Provider gets evaluated twice with configuration cache enabled

[antohaby]

A Gradle Task with some field of type Provider<*> get evaluated twice if Configuration Cache is enabled.

Expected Behavior

Provider gets evaluated only once.

Context

I want to delay the time-expensive evaluation of some data. Either during task execution (when configuration cache isn't enabled)
or capture it in configuration cache and then reuse cached value during execution.

Steps to Reproduce

// build.gradle.kts
abstract class MyTask : DefaultTask() {
    private val prop = project.provider {
        println("Expensive data calculation")
        "data"
    }

    @TaskAction
    fun action() {
        println("Results: ${prop.get()}")
    }
}

val myTask by tasks.registering(MyTask::class)

Executing this task for the first time shows that prop gets evaluated twice: during configuration phase and execution phase

$ ./gradlew myTask  --configuration-cache --rerun-tasks
Configuration cache is an incubating feature.
Calculating task graph as no configuration cache is available for tasks: myTask
Expensive data calculation

> Task :myTask
Expensive data calculation
Results: data

Executing this task second time shows that prop is recovered from cache and thus no evaluation happened.

$ ./gradlew myTask  --configuration-cache --rerun-tasks                
Configuration cache is an incubating feature.
Reusing configuration cache.

> Task :myTask
Results: data

Workaround

Kotlin lazy delegate works as expected. Although gradle configuration cache got a bit polluted with internal state of Lazy.

abstract class MyTask : DefaultTask() {
    private val prop by lazy {
        println("Expensive data calculation")
        "data"
    }

    @TaskAction
    fun action() {
        println("Results: $prop")
    }
}

val myTask by tasks.registering(MyTask::class)

1st execution:

$ ./gradlew myTask  --configuration-cache --rerun-tasks
Configuration cache is an incubating feature.
Calculating task graph as configuration cache cannot be reused because file 'build.gradle.kts' has changed.
Expensive data calculation

> Task :myTask
Results: data

2nd execution:

$ ./gradlew myTask  --configuration-cache --rerun-tasks
Configuration cache is an incubating feature.
Reusing configuration cache.

> Task :myTask
Results: data

Environment

Gradle 7.4.2

[antohaby]

One major limitation is that I can't use project inside project.provider {}.

abstract class MyTask : DefaultTask() {
    private val prop by lazy { project.scrapData() }
    /* ... */
}

This task would fail in the strict mode because project inside lambda is accessed during execution due to the original bug of double evaluation.

[Vampire]

A provider is calculated each time its value is requested, that is by design afaik.
But you can relatively easy produce a cached provider that is only calculated lazily once:

inline fun <reified T> cachedProvider(crossinline block: () -> T): Provider<T> =
    objects
        .property<T>()
        .value(provider { block() })
        .apply {
            disallowChanges()
            finalizeValueOnRead()
        }

[bamboo]

Fixed by #21985