Use taskkill to kill a process tree on Windows

* Inspired by https://stackoverflow.com/a/61113184/388803
* Using a subprocess for that is a bit heavy, OTOH we are already creating
  subprocess in ChildProcess and it only happens for a leader process on Windows.
* There seems to be no other alternative which works reliably or it would incur huge complexity.

README.md

@@ -148,8 +148,6 @@ process.leader = true
 process.start
 ```
 
-Note this does not work on Windows.
-
 #### Detach from parent
 
 ```ruby

lib/childprocess/abstract_process.rb

@@ -28,9 +28,7 @@ class AbstractProcess
     # Set this to true to make the child process the leader of a new process group
     #
     # This can be used to make sure that all grandchildren are killed
-    # when the child process dies on non-Windows.
-    #
-    # Note that grandchildren are not killed on Windows even when the process is the leader of a new process group.
+    # when the child process dies.
     #
     attr_accessor :leader
 

lib/childprocess/process_spawn_process.rb

@@ -112,19 +112,15 @@ def send_signal(sig)
       assert_started
 
       log "sending #{sig}"
-      ::Process.kill sig, _pid
-    end
-
-    def _pid
       if leader?
         if ChildProcess.unix?
-          -@pid # negative pid == process group
+          ::Process.kill sig, -@pid # negative pid == process group
         else
-          ChildProcess.logger.warn "ChildProcess#stop on leader of a new process group does not kill subprocess on Windows"
-          @pid
+          output = `taskkill /F /T /PID #{@pid}`
+          log output
         end
       else
-        @pid
+        ::Process.kill sig, @pid
       end
     end
   end

spec/childprocess_spec.rb

@@ -299,52 +299,20 @@
     }
   end
 
-  if ChildProcess.unix?
-    it 'kills the full process tree on unix' do
-      Tempfile.open('kill-process-tree') do |file|
-        process = write_pid_in_sleepy_grand_child(file.path)
-        process.leader = true
-        process.start
-
-        pid = wait_until(30) do
-          Integer(rewind_and_read(file)) rescue nil
-        end
-
-        process.stop
-        expect(alive?(process.pid)).to eql(false)
+  it 'kills the full process tree' do
+    Tempfile.open('kill-process-tree') do |file|
+      process = write_pid_in_sleepy_grand_child(file.path)
+      process.leader = true
+      process.start
 
-        wait_until(3) { expect(alive?(pid)).to eql(false) }
+      pid = wait_until(30) do
+        Integer(rewind_and_read(file)) rescue nil
       end
-    end
-  elsif ChildProcess.windows?
-    it 'does not kill the full process tree on windows' do
-      Tempfile.open('no-kill-process-tree') do |file|
-        process = write_pid_in_sleepy_grand_child(file.path)
-        process.leader = true
-        process.start
 
-        pid = wait_until(30) do
-          Integer(rewind_and_read(file)) rescue nil
-        end
+      process.stop
+      expect(process).to be_exited
 
-        log = StringIO.new
-        original_logger = ChildProcess.logger
-        begin
-          ChildProcess.logger = Logger.new(log)
-          ChildProcess.logger.level = Logger::WARN
-          process.stop
-        ensure
-          ChildProcess.logger = original_logger
-        end
-        expect(log.string).to include("ChildProcess#stop on leader of a new process group does not kill subprocess on Windows")
-
-        expect(alive?(process.pid)).to eql(false)
-
-        # The grand child is not killed on Windows:
-        expect(alive?(pid)).to eql(true)
-        Process.kill(:SIGKILL, pid)
-        wait_until(3) { expect(alive?(pid)).to eql(false) }
-      end
+      wait_until(3) { expect(alive?(pid)).to eql(false) }
     end
   end