+			// "System.Net.Http.MonoWebRequestHandler"
+			Environment.SetEnvironmentVariable (xaHandlerKey, "System.Net.Http.MonoWebRequestHandler");
+			var httpClient3 = new HttpClient ();
+			Assert.Equal ("MonoWebRequestHandler", GetInnerHandlerType (httpClient3).Name);
+
+			var handler4 = new HttpClientHandler ();
+			var httpClient4 = new HttpClient (handler4);
+			Assert.Equal ("MonoWebRequestHandler", GetInnerHandlerType (httpClient4).Name);
+
+			// "System.Net.Http.MonoWebRequestHandler, System.Net.Http"
+			Environment.SetEnvironmentVariable (xaHandlerKey, "System.Net.Http.MonoWebRequestHandler, System.Net.Http");
+			var httpClient5 = new HttpClient ();
+			Assert.Equal ("MonoWebRequestHandler", GetInnerHandlerType (httpClient5).Name);
+
+			var handler6 = new HttpClientHandler ();
+			var httpClient6 = new HttpClient (handler6);
+			Assert.Equal ("MonoWebRequestHandler", GetInnerHandlerType (httpClient6).Name);
+
+			// "System.Net.Http.HttpClientHandler"
+			Environment.SetEnvironmentVariable (xaHandlerKey, "System.Net.Http.HttpClientHandler");
+			var httpClient7 = new HttpClient ();
+			Assert.Equal ("SocketsHttpHandler", GetInnerHandlerType (httpClient7).Name);
+
+			var handler8 = new HttpClientHandler ();
+			var httpClient8 = new HttpClient (handler8);
+			Assert.Equal ("SocketsHttpHandler", GetInnerHandlerType (httpClient8).Name);
+
+			Environment.SetEnvironmentVariable (xaHandlerKey, prevHandler);
+		}
+	}
+}
diff --git a/mcs/class/System.Net.Http/UnitTests/monodroid_System.Net.Http.UnitTests_xtest.dll.sources b/mcs/class/System.Net.Http/UnitTests/monodroid_System.Net.Http.UnitTests_xtest.dll.sources
index ea8b52eca73..9334d2fcca4 100644
--- a/mcs/class/System.Net.Http/UnitTests/monodroid_System.Net.Http.UnitTests_xtest.dll.sources
+++ b/mcs/class/System.Net.Http/UnitTests/monodroid_System.Net.Http.UnitTests_xtest.dll.sources
@@ -1 +1,2 @@
 #include unit-tests.sources
+HttpClientHandlerTests.Android.cs
diff --git a/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.exclude.sources b/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.exclude.sources
index c1788b13bf2..9164cfe38ef 100644
--- a/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.exclude.sources
+++ b/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.exclude.sources
@@ -1 +1,2 @@
 HttpClient.DefaultHandler.cs
+HttpClientHandler.SocketsHandler.cs
diff --git a/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.sources b/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.sources
index eca4fd33a42..2397da00e5e 100644
--- a/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.sources
+++ b/mcs/class/System.Net.Http/monodroid_System.Net.Http.dll.sources
@@ -1,2 +1,3 @@
 #include socketshandler.sources
 System.Net.Http/HttpClient.android.cs
+HttpClientHandler.SocketsHandler.Android.cs