Kill native processes in pipeline when pipeline is disposed on Unix (#15287)

src/System.Management.Automation/engine/NativeCommandProcessor.cs

@@ -1018,9 +1018,21 @@ namespace System.Management.Automation
 
             try
             {
-                // Dispose the process if it's already created
                 if (_nativeProcess != null)
                 {
+                    // on Unix, we need to kill the process to ensure it terminates as Dispose() merely
+                    // closes the redirected streams and the processs does not exit on macOS.  However,
+                    // on Windows, a winexe like notepad should continue running so we don't want to kill it.
+#if UNIX
+                    try
+                    {
+                        _nativeProcess.Kill();
+                    }
+                    catch
+                    {
+                        // Ignore all exception since it is cleanup.
+                    }
+#endif
                     _nativeProcess.Dispose();
                 }
             }

test/powershell/Language/Scripting/NativeExecution/NativeCommandProcessor.Tests.ps1

@@ -43,6 +43,12 @@ Describe 'native commands with pipeline' -tags 'Feature' {
             $result[0] | Should -Match "pwsh"
         }
     }
+
+    It 'native command should be killed when pipeline is disposed' -Skip:($IsWindows) {
+        $yes = (Get-Process 'yes' -ErrorAction Ignore).Count
+        yes | Select-Object -First 2
+        (Get-Process 'yes' -ErrorAction Ignore).Count | Should -Be $yes
+    }
 }
 
 Describe "Native Command Processor" -tags "Feature" {