Fix #2690: NRE in IndexSettingsConverter when merge policy isn't set (#2692)

Author: gmarz

src/Nest/IndexModules/IndexSettings/Settings/IndexSettingsConverter.cs

@@ -41,13 +41,13 @@ public override void WriteJson(JsonWriter writer, object value, JsonSerializer s
 			d[UpdatableIndexSettings.TranslogFlushThresholdSize] = flush?.ThresholdSize;
 			d[UpdatableIndexSettings.TranslogFlushThresholdPeriod] = flush?.ThresholdPeriod;
 
-			d[UpdatableIndexSettings.MergePolicyExpungeDeletesAllowed] = ds.Merge?.Policy.ExpungeDeletesAllowed;
-			d[UpdatableIndexSettings.MergePolicyFloorSegment] = ds.Merge?.Policy.FloorSegment;
-			d[UpdatableIndexSettings.MergePolicyMaxMergeAtOnce] = ds.Merge?.Policy.MaxMergeAtOnce;
-			d[UpdatableIndexSettings.MergePolicyMaxMergeAtOnceExplicit] = ds.Merge?.Policy.MaxMergeAtOnceExplicit;
-			d[UpdatableIndexSettings.MergePolicyMaxMergedSegment] = ds.Merge?.Policy.MaxMergedSegment;
-			d[UpdatableIndexSettings.MergePolicySegmentsPerTier] = ds.Merge?.Policy.SegmentsPerTier;
-			d[UpdatableIndexSettings.MergePolicyReclaimDeletesWeight] = ds.Merge?.Policy.ReclaimDeletesWeight;
+			d[UpdatableIndexSettings.MergePolicyExpungeDeletesAllowed] = ds.Merge?.Policy?.ExpungeDeletesAllowed;
+			d[UpdatableIndexSettings.MergePolicyFloorSegment] = ds.Merge?.Policy?.FloorSegment;
+			d[UpdatableIndexSettings.MergePolicyMaxMergeAtOnce] = ds.Merge?.Policy?.MaxMergeAtOnce;
+			d[UpdatableIndexSettings.MergePolicyMaxMergeAtOnceExplicit] = ds.Merge?.Policy?.MaxMergeAtOnceExplicit;
+			d[UpdatableIndexSettings.MergePolicyMaxMergedSegment] = ds.Merge?.Policy?.MaxMergedSegment;
+			d[UpdatableIndexSettings.MergePolicySegmentsPerTier] = ds.Merge?.Policy?.SegmentsPerTier;
+			d[UpdatableIndexSettings.MergePolicyReclaimDeletesWeight] = ds.Merge?.Policy?.ReclaimDeletesWeight;
 
 			d[UpdatableIndexSettings.MergeSchedulerMaxThreadCount] = ds.Merge?.Scheduler?.MaxThreadCount;
 			d[UpdatableIndexSettings.MergeSchedulerAutoThrottle] = ds.Merge?.Scheduler?.AutoThrottle;

src/Tests/Reproduce/GithubIssue2690.cs

@@ -0,0 +1,21 @@
+using FluentAssertions;
+using Tests.Framework;
+
+namespace Tests.Reproduce
+{
+	public class GithubIssue2690
+	{
+		[U] public void EmptyPolicyCausesNullReferenceException()
+		{
+			var client = TestClient.GetInMemoryClient();
+			var response = client.CreateIndex("foo", c => c
+				.Settings(s => s
+					.Merge(m => m
+						.Scheduler(sch => sch.MaxThreadCount(1))
+					)
+				)
+			);
+			response.IsValid.Should().BeTrue();
+		}
+	}
+}

src/Tests/Tests.csproj

@@ -726,6 +726,7 @@
     <Compile Include="Reproduce\GithubIssue2323.cs" />
     <Compile Include="Reproduce\GithubIssue2409.cs" />
     <Compile Include="Reproduce\GithubIssue2503.cs" />
+    <Compile Include="Reproduce\GithubIssue2690.cs" />
     <Compile Include="Search\Hits\HitsSerializationTests.cs" />
     <Compile Include="Search\MultiSearch\MultiSearchApiTests.cs" />
     <Compile Include="Search\MultiSearch\MultiSearchInvalidApiTests.cs" />