def specFactory = TestFiles.execActionFactory()

    def "command line is propagated to execOperations"() {
        given:
        def spec = specFactory.newExecAction()

        when:
        createProviderOf(ProcessOutputValueSource.class) {
            it.parameters {
                it.commandLine = ["echo", "hello"]
                withDefaultEnvironment(it)
            }
        }.get()

        then:

        def configured = false

        when:
        createProviderOf(EchoValueSource) {
            configured = true
        }

        then:
        configured
    }

    def "provider forUseAtConfigurationTime is a no-op"() {

        given:
        configurationTimeBarrier.atConfigurationTime >> true
        def provider = createProviderOf(EchoValueSource) {
            it.parameters.value.set('42')
        }

        execOperations,
        TestUtil.services()
    )

    protected <T, P extends ValueSourceParameters> Provider<T> createProviderOf(Class<? extends ValueSource<T, P>> valueSourceType, Action<? super ValueSourceSpec<P>> configureAction) {
        return valueSourceProviderFactory.createProviderOf(valueSourceType, configureAction)
    }

class EnvironmentVariableValueSourceTest extends ValueSourceBasedSpec {

    def "environment variable has no value when variable name has no value"() {
        given:
        def provider = createProviderOf(EnvironmentVariableValueSource) {
            // give variable name no value
        }

        expect:
        !provider.isPresent()
    }

class SystemPropertyValueSourceTest extends ValueSourceBasedSpec {

    def "system property has no value when property name has no value"() {
        given:
        def provider = createProviderOf(SystemPropertyValueSource) {
            // give property name no value
        }

        expect:
        !provider.isPresent()
    }

 *
 * @since 6.1
 */
@ServiceScope(Scope.Build.class)
public interface ValueSourceProviderFactory {

    <T, P extends ValueSourceParameters> Provider<T> createProviderOf(
        Class<? extends ValueSource<T, P>> valueSourceType,
        Action<? super ValueSourceSpec<P>> configureAction
    );

    void addValueListener(ValueListener listener);

        if (valueSourceProviderFactory == null) {
            throw new UnsupportedOperationException();
        }
        return valueSourceProviderFactory.createProviderOf(valueSourceType, configuration);
    }

    @Override
    public <T extends Credentials> Provider<T> credentials(Class<T> credentialsType, String identity) {

        this.specInstantiator = instantiatorFactory.decorateLenientScheme().withServices(services).instantiator();
    }

    @Override
    public <T, P extends ValueSourceParameters> Provider<T> createProviderOf(Class<? extends ValueSource<T, P>> valueSourceType, Action<? super ValueSourceSpec<P>> configureAction) {
        try {
            Class<P> parametersType = extractParametersTypeOf(valueSourceType);