[Xamarin.Android.Build.Tasks] Add XA1031 error (#7448)

Fixes: #7326

Commit d3e8767 added a requirement that, under .NET 6+, the
`$(AndroidHttpClientHandlerType)` MUST inherit from
[`System.Net.Http.HttpMessageHandler`][0].  This was unfortunately a
change from Classic, which allowed/required that
[`System.Net.Http.HttpClientHandler`][1] be the base class.
Allowing `HttpClientHandler` to be in the inheritance hierarchy for
`$(AndroidHttpClientHandlerType)` would result in a
`NullReferenceException` at runtime under .NET 6.

Commit d3e8767 contained a TODO:

> TODO: [Emit a build-time warning][2] if
> `$(AndroidHttpClientHandlerType)` is set to an invalid value

Implement this TODO: add a build-time check that verifies that
`$(AndroidHttpClientHandlerType)` is a valid type for the target,
e.g. is an `HttpMessageHandler` subclass on .NET 6.

If `$(AndroidHttpClientHandlerType)` is not a valid type, then an
XA1031 error is raised:

	error XA1031: The 'AndroidHttpClientHandlerType' has an invalid value of '{0}' please check your project settings.

Additionally, if `$(AndroidHttpClientHandlerType)` is
`System.Net.Http.SocketsHttpHandler, System.Net.Http`, *and*
`$(UseNativeHttpHandler)` isn't set, then
set `$(UseNativeHttpHandler)` to false.  This is necessary to ensure
that `SocketsHttpHandler` is preserved by the linker.

[0]: https://learn.microsoft.com/en-us/dotnet/api/system.net.http.httpmessagehandler?view=net-6.0
[1]: https://learn.microsoft.com/en-us/dotnet/api/system.net.http.httpclienthandler?view=net-6.0
[2]: #7326

Documentation/guides/messages/README.md

@@ -130,6 +130,9 @@ or 'Help->Report a Problem' in Visual Studio for Mac.
 + [XA1027](xa1027.md): The 'EnableProguard' MSBuild property is set to 'true' and the 'AndroidLinkTool' MSBuild property is empty, so 'AndroidLinkTool' will default to 'proguard'.
 + [XA1028](xa1028.md): The 'AndroidEnableProguard' MSBuild property is set to 'true' and the 'AndroidLinkTool' MSBuild property is empty, so 'AndroidLinkTool' will default to 'proguard'.
 + [XA1029](xa1029.md): The 'AotAssemblies' MSBuild property is deprecated. Edit the project file in a text editor to remove this property, and use the 'RunAOTCompilation' MSBuild property instead.
++ [XA1031](xa1031.md): The 'AndroidHttpClientHandlerType' has an invalid value.
++ [XA1032](xa1032.md):Failed to resolve '{0}' from '{1}'. Please check your `AndroidHttpClientHandlerType` setting.
++ [XA1033](xa1033.md): Could not resolve '{0}'. Please check your `AndroidHttpClientHandlerType` setting.
 
 ## XA2xxx: Linker
 

Documentation/guides/messages/xa1031.md

@@ -0,0 +1,20 @@
+---
+title: Xamarin.Android error XA1031
+description: XA1031 error code
+ms.date: 10/10/2022
+---
+# Xamarin.Android error XA1031
+
+## Example messages
+
+```
+The 'AndroidHttpClientHandlerType' property value 'Foo.Bar.HttpHander, MyApp' must derive from 'System.Net.Http.HttpMessageHandler'.
+Please change the value to an assembly-qualifed type name which inherits from '{1}' or remove the property completely.
+```
+
+## Solution
+
+Edit your csproj directly and change the 'AndroidHttpClientHandlerType' to
+a valid value.
+
+Valid values can be found at `~/android/deploy-test/building-apps/build-properties.md#AndroidHttpClientHandlerType`.

Documentation/guides/messages/xa1032.md

@@ -0,0 +1,19 @@
+---
+title: Xamarin.Android error XA1032
+description: XA1032 error code
+ms.date: 10/10/2022
+---
+# Xamarin.Android error XA1032
+
+## Example messages
+
+```
+Failed to resolve '{0}' from '{1}'. Please check your `AndroidHttpClientHandlerType` setting.
+```
+
+## Solution
+
+Edit your csproj directly and change the 'AndroidHttpClientHandlerType' to
+a valid value.
+
+Valid values can be found at `~/android/deploy-test/building-apps/build-properties.md#AndroidHttpClientHandlerType`.

Documentation/guides/messages/xa1033.md

@@ -0,0 +1,19 @@
+---
+title: Xamarin.Android error XA1033
+description: XA1033 error code
+ms.date: 10/10/2022
+---
+# Xamarin.Android error XA1033
+
+## Example messages
+
+```
+Could not resolve '{0}'. Please check your `AndroidHttpClientHandlerType` setting.
+```
+
+## Solution
+
+Edit your csproj directly and change the 'AndroidHttpClientHandlerType' to
+a valid value.
+
+Valid values can be found at `~/android/deploy-test/building-apps/build-properties.md#AndroidHttpClientHandlerType`.

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.BuildOrder.targets

@@ -135,10 +135,11 @@ projects, these properties are set in Xamarin.Android.Legacy.targets.
       _IncludeLayoutBindingSources;
       AddLibraryJarsToBind;
       $(CompileDependsOn);
+      _CheckAndroidHttpClientHandlerType;
     </CompileDependsOn>
     <CoreCompileDependsOn>
       UpdateGeneratedFiles;
-      $(CoreCompileDependsOn)
+      $(CoreCompileDependsOn);
     </CoreCompileDependsOn>
     <DeferredBuildDependsOn>
       $(DeferredBuildDependsOn);

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets

@@ -95,6 +95,7 @@
     <HttpActivityPropagationSupport Condition="'$(HttpActivityPropagationSupport)' == ''">false</HttpActivityPropagationSupport>
     <InvariantGlobalization Condition="'$(InvariantGlobalization)' == ''">false</InvariantGlobalization>
     <StartupHookSupport Condition="'$(StartupHookSupport)' == ''">false</StartupHookSupport>
+    <UseNativeHttpHandler Condition=" $(AndroidHttpClientHandlerType.Contains ('System.Net.Http.SocketsHttpHandler')) And '$(UseNativeHttpHandler)' == '' ">false</UseNativeHttpHandler>
     <UseNativeHttpHandler Condition="'$(UseNativeHttpHandler)' == ''">true</UseNativeHttpHandler>
     <_AggressiveAttributeTrimming Condition="'$(_AggressiveAttributeTrimming)' == ''">true</_AggressiveAttributeTrimming>
     <NullabilityInfoContextSupport Condition="'$(NullabilityInfoContextSupport)' == ''">false</NullabilityInfoContextSupport>