Add --pid-file to ctr

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/ctr/commands/commands.go

@@ -20,6 +20,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/containerd/containerd"
@@ -99,3 +100,22 @@ func PrintAsJSON(x interface{}) {
 	}
 	fmt.Println(string(b))
 }
+
+// WritePidFile writes the pid atomically to a file
+func WritePidFile(path string, pid int) error {
+	path, err := filepath.Abs(path)
+	if err != nil {
+		return err
+	}
+	tempPath := filepath.Join(filepath.Dir(path), fmt.Sprintf(".%s", filepath.Base(path)))
+	f, err := os.OpenFile(tempPath, os.O_RDWR|os.O_CREATE|os.O_EXCL|os.O_SYNC, 0666)
+	if err != nil {
+		return err
+	}
+	_, err = fmt.Fprintf(f, "%d", pid)
+	f.Close()
+	if err != nil {
+		return err
+	}
+	return os.Rename(tempPath, path)
+}