[xabuild] Support using msbuild from mono 4.8+

[jonpryor]

tools/scripts/xabuild is an MSBuild wrapper to easily build
Xamarin.Android apps from the command-line, using the Xamarin.Android
binaries built in-tree; see e20863e.

xabuild originally invoked xbuild; commit 6805d20 extended that
to support building with msbuild by setting the MSBUILD
environment variable:

    MSBUILD=msbuild .../tools/scripts/xabuild SomeProject.csproj

This use worked with Mono 4.4 and 4.6, but broke -- unnoticed -- in
Mono 4.8 and later. When attempting to use msbuild in more recent
mono versions, the build will fail, cryptically:

    xamarin-android/bin/Debug/lib/xbuild/Xamarin/Android/Xamarin.Android.Common.targets(1007,2):
    error MSB3191: Unable to create directory "/Debug/". Access to the path "/Debug/" is denied.

The directory /Debug/ is mentioned because the
$(BaseIntermediateOutputPath) MSBuild property is not being set.
Normally we'd be creating e.g. obj/Debug, but without
$(BaseIntermediateOutputPath), this becomes /Debug, and everything
breaks.

$(BaseIntermediateOutputPath) isn't set, in turn, because
Microsoft.Common.props isn't being imported.
Microsoft.Common.props isn't being imported because xabuild
overrides the $MSBuildExtensionsPath environment variable, and
MSBuild is trying to import
$(MSBuildExtensionsPath)\...\Microsoft.Common.props, which doesn't
exist when we've overridden $MSBuildExtensionsPath to use our
in-tree build location.

A fix for this could be not overriding $MSBuildExtensionsPath
when using msbuild. There is one problem with this, though: not
overriding $MSBuildExtensionsPath means that the .targets files in
e.g. bin/Debug/lib/xbuild/Xamarin/Android will not be used.
Instead, the system files are used, which is undesirable.

Unfortunatley, this means we need to continue overriding
$MSBuildExtensionsPath, while allowing e.g.
$(MSBuildExtensionsPath)\...\Microsoft.Common.props to be found.
Square this circle by symlinking everything in the system
$MSBuildExtensionsPath directory into our $prefix/lib/xbuild.

Symlinking all these files raises another issue: we don't want all
of these symlinks, or their targets, to be added to the
oss-xamarin.android*.zip files. Support this by creating a
$prefix/lib/xbuild/.__sys_links.txt file, which contains zip -x
patterns (hence the /* suffix for directories), one per line, and
update the make package-oss target to look for and exclude the
contents of these files so that we don't "pollute" the
oss-xamarin.android*.zip file with unnecessary contents.

Finally, some cleanups:

• Only set $XBUILD_FRAMEWORK_FOLDERS_PATH when using xbuild.

• Additionally, specify $(TargetFrameworkRootPath) on msbuild,
  as that actually works.

• Always specify $(MonoAndroidBinDirectory) and
  $(MonoAndroidToolsDirectory), as this mirrors the
  Windows usage instructions.

• Don't require that $ANDROID_NDK_PATH and $ANDROID_SDK_PATH be
  set prior to executing xabuild, or that we be running in-tree
  (to use Paths.targets). The NDK, in particular, isn't always
  required, so forcing that it be set is annoying.

  The build process will error out appropriately if required
  directories cannot be found.

[jonpryor]

Question: one problem with this PR is it basically "hardcodes" 15.0. Is there a better way to symlink files? There are 49 files in e.g. /Library/Frameworks/Mono.framework/Versions/5.2.0/lib/mono/xbuild (Mono 5.2); perhaps we should just symlink all of them?

[radical]

perhaps we should just symlink all of them?
Yeah, I think we should do that for now.

[grendello]

As suggested on slack, we can use

$ echo 'Console.WriteLine (System.IO.Path.GetFullPath (System.IO.Path.GetDirectoryName (typeof (int).Assembly.Location) + "/../../../"))'|csharp
/home/grendel/devel/mono/mono-master/

Which actually runs the default mono and doesn't rely on symlinks or versions. Not pretty, but effective IMO

[jonpryor]

@grendello wrote:

Which actually runs the default mono and doesn't rely on symlinks

I don't see how it doesn't rely on symlinks. The reason for the symlinks is so that our "in-tree $MSBuildExtensionsPath environment mirrors mono's environment," without needing to copy/otherwise sync them together.

Mono's $prefix is only interesting in that it allows us to create the symlink.

[grendello]

It doesn't rely on symlinks in the sense that there isn't a requirement for them to exist before the $prefix detection code is executed.

[jonpryor]

PR updated to entirely remove any requirement for symlinks.

The "problem" with msbuild is that the $MSBuildExtensionsPath environment variable is set. However, it is not required to set it in order to use our in-tree contents. We can instead set $(TargetFrameworkRootPath).

[jonpryor]

...and eca6d9b is wrong. Well, half right.

MSBUILD=msbuild tools/scripts/xabuild /t:SignAndroidPackage /v:diag samples/HelloWorld/*.csproj > b.txt

no longer dies while attempting to create the directory /Debug. So that's a plus.

Additionally, it is trying to use the appropriate assemblies:

      Resolved file path is "/…/work/xamarin-android/bin/Debug/lib/xbuild-frameworks/MonoAndroid/v7.1/Mono.Android.dll". (TaskId:22)

However, the targets files are not used:

Target "_ComputeAndroidAssetsPaths: (TargetId:30)" in file "/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets" from project "/…/work/xamarin-android/samples/HelloWorld/HelloWorld.csproj" (target "UpdateAndroidAssets" depends on it):

Doh!

[jonpryor]

Commit ffd6376 allows the in-tree .targets files to be used, as desired. This is done by symlinking all the things, plus some additional "bookkeeping" so that we don't include the symlinks in the oss-xamarin.android*.zip file.

TODO: what happens with the .vsix?

[jonpryor]

TODO: what happens with the .vsix?

I had mistakenly believed that Jenkins would hit the make create-vsix path, but it doesn't.

Trying locally, it fails:

error VSSDK1025: Could not add the file "../../bin/Debug/lib/xbuild/Microsoft.NuGet.Build.Tasks.dll" to the zip package "bin/Debug/Xamarin.Android.Sdk.vsix".

...which isn't a good sign.

[jonpryor]

.vsix creation should be fixed in 8b8fa5c.