Update AbstractCopyTask.expand to be compatible with the Provider API

[aSemy]

I want to use AbstractCopyTask.expand() to expand variables when I am Syncing files.

When I try and use providers as inputs for expand, they are accepted, but their values are not computed. Instead, expand() will instead just call toString() on the values, resulting in invalid data.

Example

// build.gradle.kts

val projectPath: Provider<String> = provider { project.path }

val mySyncTask by tasks.registering(Sync::class) {
  from(
    resources.text.fromString(
      "This file was created by project \${projectPath}"
    ).asFile()
  )
  into(temporaryDir)
  expand(
    "projectPath" to projectPath
  )

  doLast {
    temporaryDir.walk().filter { it.isFile }.forEach {
      println(it.readText())
    }
  }
}

Expected output:

> Task :mySyncTask
This file was created by project :

Current Behavior

The actual output does not compute the value of projectPath, resulting invalid output.

> Task :mySyncTask
This file was created by project fixed(class java.lang.String, :)

Context

Reproduced on Gradle 7.6 and 8.0.2

Workaround

As a workaround I have to compute the property in a doFirst {} block. This is unintuitive and confusing.

// build.gradle.kts

val projectPath: Provider<String> = provider { project.path }

val mySyncTask by tasks.registering(Sync::class) {
  from(
    resources.text.fromString(
      "This file was created by project \${projectPath}"
    ).asFile()
  )
  into(temporaryDir)

  // redefine the variable to be compatible Gradle Config Cache
  val projectPath = projectPath

  doFirst {
    expand(
      "projectPath" to projectPath.get()
    )
  }

  doLast {
    temporaryDir.walk().filter { it.isFile }.forEach {
      println(it.readText())
    }
  }
}

This produces the correct result

This file was created by project :

[ov7a]

Thank you for your interest in Gradle!

This feature request is in the backlog of the relevant team but this area of Gradle is currently not a focus one so it might take a while before it gets implemented.

[lptr]

In Gradle 9.0 we plan to migrate to MapProperty for expand which should cover this issue.