Automatically initialize AbstractEndpointSelector

Motivation:

The `initialize()` method of `AbstractEndpointSelector` may access
member fields of subclasses, `initialize()` couldn't be called in the
constructor of `AbstractEndpointSelector`.

Although the requirement is stated in the Javadoc, many users overlooked
calling `initialize()` in their implementations.

I propose adding a fallback logic that automatically initializes
`AbstractEndpointSelector` if it hasn't been intialized by the time the
first enpoint selection occurs, so users are protected from
implmentation mistakes.

Modifications:

- Try to initialize `AbstractEndpointSelector` if it hasn't been
  initialized when `select(...)` method is called.

Result:

Make `AbstractEndpointSelector` auto-initialize to avoid user-side
implementation errors.

Author: ikhoon

core/src/main/java/com/linecorp/armeria/client/endpoint/AbstractEndpointSelector.java

@@ -21,6 +21,7 @@
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 
 import com.google.common.annotations.VisibleForTesting;
 
@@ -38,8 +39,13 @@
  */
 public abstract class AbstractEndpointSelector implements EndpointSelector {
 
+    private static final AtomicIntegerFieldUpdater<AbstractEndpointSelector> initializedUpdater =
+            AtomicIntegerFieldUpdater.newUpdater(AbstractEndpointSelector.class, "initialized");
+
     private final EndpointGroup endpointGroup;
     private final EndpointAsyncSelector asyncSelector;
+    // 0 - not initialized, 1 - initialized
+    private volatile int initialized;
 
     /**
      * Creates a new instance that selects an {@link Endpoint} from the specified {@link EndpointGroup}.
@@ -59,15 +65,33 @@ protected final EndpointGroup group() {
     /**
      * Initialize this {@link EndpointSelector} to listen to the new endpoints emitted by the
      * {@link EndpointGroup}. The new endpoints will be passed to {@link #updateNewEndpoints(List)}.
+     *
+     * <p>This method is called automatically when the first selection is made. However, if you want to
+     * start listening to the {@link EndpointGroup} earlier, you can call this method manually.
      */
-    @UnstableApi
     protected final void initialize() {
-        endpointGroup.addListener(this::refreshEndpoints, true);
+        tryInitialize();
+    }
+
+    private void tryInitialize() {
+        if (initialized == 1) {
+            return;
+        }
+
+        if (initializedUpdater.compareAndSet(this, 0, 1)) {
+            endpointGroup.addListener(this::refreshEndpoints, true);
+        }
+    }
+
+    @VisibleForTesting
+    boolean isInitialized() {
+        return initialized == 1;
     }
 
     @Override
     public CompletableFuture<Endpoint> select(ClientRequestContext ctx, ScheduledExecutorService executor,
                                               long timeoutMillis) {
+        tryInitialize();
         return asyncSelector.select(ctx, executor, endpointGroup.selectionTimeoutMillis());
     }
 
@@ -106,6 +130,7 @@ protected void onTimeout(ClientRequestContext ctx, long selectionTimeoutMillis)
         @Nullable
         @Override
         protected Endpoint selectNow(ClientRequestContext ctx) {
+            tryInitialize();
             return AbstractEndpointSelector.this.selectNow(ctx);
         }
 

core/src/main/java/com/linecorp/armeria/client/endpoint/DefaultEndpointSelector.java

@@ -52,7 +52,6 @@ final class DefaultEndpointSelector<T extends LoadBalancer<Endpoint, ClientReque
                 }
             });
         }
-        initialize();
     }
 
     @Override

core/src/test/java/com/linecorp/armeria/client/ClientRequestContextDelayedInitTest.java

@@ -89,7 +89,6 @@ class TestEndpointSelector extends AbstractEndpointSelector {
 
                 protected TestEndpointSelector(EndpointGroup endpointGroup) {
                     super(endpointGroup);
-                    initialize();
                 }
 
                 @Nullable

core/src/test/java/com/linecorp/armeria/client/endpoint/AbstractEndpointSelectorTest.java

@@ -46,8 +46,11 @@ void immediateSelection() {
         final Endpoint endpoint = Endpoint.of("foo");
         final ClientRequestContext ctx = ClientRequestContext.of(HttpRequest.of(HttpMethod.GET, "/"));
         final AbstractEndpointSelector endpointSelector = newSelector(endpoint);
+        assertThat(endpointSelector.isInitialized()).isFalse();
         assertThat(endpointSelector.select(ctx, ctx.eventLoop(), Long.MAX_VALUE))
                 .isCompletedWithValue(endpoint);
+        // The AbstractEndpointSelector should be initialized when the first selection is made.
+        assertThat(endpointSelector.isInitialized()).isTrue();
         assertThat(endpointSelector.pendingFutures()).isEmpty();
     }
 
@@ -56,8 +59,11 @@ void delayedSelection() {
         final DynamicEndpointGroup group = new DynamicEndpointGroup();
         final ClientRequestContext ctx = ClientRequestContext.of(HttpRequest.of(HttpMethod.GET, "/"));
         final AbstractEndpointSelector endpointSelector = newSelector(group);
+        assertThat(endpointSelector.isInitialized()).isFalse();
         final CompletableFuture<Endpoint> future = endpointSelector.select(ctx, ctx.eventLoop(),
                                                                            Long.MAX_VALUE);
+        // The AbstractEndpointSelector should be initialized when the first selection is made.
+        assertThat(endpointSelector.isInitialized()).isTrue();
         assertThat(future).isNotDone();
 
         final Endpoint endpoint = Endpoint.of("foo");
@@ -128,7 +134,7 @@ void testRampingUpInitialSelection() {
     }
 
     private static AbstractEndpointSelector newSelector(EndpointGroup endpointGroup) {
-        final AbstractEndpointSelector selector = new AbstractEndpointSelector(endpointGroup) {
+        return new AbstractEndpointSelector(endpointGroup) {
 
             @Nullable
             @Override
@@ -137,7 +143,5 @@ public Endpoint selectNow(ClientRequestContext ctx) {
                 return endpoints.isEmpty() ? null : endpoints.get(0);
             }
         };
-        selector.initialize();
-        return selector;
     }
 }

xds/src/main/java/com/linecorp/armeria/xds/client/endpoint/XdsEndpointGroup.java

@@ -249,7 +249,6 @@ private final class XdsEndpointGroupSelector extends AbstractEndpointSelector {
 
         XdsEndpointGroupSelector(EndpointGroup endpointGroup) {
             super(endpointGroup);
-            initialize();
         }
 
         @Override