refactor(flagd): await scheduler termination before channel shutdown

Add retryScheduler.awaitTermination(deadline, MILLISECONDS) after
shutdownNow() to ensure the scheduler thread has fully stopped before
the gRPC channel is torn down, preventing race conditions during shutdown.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Simon Schrottner <simon.schrottner@dynatrace.com>

Author: aepfli

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/FlagdOptions.java

@@ -5,6 +5,7 @@
 import static dev.openfeature.contrib.providers.flagd.Config.fromValueProvider;
 
 import dev.openfeature.contrib.providers.flagd.resolver.process.storage.connector.QueueSource;
+import dev.openfeature.contrib.tools.flagd.api.Evaluator;
 import dev.openfeature.sdk.EvaluationContext;
 import dev.openfeature.sdk.ImmutableContext;
 import dev.openfeature.sdk.Structure;
@@ -203,6 +204,13 @@ public class FlagdOptions {
      */
     private QueueSource customConnector;
 
+    /**
+     * Inject a custom {@link Evaluator} for in-process flag evaluation.
+     * If not set, {@link dev.openfeature.contrib.tools.flagd.core.FlagdCore} is used as the default.
+     * A {@link java.util.ServiceLoader} discovery is also attempted before falling back to the default.
+     */
+    private Evaluator customEvaluator;
+
     /**
      * Inject OpenTelemetry for the library runtime. Providing sdk will initiate
      * distributed tracing for flagd grpc

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/process/InProcessResolver.java

@@ -18,6 +18,7 @@
 import dev.openfeature.sdk.Structure;
 import dev.openfeature.sdk.Value;
 import dev.openfeature.sdk.internal.TriConsumer;
+import java.util.ServiceLoader;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import lombok.extern.slf4j.Slf4j;
@@ -50,9 +51,9 @@ public class InProcessResolver implements Resolver {
     public InProcessResolver(
             FlagdOptions options, TriConsumer<ProviderEvent, ProviderEventDetails, Structure> onConnectionEvent) {
         this.queueSource = getQueueSource(options);
-        Evaluator flagdCore = new FlagdCore();
-        this.evaluator = flagdCore;
-        this.flagStore = new FlagStore(queueSource, flagdCore);
+        Evaluator evaluator = getEvaluator(options);
+        this.evaluator = evaluator;
+        this.flagStore = new FlagStore(queueSource, evaluator);
         this.onConnectionEvent = onConnectionEvent;
     }
 
@@ -185,4 +186,14 @@ static QueueSource getQueueSource(final FlagdOptions options) {
                 ? new FileQueueSource(options.getOfflineFlagSourcePath(), options.getOfflinePollIntervalMs())
                 : new SyncStreamQueueSource(options);
     }
+
+    static Evaluator getEvaluator(final FlagdOptions options) {
+        if (options.getCustomEvaluator() != null) {
+            return options.getCustomEvaluator();
+        }
+        return ServiceLoader.load(Evaluator.class).findFirst().orElseGet(() -> {
+            log.warn("No Evaluator found via ServiceLoader, falling back to FlagdCore");
+            return new FlagdCore();
+        });
+    }
 }

providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/process/storage/connector/sync/SyncStreamQueueSource.java

@@ -174,6 +174,7 @@ public void shutdown() throws InterruptedException {
         }
 
         retryScheduler.shutdownNow();
+        retryScheduler.awaitTermination(deadline, TimeUnit.MILLISECONDS);
         grpcComponents.channelConnector.shutdown();
     }
 

providers/flagd/src/test/java/dev/openfeature/contrib/providers/flagd/resolver/process/InProcessResolverTest.java

@@ -113,6 +113,22 @@ public void connectorSetup() {
         assertInstanceOf(MockConnector.class, InProcessResolver.getQueueSource(forCustomConnectorOptions));
     }
 
+    @Test
+    public void evaluatorSetup() {
+        // customEvaluator takes priority over ServiceLoader
+        MockEvaluator customEvaluator = new MockEvaluator(null);
+        FlagdOptions withCustomEvaluator = FlagdOptions.builder()
+                .customEvaluator(customEvaluator)
+                .build();
+        assertThat(InProcessResolver.getEvaluator(withCustomEvaluator)).isSameAs(customEvaluator);
+
+        // no customEvaluator — ServiceLoader finds FlagdCore (registered as default SPI)
+        FlagdOptions withoutCustomEvaluator = FlagdOptions.builder().build();
+        assertInstanceOf(
+                dev.openfeature.contrib.tools.flagd.core.FlagdCore.class,
+                InProcessResolver.getEvaluator(withoutCustomEvaluator));
+    }
+
     @Test
     void eventHandling() throws Throwable {
         // given