Make sure _GenerateJavaStubs always generates native assembly files

[grendello]

Fixes: #8967

For some reason, sometimes the typemap*.ll files are sometimes removed from the obj/ directory which leads to build errors similar to:

typemaps.x86_64.ll: error: Could not open input file: no such file or directory

Files are generated by the GenerateJavaStubs tasks, which is invoked by the _GenerateJavaStubs target. However, the target doesn't specify the *.ll files in its Outputs parameter and, therefore, whenever the files are removed but the _GenerateJavaStubs.stamp file is newer than the items/files specified in the target's Inputs parameter, the native assembly files aren't regenerated leading to the above error.

To fix this, we need to add the typemap*.ll files to the target's Outputs set, thus forcing their regeneration should they be no longer where they are expected to be.

[jonathanpeppers]

This causes a test failure:

GenerateJavaStubsAndAssembly
_GenerateJavaStubs should be skipped!
Expected: True
But was:  False

I think a build with no changes should be skipped:
https://github.com/xamarin/xamarin-android/blob/f5fcd4ddc5bacdb9699c50f6dad80627b80d72c8/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs#L976

But it said:

Input file "obj/Debug/android/assets/armeabi-v7a/UnnamedProject.dll" is newer than output file "obj/Debug/android/typemaps.armeabi-v7a.ll".

If we took this change, I think we'd need to touch all the .ll files, too? Does it currently use some "copy if changed" logic?

[grendello]

@jonathanpeppers yep, it uses Files.CopyIfStreamChanged

[jonathanpeppers]

Alternatively, do we know what deleted @(_TypeMapAssemblySource)? Are they missing from FileWrites?

[grendello]

That's the problem, we have no idea why they are gone. The guess is that some earlier build deleted them (DTB perhaps?) which we don't have build logs for.

[grendello]

Regardless of what made the files disappear, I don't think a stamp file in Outputs is enough, at least in this case. The task inside the target produces files, and we must make sure that those files exist (whether they were removed by us for some reason, or by the developer) and so they must be part of the outputs.

[dellis1972]

So I wonder if it is this.

Because the _TypeMapAssemblySource are generated via a Task. If the task does not run, the _TypeMapAssemblySource will be empty.

On a DTB is _GeneratedJavaStubs is skipped the task filling in the _TypeMapAssemblySource ItemGroup will be empty. As a result the FileWrites entry will not include the files.

In the case where the Task is not run, we need a way to populate the _TypeMapAssemblySource from the files on disk to make sure they are not deleted.

[grendello]

We have this chain of dependencies:

<Target Name="_PrepareNativeAssemblySources">
  <PrepareAbiItems
      BuildTargetAbis="@(_BuildTargetAbis)"
      NativeSourcesDir="$(_NativeAssemblySourceDir)"
      InstantRunEnabled="$(_InstantRunEnabled)"
      Debug="$(AndroidIncludeDebugSymbols)"
      Mode="typemap">
    <Output TaskParameter="AssemblySources" ItemName="_TypeMapAssemblySource" />
    <Output TaskParameter="AssemblyIncludes" ItemName="_TypeMapAssemblyInclude" />
  </PrepareAbiItems>
</Target>

<PropertyGroup>
  <_GenerateJavaStubsDependsOnTargets>
    _SetLatestTargetFrameworkVersion;
    _PrepareAssemblies;
    _PrepareNativeAssemblySources;
    $(_AfterPrepareAssemblies);
  </_GenerateJavaStubsDependsOnTargets>
</PropertyGroup>

<Target Name="_GenerateJavaStubs"
    DependsOnTargets="$(_GenerateJavaStubsDependsOnTargets);$(BeforeGenerateAndroidManifest)"
    Inputs="@(_AndroidMSBuildAllProjects);@(_ResolvedUserMonoAndroidAssemblies);$(_AndroidManifestAbs);$(_AndroidBuildPropertiesCache);@(AndroidEnvironment);@(LibraryEnvironments)"
    Outputs="$(_AndroidStampDirectory)_GenerateJavaStubs.stamp">

Will _PrepareNativeAssemblySources be ignored when _GenerateJavaStubs Outputs are deemed up to date?