fix: polling 304 responses handled appropriately

pkgs/sdk/server/src/Internal/DataSources/PollingDataSource.cs

@@ -65,7 +65,7 @@ private async Task UpdateTaskAsync()
             try
             {
                 var dataAndHeaders = await _featureRequestor.GetAllDataAsync();
-                if (dataAndHeaders.DataSet is null)
+                if (dataAndHeaders is null || dataAndHeaders.DataSet is null)
                 {
                     // This means it was cached, and alreadyInited was true
                     _dataSourceUpdates.UpdateStatus(DataSourceState.Valid, null);

pkgs/sdk/server/test/Internal/DataSources/PollingDataSourceTest.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Linq;
+using LaunchDarkly.Sdk.Server.Interfaces;
 using LaunchDarkly.Sdk.Server.Internal.DataSystem;
 using LaunchDarkly.Sdk.Server.Internal.Model;
 using LaunchDarkly.Sdk.Server.Subsystems;
@@ -258,6 +259,41 @@ public void InitIsNotRepeatedIfServerReturnsNotModifiedStatus()
             }
         }
 
+        [Fact]
+        public void StatusRemainsValidIfServerReturnsNotModifiedStatus()
+        {
+            var etag = @"""abc123"""; // note that etag strings must be quoted
+            var responses = Handlers.SequentialWithLastRepeating(
+                Handlers.Header("Etag", etag).Then(PollingResponse(AllData)),
+                Handlers.Status(304)
+                );
+
+            using (var server = HttpServer.Start(responses))
+            {
+                using (var dataSource = MakeDataSource(server.Uri,
+                    c => c.DataSource(Components.PollingDataSource().PollIntervalNoMinimum(BriefInterval))))
+                {
+                    dataSource.Start();
+
+                    _updateSink.Inits.ExpectValue();
+                    server.Recorder.RequireRequest();
+                    server.Recorder.RequireRequest();
+                    server.Recorder.RequireRequest();
+
+                    // We've set it up above so that all requests except the first one return a 304
+                    // status. That just means the data is unchanged, which is a healthy state, so it
+                    // should not be reported as an interruption or logged as a failure.
+                    Assert.All(_updateSink.GetAllStatusUpdates(), status =>
+                    {
+                        Assert.Equal(DataSourceState.Valid, status.State);
+                        Assert.Null(status.LastError);
+                    });
+                    AssertLogMessageRegex(false, Logging.LogLevel.Warn,
+                        "Polling for feature flag updates failed");
+                }
+            }
+        }
+
         [Fact]
         public void ResponseWithNewEtagUpdatesEtag()
         {