feat: honor polling interval across process restarts (#282)

**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

**Related issues**

SDK-881

**Describe the solution you've provided**

Polling data source builder now uses context index timestamp to
calculate if an initial polling delay.

launchdarkly-android-client-sdk/src/androidTest/java/com/launchdarkly/sdk/android/LDClientEndToEndTest.java

@@ -38,7 +38,7 @@ public class LDClientEndToEndTest {
     private Application application;
     private MockWebServer mockPollingServer;
     private URI mockPollingServerUri;
-    private final PersistentDataStore store = new InMemoryPersistentDataStore();
+    private PersistentDataStore store;
 
     @Rule
     public final ActivityScenarioRule<TestActivity> testScenario =
@@ -65,6 +65,11 @@ public void setUp() {
                 });
     }
 
+    @Before
+    public void before() {
+        store = new InMemoryPersistentDataStore();
+    }
+
     @After
     public void after() throws IOException {
         mockPollingServer.close();

launchdarkly-android-client-sdk/src/main/java/com/launchdarkly/sdk/android/ClientContextImpl.java

@@ -32,26 +32,29 @@ final class ClientContextImpl extends ClientContext {
     private final FeatureFetcher fetcher;
     private final PlatformState platformState;
     private final TaskExecutor taskExecutor;
+    private final PersistentDataStoreWrapper.PerEnvironmentData perEnvironmentData;
 
     ClientContextImpl(
             ClientContext base,
             DiagnosticStore diagnosticStore,
             FeatureFetcher fetcher,
             PlatformState platformState,
-            TaskExecutor taskExecutor
+            TaskExecutor taskExecutor,
+            PersistentDataStoreWrapper.PerEnvironmentData perEnvironmentData
     ) {
         super(base);
         this.diagnosticStore = diagnosticStore;
         this.fetcher = fetcher;
         this.platformState = platformState;
         this.taskExecutor = taskExecutor;
+        this.perEnvironmentData = perEnvironmentData;
     }
 
     static ClientContextImpl fromConfig(
             LDConfig config,
             String mobileKey,
             String environmentName,
-            FeatureFetcher fetcher,
+            PersistentDataStoreWrapper.PerEnvironmentData perEnvironmentData, FeatureFetcher fetcher,
             LDContext initialContext,
             LDLogger logger,
             PlatformState platformState,
@@ -82,14 +85,14 @@ static ClientContextImpl fromConfig(
         if (!config.getDiagnosticOptOut()) {
             diagnosticStore = new DiagnosticStore(EventUtil.makeDiagnosticParams(baseClientContext));
         }
-        return new ClientContextImpl(baseClientContext, diagnosticStore, fetcher, platformState, taskExecutor);
+        return new ClientContextImpl(baseClientContext, diagnosticStore, fetcher, platformState, taskExecutor, perEnvironmentData);
     }
 
     public static ClientContextImpl get(ClientContext context) {
         if (context instanceof ClientContextImpl) {
             return (ClientContextImpl)context;
         }
-        return new ClientContextImpl(context, null, null, null, null);
+        return new ClientContextImpl(context, null, null, null, null, null);
     }
 
     public static ClientContextImpl forDataSource(
@@ -119,7 +122,8 @@ public static ClientContextImpl forDataSource(
                 baseContextImpl.getDiagnosticStore(),
                 baseContextImpl.getFetcher(),
                 baseContextImpl.getPlatformState(),
-                baseContextImpl.getTaskExecutor()
+                baseContextImpl.getTaskExecutor(),
+                baseContextImpl.getPerEnvironmentData()
         );
     }
 
@@ -134,7 +138,8 @@ public ClientContextImpl setEvaluationContext(LDContext context) {
             this.diagnosticStore,
             this.fetcher,
             this.platformState,
-            this.taskExecutor
+            this.taskExecutor,
+            this.perEnvironmentData
         );
     }
 
@@ -154,6 +159,10 @@ public TaskExecutor getTaskExecutor() {
         return throwExceptionIfNull(taskExecutor);
     }
 
+    public PersistentDataStoreWrapper.PerEnvironmentData getPerEnvironmentData() {
+        return throwExceptionIfNull(perEnvironmentData);
+    }
+
     private static <T> T throwExceptionIfNull(T o) {
         if (o == null) {
             throw new IllegalStateException(

launchdarkly-android-client-sdk/src/main/java/com/launchdarkly/sdk/android/ComponentsImpl.java

@@ -244,30 +244,36 @@ static final class PollingDataSourceBuilderImpl extends PollingDataSourceBuilder
             implements DiagnosticDescription, DataSourceRequiresFeatureFetcher {
         @Override
         public DataSource build(ClientContext clientContext) {
-            clientContext.getDataSourceUpdateSink().setStatus(
-                    clientContext.isInBackground() ? ConnectionInformation.ConnectionMode.BACKGROUND_POLLING :
+            ClientContextImpl clientContextImpl = ClientContextImpl.get(clientContext);
+            clientContextImpl.getDataSourceUpdateSink().setStatus(
+                    clientContextImpl.isInBackground() ? ConnectionInformation.ConnectionMode.BACKGROUND_POLLING :
                             ConnectionInformation.ConnectionMode.POLLING,
                     null
             );
-            int actualPollIntervalMillis = clientContext.isInBackground() ? backgroundPollIntervalMillis :
+
+            int pollInterval = clientContextImpl.isInBackground() ? backgroundPollIntervalMillis :
                     pollIntervalMillis;
-            int initialDelayMillis;
-            if (clientContext.isInBackground() && Boolean.FALSE.equals(clientContext.getPreviouslyInBackground())) {
-                // If we're transitioning from foreground to background, then we don't want to do a
-                // poll right away because we already have recent flag data. Start polling *after*
-                // the first background poll interval.
-                initialDelayMillis = backgroundPollIntervalMillis;
-            } else {
-                // If we're in the foreground-- or, if we're in the background but we started out
-                // that way rather than transitioning-- then we should do the first poll right away.
-                initialDelayMillis = 0;
+
+            // get the last updated timestamp for this context
+            PersistentDataStoreWrapper.PerEnvironmentData perEnvironmentData = clientContextImpl.getPerEnvironmentData();
+            String hashedContextId = LDUtil.urlSafeBase64HashedContextId(clientContextImpl.getEvaluationContext());
+            String fingerprint = LDUtil.urlSafeBase64Hash(clientContextImpl.getEvaluationContext());
+            Long lastUpdated = perEnvironmentData.getLastUpdated(hashedContextId, fingerprint);
+            if (lastUpdated == null) {
+                lastUpdated = 0L; // default to beginning of time
             }
-            ClientContextImpl clientContextImpl = ClientContextImpl.get(clientContext);
+
+            // To avoid unnecessarily frequent polling requests due to process or application lifecycle, we have added
+            // this initial delay logic. Calculate how much time has passed since the last update, if that is less than
+            // the polling interval, delay by the difference, otherwise 0 delay.
+            long elapsedSinceUpdate = System.currentTimeMillis() - lastUpdated;
+            long initialDelayMillis = Math.max(pollInterval - elapsedSinceUpdate, 0);
+
             return new PollingDataSource(
-                    clientContext.getEvaluationContext(),
-                    clientContext.getDataSourceUpdateSink(),
+                    clientContextImpl.getEvaluationContext(),
+                    clientContextImpl.getDataSourceUpdateSink(),
                     initialDelayMillis,
-                    actualPollIntervalMillis,
+                    pollInterval,
                     clientContextImpl.getFetcher(),
                     clientContextImpl.getPlatformState(),
                     clientContextImpl.getTaskExecutor(),

launchdarkly-android-client-sdk/src/main/java/com/launchdarkly/sdk/android/ConnectivityManager.java

@@ -447,7 +447,7 @@ public void onSuccess(String flagsJson) {
             @Override
             public void onError(Throwable e) {
                 logger.error("Error when attempting to get flag data: [{}] [{}]: {}",
-                        LDUtil.base64Url(contextToFetch),
+                        LDUtil.urlSafeBase64(contextToFetch),
                         contextToFetch,
                         LogValues.exceptionSummary(e));
                 resultCallback.onError(e);

launchdarkly-android-client-sdk/src/main/java/com/launchdarkly/sdk/android/ContextDataManager.java

@@ -39,8 +39,6 @@
  * deferred listener calls are done via the {@link TaskExecutor} abstraction.
  */
 final class ContextDataManager {
-    static final ContextHasher HASHER = new ContextHasher();
-
     private final PersistentDataStoreWrapper.PerEnvironmentData environmentStore;
     private final int maxCachedContexts;
     private final TaskExecutor taskExecutor;
@@ -57,14 +55,15 @@ final class ContextDataManager {
 
     @NonNull private volatile LDContext currentContext;
     @NonNull private volatile EnvironmentData flags = new EnvironmentData();
-    @NonNull private volatile ContextIndex index = null;
+    @NonNull private volatile ContextIndex index;
 
     ContextDataManager(
             @NonNull ClientContext clientContext,
             @NonNull PersistentDataStoreWrapper.PerEnvironmentData environmentStore,
             int maxCachedContexts
     ) {
         this.environmentStore = environmentStore;
+        this.index = environmentStore.getIndex();
         this.maxCachedContexts = maxCachedContexts;
         this.taskExecutor = ClientContextImpl.get(clientContext).getTaskExecutor();
         this.logger = clientContext.getBaseLogger();
@@ -120,8 +119,9 @@ private void initDataInternal(
             @NonNull EnvironmentData newData,
             boolean writeFlagsToPersistentStore
     ) {
-        List<String> removedContextIds = new ArrayList<>();
-        String contextId = hashedContextId(context);
+
+        String contextId = LDUtil.urlSafeBase64HashedContextId(context);
+        String fingerprint = LDUtil.urlSafeBase64Hash(context);
         EnvironmentData oldData;
         ContextIndex newIndex;
 
@@ -133,26 +133,27 @@ private void initDataInternal(
 
             oldData = flags;
             flags = newData;
-            if (index == null) {
-                index = environmentStore.getIndex();
-            }
-            newIndex = index.updateTimestamp(contextId, System.currentTimeMillis())
-                    .prune(maxCachedContexts, removedContextIds);
-            index = newIndex;
 
-            for (String removedContextId: removedContextIds) {
-                environmentStore.removeContextData(removedContextId);
-                logger.debug("Removed flag data for context {} from persistent store", removedContextId);
-            }
-            if (writeFlagsToPersistentStore && maxCachedContexts != 0) {
-                environmentStore.setContextData(contextId, newData);
+            if (writeFlagsToPersistentStore) {
+                List<String> removedContextIds = new ArrayList<>();
+                newIndex = index.updateTimestamp(contextId, System.currentTimeMillis())
+                        .prune(maxCachedContexts, removedContextIds);
+                index = newIndex;
+
+                for (String removedContextId: removedContextIds) {
+                    environmentStore.removeContextData(removedContextId);
+                    logger.debug("Removed flag data for context {} from persistent store", removedContextId);
+                }
+
+                environmentStore.setContextData(contextId, fingerprint, newData);
+                environmentStore.setIndex(newIndex);
+
+                if (logger.isEnabled(LDLogLevel.DEBUG)) {
+                    logger.debug("Stored context index is now: {}", newIndex.toJson());
+                }
+
                 logger.debug("Updated flag data for context {} in persistent store", contextId);
             }
-            environmentStore.setIndex(newIndex);
-        }
-
-        if (logger.isEnabled(LDLogLevel.DEBUG)) {
-            logger.debug("Stored context index is now: {}", newIndex.toJson());
         }
 
         // Determine which flags were updated and notify listeners, if any
@@ -241,8 +242,11 @@ public boolean upsert(@NonNull LDContext context, @NonNull Flag flag) {
             updatedFlags = flags.withFlagUpdatedOrAdded(flag);
             flags = updatedFlags;
 
-            String contextId = hashedContextId(context);
-            environmentStore.setContextData(contextId, updatedFlags);
+            String hashedContextId = LDUtil.urlSafeBase64HashedContextId(context);
+            String fingerprint = LDUtil.urlSafeBase64Hash(context);
+            environmentStore.setContextData(hashedContextId, fingerprint, updatedFlags);
+            index = index.updateTimestamp(hashedContextId, System.currentTimeMillis());
+            environmentStore.setIndex(index);
         }
 
         Collection<String> updatedFlag = Collections.singletonList(flag.getKey());
@@ -292,10 +296,6 @@ public Collection<FeatureFlagChangeListener> getListenersByKey(String key) {
         return res == null ? new HashSet<>() : res;
     }
 
-    public static String hashedContextId(final LDContext context) {
-        return HASHER.hash(context.getFullyQualifiedKey());
-    }
-
     /**
      * Attempts to retrieve data for the specified context, if any, from the persistent store. This
      * does not affect the current context/flags state.
@@ -305,7 +305,7 @@ public static String hashedContextId(final LDContext context) {
      */
     @VisibleForTesting
     public @Nullable EnvironmentData getStoredData(LDContext context) {
-        return environmentStore.getContextData(hashedContextId(context));
+        return environmentStore.getContextData(LDUtil.urlSafeBase64HashedContextId(context));
     }
 
     private void notifyFlagListeners(Collection<String> updatedFlagKeys) {

launchdarkly-android-client-sdk/src/main/java/com/launchdarkly/sdk/android/HttpFeatureFlagFetcher.java

@@ -138,7 +138,7 @@ private Request getDefaultRequest(LDContext ldContext) throws IOException {
         // and methods like Uri.withAppendedPath, simply to minimize the amount of code that relies on
         // Android-specific APIs so our components are more easily unit-testable.
         URI uri = HttpHelpers.concatenateUriPath(pollUri, StandardEndpoints.POLLING_REQUEST_GET_BASE_PATH);
-        uri = HttpHelpers.concatenateUriPath(uri, LDUtil.base64Url(ldContext));
+        uri = HttpHelpers.concatenateUriPath(uri, LDUtil.urlSafeBase64(ldContext));
         if (evaluationReasons) {
             uri = URI.create(uri.toString() + "?withReasons=true");
         }

launchdarkly-android-client-sdk/src/main/java/com/launchdarkly/sdk/android/LDClient.java

@@ -330,7 +330,7 @@ protected LDClient(
         FeatureFetcher fetcher = null;
         if (config.dataSource instanceof ComponentsImpl.DataSourceRequiresFeatureFetcher) {
             ClientContextImpl minimalContext = ClientContextImpl.fromConfig(config, mobileKey,
-                    environmentName, null, initialContext, logger, platformState, environmentReporter, taskExecutor
+                    environmentName, environmentStore, null, initialContext, logger, platformState, environmentReporter, taskExecutor
             );
             fetcher = new HttpFeatureFlagFetcher(minimalContext);
         }
@@ -339,7 +339,7 @@ protected LDClient(
                 config,
                 mobileKey,
                 environmentName,
-                fetcher,
+                environmentStore, fetcher,
                 initialContext,
                 logger,
                 platformState,

launchdarkly-android-client-sdk/src/main/java/com/launchdarkly/sdk/android/LDUtil.java

@@ -145,7 +145,15 @@ static String urlSafeBase64Hash(String input) {
         }
     }
 
-    static String base64Url(final LDContext context) {
+    public static String urlSafeBase64HashedContextId(LDContext context) {
+        return urlSafeBase64Hash(context.getFullyQualifiedKey());
+    }
+
+    static String urlSafeBase64Hash(LDContext context) {
+        return urlSafeBase64Hash(JsonSerialization.serialize(context));
+    }
+
+    static String urlSafeBase64(LDContext context) {
         return Base64.encodeToString(JsonSerialization.serialize(context).getBytes(),
                 Base64.URL_SAFE + Base64.NO_WRAP);
     }