From 6c932e15420249975c06469db68b33606b716bb2 Mon Sep 17 00:00:00 2001
From: Todd Anderson <tanderson@launchdarkly.com>
Date: Thu, 3 Sep 2026 15:14:14 -0400
Subject: [PATCH] fix: Avoid NullReferenceException on polling 304 responses

FeatureRequestor.GetAllDataAsync() signals an unchanged payload by
returning a null DataSetWithHeaders, but PollingDataSource dereferenced
dataAndHeaders.DataSet without first checking the wrapper itself. Every
poll that received a 304 therefore threw a NullReferenceException, which
was swallowed by the generic handler, logged as a warning, and reported
as DataSourceState.Interrupted -- leaving a polling client with stable
flags permanently in an Interrupted state.

The inner DataSet null check is retained since it is harmless, but note
that it is currently unreachable: DataSetWithHeaders is only ever
constructed with a non-null data set.
---
 .../Internal/DataSources/PollingDataSource.cs |  2 +-
 .../DataSources/PollingDataSourceTest.cs      | 36 +++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/pkgs/sdk/server/src/Internal/DataSources/PollingDataSource.cs b/pkgs/sdk/server/src/Internal/DataSources/PollingDataSource.cs
index 9048c9fa..a71ab630 100644
--- a/pkgs/sdk/server/src/Internal/DataSources/PollingDataSource.cs
+++ b/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);
diff --git a/pkgs/sdk/server/test/Internal/DataSources/PollingDataSourceTest.cs b/pkgs/sdk/server/test/Internal/DataSources/PollingDataSourceTest.cs
index af92f2b7..069e07ff 100644
--- a/pkgs/sdk/server/test/Internal/DataSources/PollingDataSourceTest.cs
+++ b/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()
         {