Add test for Binding Projects Incremental Build

[dellis1972]

Fixes #8658
Fixes #8698

The PR fixes an issue where the bindings files in intermediate generated folder get deleted on subsequent builds. This causes the entire binding process to run again, slowing down builds.
The root fo the problem is the _ClearGeneratedManagedBindings target. It was designed to clean out the generated folder in the case where no binding libraries were present. However it turns out if was running during a design time build! During design time builds the binding library item groups are not evaludated, so the _ClearGeneratedManagedBindings would run.. deleting everything.

To fix this issue we only run the _ClearGeneratedManagedBindings in a standard build. The good thing is this allowed us time to take a look at our incremental build process for bindings. The binding generator does produce a projitems file which lists all the files it generated. We can use that data to incrementally clean up the generated folder of old files. This way code that is no longer needed can be removed. While we know the linker will eventually remove unused classes and types, it is better to not have to compile this stuff in the first place.

[jonpryor]

I'm surprised that this works, given that the generator-generated .projitems file uses XML Namespaces:

• https://github.com/xamarin/java.interop/blob/651de42732d194cee5a45fae45feda37706a8c16/tools/generator/GenerationInfo.cs#L40-L68
• https://github.com/xamarin/java.interop/blob/651de42732d194cee5a45fae45feda37706a8c16/tests/generator-Tests/expected.ji/TestInterface/Mono.Android.projitems

Wouldn't compileItems always be an empty iterator right now?

[dellis1972]

It worked in my tests?

[jonpryor]

Not sure why it worked in the tests; if I apply this patch:

diff --git a/src/Xamarin.Android.Build.Tasks/Tasks/Generator.cs b/src/Xamarin.Android.Build.Tasks/Tasks/Generator.cs
index a3ee0e211..7b58ed3f4 100644
--- a/src/Xamarin.Android.Build.Tasks/Tasks/Generator.cs
+++ b/src/Xamarin.Android.Build.Tasks/Tasks/Generator.cs
@@ -142,11 +142,13 @@ namespace Xamarin.Android.Tasks
 			var result = base.RunTask ();
 			List<ITaskItem> files = new List<ITaskItem> ();
 			if (result && GeneratedFileListFile != null && File.Exists (GeneratedFileListFile)) {
+				Log.LogDebugMessage ($"# jonp: SHOULD process {GeneratedFileListFile}");
 				var doc = XDocument.Load (GeneratedFileListFile);
 				var compileItems = doc.XPathSelectElements ("//Project/ItemGroup/Compile");
 				foreach (var item in compileItems) {
 					var file = item.Attribute ("Include");
 					if (file != null && File.Exists (file.Value)) {
+						Log.LogDebugMessage ($"# jonp: found //Compile/@Include value: {file.Value}");
 						files.Add (new TaskItem (file.Value));
 					}
 				}

then build this branch locally, and do the equivalent of:

% dotnet new androidlib
% cat > Example.java <<EOF
package e;

public class Example {
    public static void e() {
    }
}
EOF
% dotnet build -v:diag > b.txt

Then I see:

# jonp: SHOULD process obj/Debug/net9.0-android/generated/src/gxa-8706.projitems

but I don't see # jonp: found //Compile/@Include value. I also believe that the GeneratedFiles output property is empty, as it doesn't appear in the diagnostic log.

The contents of gxa-8706.projitems:

<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <DefineConstants>$(DefineConstants);ANDROID_1;ANDROID_2;ANDROID_3;ANDROID_4;ANDROID_5;ANDROID_6;ANDROID_7;ANDROID_8;ANDROID_9;ANDROID_10;ANDROID_11;ANDROID_12;ANDROID_13;ANDROID_14;ANDROID_15;ANDROID_16;ANDROID_17;ANDROID_18;ANDROID_19;ANDROID_20;ANDROID_21;ANDROID_22;ANDROID_23;ANDROID_24;ANDROID_25;ANDROID_26;ANDROID_27;ANDROID_28;ANDROID_29;ANDROID_30;ANDROID_31;ANDROID_32;ANDROID_33;ANDROID_34</DefineConstants>
  </PropertyGroup>
  <!-- Classes -->
  <ItemGroup>
    <Compile Include="$(MSBuildThisFileDirectory)E.Example.cs" />
    <Compile Include="$(MSBuildThisFileDirectory)Java.Interop.__TypeRegistrations.cs" />
    <Compile Include="$(MSBuildThisFileDirectory)__NamespaceMapping__.cs" />
  </ItemGroup>
  <!-- Enums -->
  <ItemGroup />
</Project>

[jonpryor]

…so I use this stackoverflow answer to add use of an XmlNamespaceManager so that the XPath query can. use namespaces, resulting in:

		void AddGeneratedFiles ()
		{
			List<ITaskItem> files = new List<ITaskItem> ();
			Log.LogDebugMessage ($"# jonp: SHOULD process {GeneratedFileListFile}");
			var doc = XDocument.Load (GeneratedFileListFile);

			var nsmgr = new XmlNamespaceManager(new NameTable());
			nsmgr.AddNamespace ("msb", "http://schemas.microsoft.com/developer/msbuild/2003");

			var compileItems = doc.XPathSelectElements ("//msb:Project/msb:ItemGroup/msb:Compile", nsmgr);
			foreach (var item in compileItems) {
				var file = item.Attribute ("Include");
				if (file != null && File.Exists (file.Value)) {
					Log.LogDebugMessage ($"# jonp: found //Compile/@Include value: {file.Value}");
					files.Add (new TaskItem (file.Value));
				}
			}
			GeneratedFiles = files.ToArray ();
		}

and realize, to my horror, that it still doesn't work, because of the File.Exists() check: the value of file.Attribute("Include").Value will be $(MSBuildThisFileDirectory)E.Example.cs, which will not exist. It will (almost certainly) never exist, because a directory literally named $(MSBuildThisFileDirectory) will (almost certainly) never exist. (Unless you're bananas and do mkdir '$(MSBuildThisFileDirectory)', but who would do such a thing?)

This in turn means that, as-is, @(_GeneratedBindingFiles) will always be empty, which suggests that most of the changes to the GenerateBindings target are in fact dead code.

[jonpryor]

…which is a long-winded way of explaining that I think this doesn't work, and is effectively dead (unexecuted) code.

I'm trying to remove it; let's see if anything breaks.

[jonpryor]

Draft commit message:

Fixes: https://github.com/xamarin/xamarin-android/issues/8658
Fixes: https://github.com/xamarin/xamarin-android/issues/8698

Design-time builds don't play nicely with binding project builds:

	% dotnet new androidlib
	% cat > Example.java <<EOF
	package e;

	public class Example {
	    public static void e() {
	    }
	}
	EOF
	% dotnet build -p:DesignTimeBuild=true -v:diag

After this initial Design-Time build, we have the following generated
source code for the binding:

	% find obj -iname \*.cs | xargs ls -l
	-rw-r--r--  1 user staff   197 Mar 25 19:22 obj/Debug/net8.0-android/.NETCoreApp,Version=v8.0.AssemblyAttributes.cs
	-rw-r--r--  1 user staff   441 Mar 25 19:22 obj/Debug/net8.0-android/__Microsoft.Android.Resource.Designer.cs
	-rw-r--r--  1 user staff  2975 Mar 25 19:22 obj/Debug/net8.0-android/generated/src/E.Example.cs
	-rw-r--r--  1 user staff  1518 Mar 25 19:22 obj/Debug/net8.0-android/generated/src/Java.Interop.__TypeRegistrations.cs
	-rw-r--r--  1 user staff   696 Mar 25 19:22 obj/Debug/net8.0-android/generated/src/__NamespaceMapping__.cs
	-rw-r--r--  1 user staff  1094 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.AssemblyInfo.cs
	-rw-r--r--  1 user staff   407 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.GlobalUsings.g.cs

Run a Design-Time build *again*:

	% dotnet build -p:DesignTimeBuild=true -v:diag

…and we're now missing files (?!):

	% find obj -iname \*.cs | xargs ls -l                     
	-rw-r--r--  1 user staff   197 Mar 25 19:22 obj/Debug/net8.0-android/.NETCoreApp,Version=v8.0.AssemblyAttributes.cs
	-rw-r--r--  1 user staff   441 Mar 25 19:22 obj/Debug/net8.0-android/__Microsoft.Android.Resource.Designer.cs
	-rw-r--r--  1 user staff  1094 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.AssemblyInfo.cs
	-rw-r--r--  1 user staff   407 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.GlobalUsings.g.cs

In particular, `$(IntermediateOutputPath)generated/*/**.cs` is gone,
including `E.Example.cs`!

The result of this is that Design-Time builds and "normal" builds
"fight" each other, constantly generating and deleting files, slowing
down incremental builds.

The root of the problem is the `_ClearGeneratedManagedBindings`
target: It was designed to clean out the `generated` folder in the
case where no binding libraries were present.  However, it turns out
it was running during a design time build!  During design time builds
the binding library item groups are not evaluated, so the
`_ClearGeneratedManagedBindings` target would run, deleting everything.

Fix this by ensuring we only run the `_ClearGeneratedManagedBindings`
target in in "standard"/*non*-Design-Time builds.

<<<
(jonp: TODO: remove this section?  Since the code to evaluate a
`.projitems` file doesn't appear to *work*, we can either:

 1. Remove the "dead"/non-executed code, or
 2. Leave it in but mention that it doesn't work yet.)

The good thing is this allowed us time to take a look at our
incremental build process for bindings: The binding generator does
produce a `.projitems` file which lists all the files it generated.
We can use that data to incrementally clean up the `generated` folder
of old files.  This way code that is no longer needed can be removed.
While we know the linker will eventually remove unused classes and
types, it is better to not have to compile this stuff in the first
place. 
>>>