feat: In-memory provider (#208)

[mstepien]

InMemoryProvider, Gherkin based tests

This PR introduces the InMemoryProvider for the OpenFeature Kotlin SDK and validates its implementation using the official OpenFeature specification.

• Adds the InMemoryProvider for managing and evaluating flags entirely in memory, including ContextEvaluator and Flag models.
• Links the open-feature/spec repository as a Git submodule to utilize the official cross-platform test suite.
• Adds End-to-End (E2E) testing suite (EvaluationSteps, GherkinSpecTest) using Cucumber to verify the new provider adheres to the OpenFeature Gherkin specification. Evaluation targets evaluation.feature file and ignores evaluation_v2.feature.
• Updates the GitHub Actions ci.yaml workflow to ensure Git submodules are checked out during CI test runs.

Related Issues

Fixes #208

Notes

Because this PR introduces the open-feature/spec as a Git submodule, developers pulling or checking out this branch locally may need to initialize submodules by running:

git submodule update --init --recursive

[gemini-code-assist]

Code Review

This pull request introduces an InMemoryProvider implementation for the Kotlin SDK, along with supporting classes like Flag and ContextEvaluator. It also integrates Gherkin-based E2E tests to ensure compliance with the OpenFeature specification. Feedback focuses on improving thread safety for the provider's internal state, ensuring type safety during flag evaluation (especially when using custom evaluators), resolving nullability mismatches in provider metadata, and enforcing invariants in the Flag data class to prevent invalid configurations.

[dd-oleksii]

This is user-facing API change, so the PR name should use feat: prefix (test: is for test-only changes)

[typotter]

Solid implementation with a clean InMemoryProvider and a nice Flag/Builder API. Nothing major from my pass outside of what has already been flagged/is in discussion.

[typotter]

A few things I noticed on the latest commits — great progress overall.

[typotter]

A few paths still don't have unit test coverage:

• The disabled flag path (return defaultValue with Reason.DISABLED)
• contextEvaluator returning null (fallback to defaultVariant with Reason.DEFAULT)
• updateFlags/updateFlag event emission (that ProviderConfigurationChanged fires with the correct flagsChanged set)

[typotter]

Thanks for the changes, Marcin

Sorry for missing this earlier, but I think we need to relook at the ContextEvaluator interface.

[typotter]

Code review

Found 2 issues:

1. kotlin.concurrent.Volatile is JVM-only — will fail to compile on linuxX64, js, iosArm64, iosSimulatorArm64. Replace with AtomicRef from kotlinx-atomicfu (move from commonTest to commonMain), or restructure to avoid the field entirely.

kotlin-sdk/kotlin-sdk/src/commonMain/kotlin/dev/openfeature/kotlin/sdk/providers/memory/InMemoryProvider.kt

Lines 25 to 35 in eaa5335

	import kotlin.concurrent.Volatile
	class InMemoryProvider(initialFlags: Map<String, Flag<*>> = emptyMap()) : FeatureProvider {
	override val hooks: List<Hook<*>> = emptyList()
	override val metadata: ProviderMetadata = InMemoryProviderMetadata("InMemoryProvider")
	private val flagsState = MutableStateFlow<Map<String, Flag<*>>>(initialFlags.toMap())
	@Volatile
	private var state: OpenFeatureStatus = OpenFeatureStatus.NotReady

2. context-aware E2E fixture: contextEvaluator returns "INTERNAL"/"EXTERNAL" (uppercase) but the variant keys are "internal"/"external" (lowercase). containsKey always misses — every evaluation falls back to defaultVariant with Reason.ERROR instead of producing Reason.TARGETING_MATCH. Fix: return lowercase strings from the evaluator.

kotlin-sdk/kotlin-sdk/src/jvmTest/kotlin/dev/openfeature/kotlin/sdk/e2e/EvaluationSteps.kt

Lines 41 to 47 in eaa5335

	"context-aware" to Flag.builder<String>()
	.variant("internal", "INTERNAL")
	.variant("external", "EXTERNAL")
	.defaultVariant("external")
	.contextEvaluator { _, ctx ->
	if (ctx?.getValue("customer")?.asBoolean() == false) "INTERNAL" else "EXTERNAL"
	}

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[mstepien]

kotlin-sdk/kotlin-sdk/src/commonMain/kotlin/dev/openfeature/kotlin/sdk/providers/memory/InMemoryProvider.kt

Volatile should work without changes. And should take effects on platform with multithreading. Project compiles fine on all non-JVM targets without failing.
Starting with Kotlin 1.9.0, kotlin.concurrent.Volatile was introduced into the common standard library and made stable for Kotlin multiplatform.

kotlin-sdk/kotlin-sdk/src/jvmTest/kotlin/dev/openfeature/kotlin/sdk/e2e/EvaluationSteps.kt

Good catch, fixed case sensitivity issue