Ensure serializer reflection-dependency sentinel is accurate

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -9,7 +9,8 @@
     <Nullable>enable</Nullable>
     <IncludeInternalObsoleteAttribute>true</IncludeInternalObsoleteAttribute>
     <IsPackable>true</IsPackable>
-    <ServicingVersion>3</ServicingVersion>
+    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+    <ServicingVersion>4</ServicingVersion>
     <PackageDescription>Provides high-performance and low-allocating types that serialize objects to JavaScript Object Notation (JSON) text and deserialize JSON text to objects, with UTF-8 support built-in. Also provides types to read and write JSON text encoded as UTF-8, and to create an in-memory document object model (DOM), that is read-only, for random access of the JSON elements within a structured view of the data.
 
 Commonly Used Types:

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Converters.cs

@@ -10,6 +10,7 @@
 using System.Text.Json.Serialization;
 using System.Text.Json.Serialization.Converters;
 using System.Text.Json.Serialization.Metadata;
+using System.Threading;
 
 namespace System.Text.Json
 {
@@ -30,10 +31,10 @@ public sealed partial class JsonSerializerOptions
         [RequiresUnreferencedCode(JsonSerializer.SerializationUnreferencedCodeMessage)]
         private void RootBuiltInConverters()
         {
-            if (s_defaultSimpleConverters is null)
+            if (Volatile.Read(ref s_defaultFactoryConverters) is null)
             {
                 s_defaultSimpleConverters = GetDefaultSimpleConverters();
-                s_defaultFactoryConverters = new JsonConverter[]
+                Volatile.Write(ref s_defaultFactoryConverters, new JsonConverter[]
                 {
                     // Check for disallowed types.
                     new UnsupportedTypeConverterFactory(),
@@ -48,7 +49,7 @@ private void RootBuiltInConverters()
                     new IEnumerableConverterFactory(),
                     // Object should always be last since it converts any type.
                     new ObjectConverterFactory()
-                };
+                });
             }
         }