Fix thread safety issue in JsonObject.InitializeDictionary causing GetPath failures (#120619)

* Initial plan

* Fix thread safety issue in JsonObject.InitializeDictionary using Interlocked.CompareExchange

Co-authored-by: stephentoub <[email protected]>

* Remove unnecessary Interlocked.MemoryBarrier since CompareExchange is already a full barrier

Co-authored-by: stephentoub <[email protected]>

* Update src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.IDictionary.cs

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: stephentoub <[email protected]>
Co-authored-by: Stephen Toub <[email protected]>

Author: Copilot

src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonObject.IDictionary.cs

@@ -250,7 +250,7 @@ public bool Remove(string propertyName)
 
             if (dictionary is null)
             {
-                dictionary = CreateDictionary(Options);
+                OrderedDictionary<string, JsonNode?> newDictionary = CreateDictionary(Options);
 
                 if (jsonElement.HasValue)
                 {
@@ -259,14 +259,24 @@ public bool Remove(string propertyName)
                         JsonNode? node = JsonNodeConverter.Create(jElementProperty.Value, Options);
                         node?.Parent = this;
 
-                        dictionary.Add(jElementProperty.Name, node);
+                        newDictionary.Add(jElementProperty.Name, node);
                     }
                 }
 
-                // Ensure _jsonElement is written to after _dictionary
-                _dictionary = dictionary;
-                Interlocked.MemoryBarrier();
-                _jsonElement = null;
+                // Ensure only one dictionary instance is published using CompareExchange
+                OrderedDictionary<string, JsonNode?>? exchangedDictionary = Interlocked.CompareExchange(ref _dictionary, newDictionary, null);
+                if (exchangedDictionary is null)
+                {
+                    // We won the race and published our dictionary
+                    // Ensure _jsonElement is written to after _dictionary
+                    _jsonElement = null;
+                    dictionary = newDictionary;
+                }
+                else
+                {
+                    // Another thread won the race, use their dictionary
+                    dictionary = exchangedDictionary;
+                }
             }
 
             return dictionary;

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/JsonNode/JsonObjectTests.cs

@@ -1789,5 +1789,26 @@ public static void Deserialize_WrongType(string json)
         {
             Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<JsonObject>(json));
         }
+
+        [Fact]
+        public static void GetPath_IsThreadSafe()
+        {
+            for (int attempt = 0; attempt < 20; attempt++)
+            {
+                var tree = (JsonNode.Parse(
+                    """
+                    {
+                      "oh": "noes"
+                    }
+                    """
+                ) as JsonObject)!;
+
+                Parallel.ForEach(Enumerable.Range(0, 100), new ParallelOptions { MaxDegreeOfParallelism = 16 }, _ =>
+                {
+                    string path = tree.First().Value!.GetPath();
+                    Assert.Equal("$.oh", path);
+                });
+            }
+        }
     }
 }