[native-image] Problems during compilation of http4s with http client

[lbialy]

I was trying to compile my POC service and got this:

    analysis:  14,013.19 ms
error: Must not have a started Thread in the image heap.
Detailed message:
Error: Must not have a started Thread in the image heap.
Trace:  object org.http4s.blaze.util.TickWheelExecutor
        object org.http4s.client.blaze.bits$
        field org.http4s.client.blaze.bits$.MODULE$

Error: Processing image build request failed

How to reproduce -

1. pull https://github.com/lbialy/blockchain-explorer-on-cats
2. run sbt assembly
3. run native-image -H:+ReportUnsupportedElementsAtRuntime -jar blockchain-explorer-on-cats-assembly-0.0.1-SNAPSHOT.jar

[lbialy]

What does this Must not have a started Thread in the image heap. error mean?

[cstancu]

That error means that you start a thread in a static initializer. This is not allowed. The error message has been improved since the release.

[thomaswue]

One way to solve these issues could be to run specific static initializers not at boot image generation time, but make them part of the native image.

[lbialy]

I was able to pinpoint the problem in http4s' blaze-based client here:
https://github.com/http4s/http4s/blob/b9a28ddcd335cf5dee60a9f7d3be5bdc12523179/blaze-client/src/main/scala/org/http4s/client/blaze/bits.scala#L20
changing that val to lazy val seems not to break anything and makes native compilation possible again.

I think we can close this issue. Is this limitation in documentation somewhere? I guess it's quite important.

[cstancu]

@lbialy This limitation is not explicitly in the documentation. I'll fix it.
@thomaswue that should be possible, however it requires more investigation. If we push some initializer to runtime we need to push all other initializers that might depend on it and need to be careful about the initialization order.