Add _NET_WM_PID atom to Linux X11 window (#17470)

* Add `_NET_WM_PID` atom to Linux X11 window

https://github.com/AvaloniaUI/Avalonia/issues/17444

* Also append WM_CLIENT_MACHINE

* Get the host name from uname method to avoid read the file

* Fix build fail on .NET Standard 2.0

* The render window is the child window. And it should not append pid atom.

* Using the atom from atoms

* Replace UtsName with gethostname

src/Avalonia.X11/X11Window.cs

@@ -168,15 +168,20 @@ namespace Avalonia.X11
                 (int)CreateWindowArgs.InputOutput, 
                 visual,
                 new UIntPtr((uint)valueMask), ref attr);
+            AppendPid(_handle);
 
             if (_useRenderWindow)
+            {
                 _renderHandle = XCreateWindow(_x11.Display, _handle, 0, 0, defaultWidth, defaultHeight, 0, depth,
                     (int)CreateWindowArgs.InputOutput,
                     visual,
                     new UIntPtr((uint)(SetWindowValuemask.BorderPixel | SetWindowValuemask.BitGravity |
                                        SetWindowValuemask.WinGravity | SetWindowValuemask.BackingStore)), ref attr);
+            }
             else
+            {
                 _renderHandle = _handle;
+            }
 
             Handle = new PlatformHandle(_handle, "XID");
 
@@ -335,6 +340,55 @@ namespace Avalonia.X11
                 PropertyMode.Replace, ref hints, 5);
         }
 
+        /// <summary>
+        /// Append `_NET_WM_PID` atom to X11 window
+        /// </summary>
+        /// <param name="windowXId"></param>
+        private void AppendPid(IntPtr windowXId)
+        {
+            // See https://github.com/AvaloniaUI/Avalonia/issues/17444
+            var pid = (uint)s_pid;
+            // The type of `_NET_WM_PID` is `CARDINAL` which is 32-bit unsigned integer, see https://specifications.freedesktop.org/wm-spec/1.3/ar01s05.html
+            XChangeProperty(_x11.Display, windowXId,
+                _x11.Atoms._NET_WM_PID, _x11.Atoms.XA_CARDINAL, 32,
+                PropertyMode.Replace, ref pid, 1);
+
+            const int maxLength = 1024;
+            var name = stackalloc byte[maxLength];
+            var result = gethostname(name, maxLength);
+            if (result != 0)
+            {
+                // Fail
+                return;
+            }
+
+            var length = 0;
+            while (length < maxLength && name[length] != 0)
+            {
+                length++;
+            }
+
+            XChangeProperty(_x11.Display, windowXId,
+                _x11.Atoms.XA_WM_CLIENT_MACHINE, _x11.Atoms.XA_STRING, 8,
+                PropertyMode.Replace, name, length);
+        }
+
+        [DllImport("libc")]
+        private static extern int gethostname(byte* name, int len);
+
+        private static readonly int s_pid = GetProcessId();
+
+        private static int GetProcessId()
+        {
+#if NET6_0_OR_GREATER
+            var pid = Environment.ProcessId;
+#else
+            using var currentProcess = Process.GetCurrentProcess();
+            var pid = currentProcess.Id;
+#endif
+            return pid;
+        }
+
         private void UpdateSizeHints(PixelSize? preResize, bool forceDisableResize = false)
         {
             if (_overrideRedirect)