Fix instrumentation of newly loaded classes

[eupp]

Before this commit, the instrumentation would only be triggered for re-transformed or re-defined classes.
With this change, instrumentation is also applied to new classes loaded during Lincheck test execution.

[eupp]

This is a part of #505

[dmitrii-artuhov]

Two nits: I would remove the mentioned if-statements, but leaving them is also ok

[eupp]

nit: retransformClasses on empty array is guaranteed no-op, so this if is exhaustive

I also thought it should be no-op, but unfortunately without the surrounding if statement I can observe NullPointerException thrown from retransformClasses on Java 8, leading to failed tests.

I don't know what exactly is the reason, could be some bug in ByteBuddy or Kotlin compiler (e.g., maybe it decides to convert vararg argument with 0 provided arguments to null instead of empty array). I did not dig further to be honest, as simply adding the if statement seem to solve the problem.

I've added a comment to the code about this problem.

[ndkoval]

@eupp, should a test that verifies the fix be added to this PR?

[eupp]

should a test that verifies the fix be added to this PR?

This issue manifests on the coroutines related test that I am going to add under #505.

I've tried to minimize the bug but so far with no success.
It seems that the problem can only manifest if several non-trivial conditions are satisfied, including some interplay of lazy class transformation logic and ignored sections.

Normally in most cases, the lazy instrumentation would be applied to a class, for example, before the object is created:

override fun beforeNewObjectCreation(className: String) = runInIgnoredSection {
    LincheckJavaAgent.ensureClassHierarchyIsTransformed(className)
}

// `ensureClassHierarchyIsTransformed` first calls `Class.forName(canonicalClassName)` to ensure class is loaded

So at the point when LincheckClassFileTransformer::transform is called, the class is already loaded.

But if some object is created inside an ignored section, or its creation is untracked by some other reason, the beforeNewObjectCreation injection will not be called, and the class will not be instrumented. In such cases, instrumenting the class when it is loaded could be the only opportunity to instrument it.