SBT recompiles test classes on sbt gatling:testOnly and creates a duplicate of artifacts

Scala 2.12.6 SBT 1.1.6 Gatling SBT plugin 2.2.2 Gatling 2.3.1

I am creating a Docker image with the Gatling code repo and compile the test sources on image creation with sbt test:compile. I see that compilation indeed succeeds (though runs suspiciously fast):

[info] Compiling 22 Scala sources and 2 Java sources to /src/target/scala-2.12/test-classes ...
[info] Non-compiled module 'compiler-bridge_2.12' for Scala 2.12.6. Compiling...
[info]   Compilation completed in 6.069s.
[info] Done compiling.

and when I check the target/ directory, I see that test classes were compiled as expected:

docker run -it performance-test:latest
root@ee4f91c4dccf:/src# ls /src/target/scala-2.12/test-classes/com/company/scenarios/
AgentEnrollStressTest.class  AgentStressTest.class        BaseTest.class               DeviceList.class         UserCRUD.class               VerdictRequest.class

but on sbt gatling:testOnly dummyscenario I see that SBT recompiles them (and takes considerably longer time):

root@a44495c45b1e:/src# sbt "gatling:testOnly com.company.scenarios.Dummy"
[info] Loading settings from plugins.sbt ...
[info] Loading project definition from /src/project
[info] Loading settings from build.sbt ...
[info] Set current project to performance (in build file:/src/)
[info] Updating ...
[info] Done updating.
[info] Compiling 22 Scala sources and 2 Java sources to /src/target/scala-2.12/test-classes ...
[info] Done compiling.
[info] Compiling 22 Scala sources and 2 Java sources to /src/target/scala-2.12/gatling-classes ...
[info] Done compiling.
[info] Simulation(s) execution ended.
[info] No tests to run for Gatling / testOnly
[success] Total time: 39 s, completed Jul 5, 2018 1:58:33 PM

• creates a duplicate in the target/scala-2.12/gatling-classes directory:

root@a44495c45b1e:/src# ls /src/target/scala-2.12/test-classes/com/company/scenarios/
AgentEnrollStressTest.class  AgentStressTest.class        BaseTest.class               DeviceList.class         UserCRUD.class               VerdictRequest.class
root@a44495c45b1e:/src# ls /src/target/scala-2.12/gatling-classes/com/company/scenarios/
AgentEnrollStressTest.class  AgentStressTest.class        BaseTest.class               DeviceList.class         UserCRUD.class               VerdictRequest.class

Public repository to reproduce: https://github.com/EugeneAbramchuk/performance-public

To build Docker image: docker build -t performance-test:latest --file ./Dockerfile ./ --no-cache To start in interactive mode: docker run -it performance-test:latest To trigger the duplicate compilation: sbt "gatling:testOnly dummy"

[slandelle]

Regression happened in sbt 1.1.0.

[EugeneAbramchuk]

@slandelle looks like the roots grow deeper.

target/scala-2.12/gatling-classes are generated on gatling:testOnly with sbt 1.0.4 too with existing target/scala-2.12/test-classes.

Granted, this is an improvement since testOnly does not compile twice, but it still occurs when it already has sbt compile artifacts.