chore: handles offline mode, no initializers nor synchronizers (#117)

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [x] I have validated my changes against all supported platform
versions


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes affect initialization and offline data source selection, which
can alter client connectivity/status behavior. Persistence propagation
tweaks also impact whether updates are written to persistent stores.
> 
> **Overview**
> **FDv2 now behaves cleanly in offline and no-source configurations.**
`FDv2DataSystem` builds an `externalUpdatesOnly` data source when
`config.offline` is true, and `FDv2DataSource` explicitly marks the
client `VALID` (with a log) when there are no initializers/synchronizers
instead of trying to connect.
> 
> **Persistence signaling is now consistently propagated.**
`DataSourceSynchronizerAdapter` includes `shouldPersist` when converting
FDv1 `init` payloads into FDv2 `ChangeSet`s and defaults FDv1 `upsert`
conversions to persist, while `WriteThroughStore` simplifies/guards
persistent-store calls using `hasPersistence`.
> 
> Adds a regression test to ensure an offline client using the FDv2 data
system initializes successfully and reports `VALID` status.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
6ae877b. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Ryan Lamb <4955475+kinyoklion@users.noreply.github.com>

Author: tanderson-ld

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/DataSourceSynchronizerAdapter.java

@@ -133,7 +133,9 @@ public boolean init(DataStoreTypes.FullDataSet<DataStoreTypes.ItemDescriptor> al
                             DataStoreTypes.ChangeSetType.Full,
                             Selector.EMPTY,
                             allData.getData(),
-                            null);
+                            null,
+                            allData.shouldPersist()
+                        );
             resultQueue.put(FDv2SourceResult.changeSet(changeSet, false));
             return true;
         }
@@ -153,7 +155,9 @@ public boolean upsert(DataStoreTypes.DataKind kind, String key, DataStoreTypes.I
                             DataStoreTypes.ChangeSetType.Partial,
                             Selector.EMPTY,
                             data,
-                            null);
+                            null,
+                            true // default to true as this adapter is used for adapting FDv1 data sources which are always persistent
+                        );
             resultQueue.put(FDv2SourceResult.changeSet(changeSet, false));
             return true;
         }

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/FDv2DataSource.java

@@ -117,6 +117,7 @@ private void run() {
             if (!sourceManager.hasAvailableSources()) {
                 // There are not any initializer or synchronizers, so we are at the best state that
                 // can be achieved.
+                logger.info("LaunchDarkly client will not connect to Launchdarkly for feature flag data due to no initializers or synchronizers");
                 dataSourceUpdates.updateStatus(DataSourceStatusProvider.State.VALID, null);
                 startFuture.complete(true);
                 return;

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/FDv2DataSystem.java

@@ -15,7 +15,6 @@
 import com.launchdarkly.sdk.server.subsystems.DataStore;
 import com.launchdarkly.sdk.server.subsystems.LoggingConfiguration;
 import com.launchdarkly.sdk.server.subsystems.DataSystemConfiguration;
-import com.launchdarkly.sdk.server.subsystems.ComponentConfigurer;
 
 import java.io.Closeable;
 import java.io.IOException;
@@ -167,15 +166,21 @@ static FDv2DataSystem create(
       };
     }
 
-    DataSource dataSource = new FDv2DataSource(
-      initializerFactories,
-      synchronizerFactories,
-      fdv1FallbackFactory,
-      dataSourceUpdates,
-      config.threadPriority,
-      clientContext.getBaseLogger().subLogger(Loggers.DATA_SOURCE_LOGGER_NAME),
-      clientContext.sharedExecutor
-    );
+    final DataSource dataSource;
+    if (config.offline) {
+      dataSource = Components.externalUpdatesOnly().build(clientContext.withDataSourceUpdateSink(dataSourceUpdates));
+    } else {
+      dataSource = new FDv2DataSource(
+        initializerFactories,
+        synchronizerFactories,
+        fdv1FallbackFactory,
+        dataSourceUpdates,
+        config.threadPriority,
+        clientContext.getBaseLogger().subLogger(Loggers.DATA_SOURCE_LOGGER_NAME),
+        clientContext.sharedExecutor
+      );
+    }
+
     DataSourceStatusProvider dataSourceStatusProvider = new DataSourceStatusProviderImpl(
       dataSourceStatusBroadcaster,
       dataSourceUpdates);

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/WriteThroughStore.java

@@ -58,10 +58,8 @@ public void init(FullDataSet<ItemDescriptor> allData) {
     maybeSwitchStore();
 
     // Only write to persistent store if shouldPersist is true and store is in READ_WRITE mode
-    if (persistenceMode == DataStoreMode.READ_WRITE && allData.shouldPersist()) {
-      if (persistentStore != null) {
-        persistentStore.init(allData);
-      }
+    if (hasPersistence && persistenceMode == DataStoreMode.READ_WRITE && allData.shouldPersist()) {
+      persistentStore.init(allData);
     }
   }
 
@@ -97,12 +95,12 @@ public boolean isInitialized() {
 
   @Override
   public boolean isStatusMonitoringEnabled() {
-    return persistentStore != null ? persistentStore.isStatusMonitoringEnabled() : false;
+    return hasPersistence ? persistentStore.isStatusMonitoringEnabled() : false;
   }
 
   @Override
   public CacheStats getCacheStats() {
-    return persistentStore != null ? persistentStore.getCacheStats() : null;
+    return hasPersistence ? persistentStore.getCacheStats() : null;
   }
 
   @Override
@@ -135,7 +133,7 @@ public Selector getSelector() {
   @Override
   public void close() throws IOException {
     memoryStore.close();
-    if (persistentStore != null) {
+    if (hasPersistence) {
       persistentStore.close();
     }
   }

lib/sdk/server/src/test/java/com/launchdarkly/sdk/server/LDClientOfflineTest.java

@@ -3,6 +3,7 @@
 import com.google.common.collect.ImmutableMap;
 import com.launchdarkly.sdk.LDContext;
 import com.launchdarkly.sdk.LDValue;
+import com.launchdarkly.sdk.server.integrations.DataSystemModes;
 import com.launchdarkly.sdk.server.interfaces.DataSourceStatusProvider;
 import com.launchdarkly.sdk.server.subsystems.DataStore;
 
@@ -78,4 +79,17 @@ public void offlineClientGetsFlagsStateFromDataStore() throws IOException {
       assertEquals(ImmutableMap.<String, LDValue>of("key", LDValue.of(true)), state.toValuesMap());
     }
   }
-}
+
+  @Test
+  public void offlineWithFDv2DataSystemIsInitializedAndReportsValidStatus() throws IOException {
+    LDConfig config = baseConfig()
+        .dataSystem(new DataSystemModes().defaultMode())
+        .offline(true)
+        .build();
+    try (LDClient client = new LDClient("SDK_KEY", config)) {
+      assertTrue(client.dataSystem instanceof FDv2DataSystem);
+      assertTrue(client.isInitialized());
+      assertEquals(DataSourceStatusProvider.State.VALID, client.getDataSourceStatusProvider().getStatus().getState());
+    }
+  }
+}