Fix empty string received by .NET 8 users on Linux on userName (#199)

* Fix UserName for non-Windows users

* Add unit test for UserName and apply style suggestions

* Add docs

* Fix test

* Extend try and upgrade docs

Author: gdziadkiewicz

src/log4net.Tests/Core/LoggingEventTest.cs

@@ -142,4 +142,15 @@ static string ReviseThreadName(string? name)
     static void AssertIsCurrentThreadId(string name)
       => Assert.That(SystemInfo.CurrentThreadId.ToString(CultureInfo.InvariantCulture), Is.EqualTo(name));
   }
+
+  [Test]
+  public void UserNameTest()
+  {
+    string expectedUserName =
+                Environment.OSVersion.VersionString.StartsWith("Microsoft Windows")?
+                  $"{Environment.UserDomainName}\\{Environment.UserName}"
+                : Environment.UserName;
+    LoggingEvent sut = new();
+    Assert.That(sut.UserName, Is.EqualTo(expectedUserName));
+  }
 }

src/log4net/Core/LoggingEvent.cs

@@ -701,8 +701,8 @@ private static string ReviseThreadName(string? threadName)
   /// </value>
   /// <remarks>
   /// <para>
-  /// Calls <c>WindowsIdentity.GetCurrent().Name</c> to get the name of
-  /// the current windows user.
+  /// On Windows it calls <c>WindowsIdentity.GetCurrent().Name</c> to get the name of
+  /// the current windows user. On other OSes it calls Environment.UserName.
   /// </para>
   /// <para>
   /// To improve performance, we could cache the string representation of 
@@ -743,17 +743,26 @@ private static string ReviseThreadName(string? threadName)
 
   private string? TryGetCurrentUserName()
   {
-    if (_platformDoesNotSupportWindowsIdentity)
-    {
-      // we've already received one PlatformNotSupportedException
-      // and it's highly unlikely that will change
-      return Environment.UserName;
-    }
-
     try
     {
-      return _cachedWindowsIdentityUserName ??=
-          TryReadWindowsIdentityUserName();
+      if (_platformDoesNotSupportWindowsIdentity)
+      {
+        // we've already received one PlatformNotSupportedException or null from TryReadWindowsIdentityUserName
+        // and it's highly unlikely that will change
+        return Environment.UserName;
+      }
+    
+      if (_cachedWindowsIdentityUserName is not null)
+      {
+        return _cachedWindowsIdentityUserName;
+      }
+      if (TryReadWindowsIdentityUserName() is string userName)
+      {
+        _cachedWindowsIdentityUserName = userName;
+        return _cachedWindowsIdentityUserName;
+      }
+      _platformDoesNotSupportWindowsIdentity = true;
+      return Environment.UserName;
     }
     catch (PlatformNotSupportedException)
     {
@@ -777,12 +786,21 @@ private static string ReviseThreadName(string? threadName)
   }
 
   private string? _cachedWindowsIdentityUserName;
-  private static string TryReadWindowsIdentityUserName()
+  
+  /// <returns>
+  ///  On Windows: UserName in case of success, empty string for unexpected null in identity or Name
+  ///  <para/>
+  ///  On other OSes: null
+  /// </returns>
+  /// <exception cref="PlatformNotSupportedException">Thrown on non-Windows platforms on net462</exception>
+  private static string? TryReadWindowsIdentityUserName()
   {
-#if !NET462_OR_GREATER
+    // According to docs RuntimeInformation.IsOSPlatform is supported from netstandard1.1,
+    // but it's erroring in runtime on < net471
+#if NET471_OR_GREATER || NETSTANDARD2_0_OR_GREATER
     if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
     {
-      return string.Empty;
+      return null;
     }
 #endif
     using var identity = WindowsIdentity.GetCurrent();