[Xamarin.Android.Build.Tasks] Use GPL'd ProGuard (#386)

**Background**: many Android SDK features rely on [ProGuard][0], a
Java bytecode manipulation library. Some of these features include
linking (shrinking) bytecode and [multidex][1].

Xamarin.Android has historically relied on the version of ProGuard
bundled with the Android SDK, which is how the `<Proguard/>` task and
`@(ProguardConfiguration)` build actions work.

Unfortunately, the version of ProGuard bundled with the Android SDK
is, at this point, *ancient* (ProGuard v4.7; current version is v5.3),
and (more importantly) ProGuard v4.7 doesn't support JDK 1.8 bytecode.

Meaning if the end-developer references a Java library targeting
JDK 1.8 -- e.g. the Android Support Library v25 -- then the version of
ProGuard bundled with the Android SDK *cannot process that library*.
This in turn means that the `<Proguard/>` task and
`@(ProguardConfiguration)` build actions don't work/are useless.

Native Java development "works around" this problem by using Gradle as
the build system, and Java projects are configured to download and
install a newer version of ProGuard via Gradle.

Xamarin.Android projects do not use Gradle, nor will they.
(At least not in the foreseeable future.)

**Solution**: We have communicated with Eric Lafortune, maintainer of
ProGuard, about distributing a newer ProGuard with Xamarin.Android.
He replied:

> From a licensing point of view, you shouldn't need an exception to
> the GPL in many practical configurations:
>
> * if you indeed run ProGuard in a separate JVM

The `<Proguard/>` task is a [`ToolTask`][2], which operates by
creating a new process to invoke the specified tool. We thus are
*already* using ProGuard within a "separate JVM", and no architectural
change is required to comply with ProGuard's licensing without
altering Xamarin.Android's licensing.

The current ProGuard sources (v5.3.2) have been imported into the
[xamarin/proguard][3] module, and we will be using the
`xamarin/proguard` module as a git submodule for build purposes.

[0]: https://www.guardsquare.com/en/proguard
[1]: https://developer.android.com/studio/build/multidex.html
[2]: https://msdn.microsoft.com/en-us/library/microsoft.build.utilities.tooltask.aspx
[3]: http://github.com/xamarin/proguard/

Author: jonpryor

.gitmodules

@@ -32,3 +32,7 @@
 [submodule "external/linker"]
 	path = external/linker
 	url = https://github.com/mono/linker.git
+[submodule "external/proguard"]
+	path = external/proguard
+	url = https://github.com/xamarin/proguard.git
+	branch = master

Configuration.props

@@ -49,6 +49,7 @@
     <OpenTKSourceDirectory>$(MSBuildThisFileDirectory)external\opentk</OpenTKSourceDirectory>
     <LibZipSourceDirectory Condition=" '$(LibZipSourceDirectory)' == '' ">$(MSBuildThisFileDirectory)external\libzip</LibZipSourceDirectory>
     <LibZipSharpSourceDirectory Condition=" '$(LibZipSharpSourceDirectory)' == '' ">$(MSBuildThisFileDirectory)external\LibZipSharp</LibZipSharpSourceDirectory>
+    <ProGuardSourceDirectory Condition=" '$(ProGuardSourceDirectory)' == '' ">$(MSBuildThisFileDirectory)external\proguard</ProGuardSourceDirectory>
     <SqliteSourceDirectory Condition=" '$(SqliteSourceDirectory)' == '' ">$(MSBuildThisFileDirectory)external\sqlite</SqliteSourceDirectory>
     <XamarinAndroidSourcePath>$(MSBuildThisFileDirectory)</XamarinAndroidSourcePath>
     <AllSupported32BitTargetAndroidAbis>armeabi;armeabi-v7a;x86</AllSupported32BitTargetAndroidAbis>
@@ -73,6 +74,7 @@
     <OpenTKSourceFullPath>$([System.IO.Path]::GetFullPath ('$(OpenTKSourceDirectory)'))</OpenTKSourceFullPath>
     <LibZipSourceFullPath>$([System.IO.Path]::GetFullPath ('$(LibZipSourceDirectory)'))</LibZipSourceFullPath>
     <LibZipSharpSourceFullPath>$([System.IO.Path]::GetFullPath ('$(LibZipSharpSourceDirectory)'))</LibZipSharpSourceFullPath>
+    <ProGuardSourceFullPath>$([System.IO.Path]::GetFullPath ('$(ProGuardSourceDirectory)'))</ProGuardSourceFullPath>
   </PropertyGroup>
   <PropertyGroup>
     <AdbToolPath Condition=" '$(AdbToolPath)' == '' ">$(AndroidSdkFullPath)\platform-tools</AdbToolPath>

Xamarin.Android.sln

@@ -99,6 +99,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Xamarin.Android.Cecil.Mdb",
 EndProject
 Project("{9344BDBB-3E7F-41FC-A0DD-8665D75EE146}") = "dependencies", "build-tools\dependencies\dependencies.mdproj", "{C845ECC0-2ED3-498E-8EA8-02EF7AC6E9AD}"
 EndProject
+Project("{9344BDBB-3E7F-41FC-A0DD-8665D75EE146}") = "proguard", "src\proguard\proguard.mdproj", "{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|AnyCPU = Debug|AnyCPU
@@ -457,6 +459,14 @@ Global
 		{C845ECC0-2ED3-498E-8EA8-02EF7AC6E9AD}.XAIntegrationDebug|AnyCPU.Build.0 = Debug|Any CPU
 		{C845ECC0-2ED3-498E-8EA8-02EF7AC6E9AD}.XAIntegrationRelease|AnyCPU.ActiveCfg = Debug|Any CPU
 		{C845ECC0-2ED3-498E-8EA8-02EF7AC6E9AD}.XAIntegrationRelease|AnyCPU.Build.0 = Debug|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.Debug|AnyCPU.ActiveCfg = Debug|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.Debug|AnyCPU.Build.0 = Debug|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.Release|AnyCPU.ActiveCfg = Release|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.Release|AnyCPU.Build.0 = Release|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.XAIntegrationDebug|AnyCPU.ActiveCfg = Debug|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.XAIntegrationDebug|AnyCPU.Build.0 = Debug|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.XAIntegrationRelease|AnyCPU.ActiveCfg = Debug|Any CPU
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}.XAIntegrationRelease|AnyCPU.Build.0 = Debug|Any CPU
 	EndGlobalSection
 	GlobalSection(NestedProjects) = preSolution
 		{8FF78EB6-6FC8-46A7-8A15-EBBA9045C5FA} = {E351F97D-EA4F-4E7F-AAA0-8EBB1F2A4A62}
@@ -504,6 +514,7 @@ Global
 		{C0487169-8F81-497F-919E-EB42B1D0243F} = {864062D3-A415-4A6F-9324-5820237BA058}
 		{C876DA71-8573-4CEF-9149-716D72455ED4} = {E351F97D-EA4F-4E7F-AAA0-8EBB1F2A4A62}
 		{C845ECC0-2ED3-498E-8EA8-02EF7AC6E9AD} = {E351F97D-EA4F-4E7F-AAA0-8EBB1F2A4A62}
+		{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1} = {04E3E11E-B47D-4599-8AFC-50515A95E715}
 	EndGlobalSection
 	GlobalSection(MonoDevelopProperties) = preSolution
 		Policies = $0

external/proguard

@@ -633,6 +633,15 @@
       <Name>Xamarin.Android.Tools.BootstrapTasks</Name>
       <ReferenceOutputAssembly>False</ReferenceOutputAssembly>
     </ProjectReference>
+    <!--
+      *Logically* needs to be built first because executing the <Proguard/>
+      task will require the outputs of `proguard.mdproj`.
+      -->
+    <ProjectReference Include="..\proguard\proguard.mdproj">
+      <Project>{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}</Project>
+      <Name>proguard</Name>
+      <ReferenceOutputAssembly>False</ReferenceOutputAssembly>
+    </ProjectReference>
     <ProjectReference Include="$(LibZipSharpSourceFullPath)\libZipSharp.csproj">
       <Project>{E248B2CA-303B-4645-ADDC-9D4459D550FD}</Project>
       <Name>libZipSharp</Name>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

@@ -756,7 +756,7 @@ because xbuild doesn't support framework reference assemblies.
 		/>
 	</CreateProperty>
 
-	<CreateProperty Value="$(AndroidSdkDirectory)\tools\proguard\">
+	<CreateProperty Value="$(MonoAndroidToolsDirectory)\proguard\">
 		<Output TaskParameter="Value" PropertyName="ProguardToolPath"
 				Condition="'$(ProguardToolPath)' == ''"
 		/>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -0,0 +1,22 @@
+<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ItemType>GenericProject</ItemType>
+    <ProjectGuid>{4B9D96BB-95AB-44E8-9F87-13B12C8BCED1}</ProjectGuid>
+    <OutputPath>..\..\bin\$(Configuration)\lib\mandroid\proguard</OutputPath>
+  </PropertyGroup>
+  <Import Project="..\..\Configuration.props" />
+  <PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
+  </PropertyGroup>
+  <Import Project="$(MSBuildBinPath)\Microsoft.Common.targets" />
+  <PropertyGroup>
+    <BuildDependsOn>
+      ResolveReferences;
+      _BuildProGuard
+    </BuildDependsOn>
+  </PropertyGroup>
+  <Import Project="proguard.targets" />
+</Project>

src/proguard/proguard.mdproj

@@ -0,0 +1,37 @@
+<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Target Name="_BuildProGuard">
+    <Exec
+        Command="$(AntDirectory)\bin\ant -verbose -f buildscripts\build.xml proguard"
+        WorkingDirectory="$(ProGuardSourceFullPath)"
+    />
+    <MakeDir Directories="$(OutputPath)" />
+    <Copy
+        SourceFiles="..\..\external\proguard\lib\proguard.jar"
+        DestinationFiles="$(OutputPath)\lib\proguard.jar"
+    />
+    <Copy
+        SourceFiles="..\..\external\proguard\bin\proguard.bat"
+        DestinationFiles="$(OutputPath)\bin\proguard.bat"
+    />
+    <Copy
+        SourceFiles="..\..\external\proguard\bin\proguard.sh"
+        DestinationFiles="$(OutputPath)\bin\proguard.sh"
+    />
+    <Copy
+        SourceFiles="..\..\external\proguard\docs\license.html"
+        DestinationFiles="$(OutputPath)\license.html"
+    />
+  </Target>
+  <Target Name="_CleanProGuard"
+      AfterTargets="Clean">
+    <Delete Files="$(OutputPath)\license.html" />
+    <Delete Files="$(OutputPath)\lib\proguard.jar" />
+    <Delete Files="$(OutputPath)\bin\proguard.sh" />
+    <Delete Files="$(OutputPath)\bin\proguard.bat" />
+    <Exec
+        Command="$(AntDirectory)\bin\ant -f buildscripts\build.xml clean"
+        WorkingDirectory="$(ProGuardSourceFullPath)"
+    />
+  </Target>
+</Project>
+