Fix Creating Undermined Culture (#115919)

* Fix Creating Undermined Culture

* Update src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.cs

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

* Update to work on OSX

* restore removed empty line

* Feedback

---------

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

Author: tarekgh

src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.cs

@@ -667,8 +667,11 @@ private static CultureData CreateCultureWithInvariantData()
 
         internal static CultureData? GetCultureData(string? cultureName, bool useUserOverride)
         {
+            // The undetermined culture name "und" is not a real culture, but it resolves to the invariant culture because ICU typically normalizes it to an empty string.
+            const string UndeterminedCultureName = "und";
+
             // First do a shortcut for Invariant
-            if (string.IsNullOrEmpty(cultureName))
+            if (string.IsNullOrEmpty(cultureName) || cultureName.Equals(UndeterminedCultureName, StringComparison.OrdinalIgnoreCase))
             {
                 return Invariant;
             }

src/libraries/System.Private.CoreLib/src/System/Globalization/CultureInfo.cs

@@ -1052,7 +1052,8 @@ public static CultureInfo GetCultureInfo(string name)
 
             lock (nameTable)
             {
-                nameTable[name] = result;
+                // add only if it wasn't already added
+                nameTable.TryAdd(name, result);
             }
 
             return result;

src/libraries/System.Runtime/tests/System.Globalization.Tests/CultureInfo/CultureInfoCtor.cs

@@ -417,6 +417,18 @@ public void CtorUndeterminedLanguageTag(string cultureName, string expectedCultu
             Assert.Equal(expectedSortName, culture.CompareInfo.Name);
         }
 
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsIcuGlobalization))]
+        public void UndeterminedCultureTest()
+        {
+            CultureInfo invariant1 = CultureInfo.GetCultureInfo("");
+
+            CultureInfo undetermined1 = CultureInfo.GetCultureInfo("und");
+            Assert.Equal("", undetermined1.Name);
+
+            CultureInfo invariant2 = CultureInfo.GetCultureInfo("");
+            Assert.True(object.ReferenceEquals(invariant1, invariant2), "CultureInfo.GetCultureInfo(\"\") is not returning the previously cached instance");
+        }
+
         [Theory]
         [MemberData(nameof(Ctor_String_TestData))]
         public void Ctor_String(string name, string[] expectedNames)