Support running KSP tests in parallel

[ansman]

This issue might actually be related to KSP itself, but in my project auto-dagger I run my tests concurrently using Junit 5. This works fine for tests that use a regular annotation processor but fails for KSP tests (you essentially get a bunch of errors during type resolution like org.jetbrains.kotlin.resolve.lazy.NoDescriptorForDeclarationException: Descriptor wasn't found for declaration CLASS).

To repro, comment out this line: https://github.com/ansman/auto-dagger/blob/main/compiler/src/test/kotlin/se/ansman/dagger/auto/compiler/KspTest.kt#L6
And then run the tests.

Again this issue might be with KSP and if so please just close this issue and I'll open an issue with KSP.

[tschuchortdev]

I'm surprised that it works at all. KotlinCompilation uses a thread-local singleton object to register KAPT processors, ComponentRegistrars and so on, since the compiler can only find them through classpath scanning. Parallel execution should thus always come with a risk of race conditions unless you can guarantee a dedicated thread per compilation. Does the KSP source code itself use parallel test execution?

[ansman]

Since it's all synchronous otherwise I think each test would run in a dedicated thread. But the threads would probably be reused but that must work or it wouldn't work running multiple tests.

I spawn no new threads in my processor or tests. In fact I didn't even think it was possible.