Gradle: Allow setting custom sourceset

[stashymane]

Environment:

• Jib version: 3.4.3
• Build tool: Gradle 8.7

Description of the issue:
Currently, the main sourceset is hardcoded into the gradle plugin. With the default Java plugin this works well, however running anything else makes using Jib difficult.
My use-case is Kotlin Multiplatform - it does generate a JVM target, however it does not create a main sourceset. This results in Jib throwing an exception:

org.gradle.api.ProjectConfigurationException: A problem occurred configuring project ':server'.
Caused by: org.gradle.api.UnknownDomainObjectException: SourceSet with name 'main' not found.
	at com.google.cloud.tools.jib.gradle.JibPlugin.lambda$apply$9(JibPlugin.java:189)

Having to manually set up the main sourceset as a "mirror" is error-prone and cumbersome. A very simple choice of sourceset in the Jib configuration would solve most issues.

This exact issue was already mentioned in #1778, and even though the issue was closed, only the configuration was allowed to be configurable. The sourceset still isn't.

[yluom]

Out of curiosity, @stashymane could you explain how you bypassed this problem ?

[stashymane]

@yluom specifically for Kotlin Multiplatform, turns out you can run withJava() on the JVM configuration block. I avoided this earlier due to some issues being associated with it, but I'm not sure what that was anymore.
In case of any other configuration, no clue.

[chris-hatton]

With Kotlin 2.1.20 now incompatible with the application plugin and the withJava() API now deprecated, it could become relevant to consider adding configurations to Jib, to support KMP/JVM users who have an e.g. jvmMain source-set instead of plain main.

[osipxd]

It seems like the only way to bypass this problem is to create a new JVM-only project with JIB applied and add the KMP project as a dependency to it.

[nathanfallet]

Maybe there is a way to configure this, or at least check for multiple targets? (Like you check for "main" ?: "jvmMain" ?: "commonMain", you get the idea)
EDIT: I inspected the code, I'm making a PR that allows to configure it via the configuration extension of the Gradle plugin

[LiYing2010]

I think I found a workaround for this error

in the build.gradle.kts file of your Kotlin Multiplatform Project, add the following setting:

sourceSets {    <--- this should be outside of `kotlin { sourceSets { ... } }` block
    // !!! JIB need this `main` source set !!!
    create("main") {  <--- JIB plugin need this "main" source set
        (this.output.classesDirs as? ConfigurableFileCollection)
            ?.from("build/classes/kotlin/jvm/main")   <-- this is the output classes dir of `jvmMain` source set, we need to add it to the output classes dir of "main" source set
    }
}

BTW
I think the jvmMain Source Set in Kotlin Multiplatform Project CAN NOT be used in JIB plugin simply,
since JIB works with org.gradle.api.tasks.SourceSet type, but Kotlin Source Set is org.jetbrains.kotlin.gradle.plugin.KotlinSourceSet type

see: #4396 (comment)

[stashymane]

BTW I think the jvmMain Source Set in Kotlin Multiplatform Project CAN NOT be used in JIB plugin simply, since JIB works with org.gradle.api.tasks.SourceSet type, but Kotlin Source Set is org.jetbrains.kotlin.gradle.plugin.KotlinSourceSet type

see: #4396 (comment)

kotlin source sets eventually get transformed into gradle source sets, what you're referencing is the internal/DSL state before it's passed to gradle