Terminate child processes on Windows (#4723)

Windows doesn't have signals, the default Kill doesn't actually kill
anything, and other forms of termination don't allow cleanup. So we
spawn preview processes in a new process group and send them a
CTRL_BREAK_EVENT to terminate.

However, we only do this for "pwsh" (PowerShell 7+) and unknown/
posix-ish shells, because cmd.exe and Windows PowerShell
("powershell.exe") don't always exit on Ctrl-Break. pwsh also needs
the -NonInteractive flag to exit on Ctrl-Break.

If the process wasn't given its own group, or if sending the console
control event fails, we fall back to the standard Kill (which likely
won't help, but doesn't hurt to try).

Fix #3134

src/util/util_windows.go

@@ -11,6 +11,8 @@ import (
 	"strings"
 	"sync/atomic"
 	"syscall"
+
+	"golang.org/x/sys/windows"
 )
 
 type shellType int
@@ -19,6 +21,7 @@ const (
 	shellTypeUnknown shellType = iota
 	shellTypeCmd
 	shellTypePowerShell
+	shellTypePwsh
 )
 
 var escapeRegex = regexp.MustCompile(`[&|<>()^%!"]`)
@@ -46,7 +49,10 @@ func NewExecutor(withShell string) *Executor {
 	} else if strings.HasPrefix(basename, "cmd") {
 		shellType = shellTypeCmd
 		args = []string{"/s/c"}
-	} else if strings.HasPrefix(basename, "pwsh") || strings.HasPrefix(basename, "powershell") {
+	} else if strings.HasPrefix(basename, "pwsh") {
+		shellType = shellTypePwsh
+		args = []string{"-NoProfile", "-Command"}
+	} else if strings.HasPrefix(basename, "powershell") {
 		shellType = shellTypePowerShell
 		args = []string{"-NoProfile", "-Command"}
 	} else {
@@ -56,8 +62,12 @@ func NewExecutor(withShell string) *Executor {
 }
 
 // ExecCommand executes the given command with $SHELL
-// FIXME: setpgid is unused. We set it in the Unix implementation so that we
-// can kill preview process with its child processes at once.
+//
+// On Windows, setpgid controls whether the spawned process is placed in a new
+// process group (so that it can be signaled independently, e.g. for previews).
+// However, we only do this for "pwsh" and non-standard shells, because cmd.exe
+// and Windows PowerShell ("powershell.exe") don't always exit on Ctrl-Break.
+//
 // NOTE: For "powershell", we should ideally set output encoding to UTF8,
 // but it is left as is now because no adverse effect has been observed.
 func (x *Executor) ExecCommand(command string, setpgid bool) *exec.Cmd {
@@ -73,19 +83,31 @@ func (x *Executor) ExecCommand(command string, setpgid bool) *exec.Cmd {
 		}
 		x.shellPath.Store(shell)
 	}
+
+	var creationFlags uint32
+	// Set new process group for pwsh (PowerShell 7+) and unknown/posix-ish shells
+	if setpgid && (x.shellType == shellTypePwsh || x.shellType == shellTypeUnknown) {
+		creationFlags = windows.CREATE_NEW_PROCESS_GROUP
+	}
+
 	var cmd *exec.Cmd
 	if x.shellType == shellTypeCmd {
 		cmd = exec.Command(shell)
 		cmd.SysProcAttr = &syscall.SysProcAttr{
 			HideWindow:    false,
 			CmdLine:       fmt.Sprintf(`%s "%s"`, strings.Join(x.args, " "), command),
-			CreationFlags: 0,
+			CreationFlags: creationFlags,
 		}
 	} else {
-		cmd = exec.Command(shell, append(x.args, command)...)
+		args := x.args
+		if setpgid && x.shellType == shellTypePwsh {
+			// pwsh needs -NonInteractive flag to exit on Ctrl-Break
+			args = append([]string{"-NonInteractive"}, x.args...)
+		}
+		cmd = exec.Command(shell, append(args, command)...)
 		cmd.SysProcAttr = &syscall.SysProcAttr{
 			HideWindow:    false,
-			CreationFlags: 0,
+			CreationFlags: creationFlags,
 		}
 	}
 	return cmd
@@ -156,7 +178,7 @@ func (x *Executor) QuoteEntry(entry string) string {
 		   fd -H --no-ignore -td -d 4 | fzf --preview ".\eza.exe --color=always --tree --level=3 --icons=always {}" --with-shell "powershell -NoProfile -Command"
 		*/
 		return escapeArg(entry)
-	case shellTypePowerShell:
+	case shellTypePowerShell, shellTypePwsh:
 		escaped := strings.ReplaceAll(entry, `"`, `\"`)
 		return "'" + strings.ReplaceAll(escaped, "'", "''") + "'"
 	default:
@@ -166,6 +188,21 @@ func (x *Executor) QuoteEntry(entry string) string {
 
 // KillCommand kills the process for the given command
 func KillCommand(cmd *exec.Cmd) error {
+	// Safely handle nil command or process.
+	if cmd == nil || cmd.Process == nil {
+		return nil
+	}
+	// If it has its own process group, we can send it Ctrl-Break
+	if cmd.SysProcAttr != nil && cmd.SysProcAttr.CreationFlags&windows.CREATE_NEW_PROCESS_GROUP != 0 {
+		if err := windows.GenerateConsoleCtrlEvent(windows.CTRL_BREAK_EVENT, uint32(cmd.Process.Pid)); err == nil {
+			return nil
+		}
+	}
+	// If it's the same process group, or if sending the console control event
+	// fails (e.g., no console, different console, or process already exited),
+	// fall back to a standard kill.  This probably won't *help* if there's I/O
+	// going on, because Wait() will still hang until the I/O finishes unless we
+	// hard-kill the entire process group.  But it doesn't hurt to try!
 	return cmd.Process.Kill()
 }