FixAbstractMethods bails out unless assembly references Java.Lang.Object

[sbomer]

FixAbstractMethodsStep won't do any work unless it's running on an assembly that references Java.Lang.Object:

https://github.com/xamarin/xamarin-android/blob/9add575ba0e25903b6894f40fcd53e6d51785fae/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/FixAbstractMethodsStep.cs#L86-L89

From @jonpryor in https://github.com/xamarin/xamarin-android/pull/5748/files#r641035596:

#5965
#4225 (comment)
There is no requirement that an assembly which contains types which need to have abstract methods "fixed up" also contain a type reference to Java.Lang.Object, much less a reference to Mono.Android.dll! Such is the glory of transitive references.

[jonathanpeppers]

The only concern with fixing this would be build performance.

I bet it would cause this step to traverse all the types on every assembly -- unless it had some check to skip them.

[jpobst]

While this may be technically true, I suspect it is impossible or nearly impossible to hit in the wild.

[jonpryor]

It's "easy" to check for locally!

% for f in `find $HOME/.nuget/packages/xamarin.androidx.* -iname \*.dll `; do
    # hoping to find an mscorlib ref to 2.0.5.0, on the assumption this *won't* be a desktop profile assembly
    if ! monodis --assemblyref $f | grep Version=2.0.5.0 >/dev/null 2>&1 ; then
        continue;
    fi
    if ! monodis --typeref $f | grep Java.Lang.Object > /dev/null 2>&1 ; then
        echo "missing Java.Lang.Object in: $f" 
    fi
done

On my machine this finds 7 assemblies:

missing Java.Lang.Object in: $HOME/.nuget/packages/xamarin.androidx.legacy.support.v4/1.0.0/lib/monoandroid90/Xamarin.AndroidX.Legacy.Support.V4.dll
missing Java.Lang.Object in: $HOME/.nuget/packages/xamarin.androidx.legacy.support.v4/1.0.0.1/lib/monoandroid90/Xamarin.AndroidX.Legacy.Support.V4.dll
missing Java.Lang.Object in: $HOME/.nuget/packages/xamarin.androidx.multidex/2.0.1.1/lib/monoandroid90/Xamarin.AndroidX.MultiDex.dll
missing Java.Lang.Object in: $HOME/.nuget/packages/xamarin.androidx.multidex/2.0.1/lib/monoandroid90/Xamarin.AndroidX.MultiDex.dll
missing Java.Lang.Object in: $HOME/.nuget/packages/xamarin.androidx.multidex/2.0.1-rc3/lib/monoandroid90/Xamarin.AndroidX.MultiDex.dll
missing Java.Lang.Object in: $HOME/.nuget/packages/xamarin.androidx.multidex/2.0.1-preview02/lib/monoandroid90/Xamarin.AndroidX.MultiDex.dll
missing Java.Lang.Object in: $HOME/.nuget/packages/xamarin.androidx.multidex/2.0.1.5/lib/monoandroid90/Xamarin.AndroidX.MultiDex.dll

"Missing" abstract methods is a fairly rare occurrence, so this might not be the cause of any actual crashes, but there are assemblies "in the wild" -- which we produce! -- which lack a type reference to Java.Lang.Object.

[jpobst]

Both of those assemblies reference Mono.Android.dll, however they are empty assemblies that do not contain any types, so I guess they don't need a type reference to Java.Lang.Object. 😉