Add -pidfile

Fixes #200

vendor/github.com/facebookgo/pidfile/pidfile.go

@@ -0,0 +1,89 @@
+// Package pidfile manages pid files.
+package pidfile
+
+import (
+	"bytes"
+	"errors"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+
+	"github.com/facebookgo/atomicfile"
+)
+
+var (
+	errNotConfigured = errors.New("pidfile not configured")
+	pidfile          = flag.String("pidfile", "", "If specified, write pid to file.")
+)
+
+// IsNotConfigured returns true if the error indicates the pidfile location has
+// not been configured.
+func IsNotConfigured(err error) bool {
+	if err == errNotConfigured {
+		return true
+	}
+	return false
+}
+
+// GetPidfilePath returns the configured pidfile path.
+func GetPidfilePath() string {
+	return *pidfile
+}
+
+// SetPidfilePath sets the pidfile path.
+func SetPidfilePath(p string) {
+	*pidfile = p
+}
+
+// Write the pidfile based on the flag. It is an error if the pidfile hasn't
+// been configured.
+func Write() error {
+	if *pidfile == "" {
+		return errNotConfigured
+	}
+
+	if err := os.MkdirAll(filepath.Dir(*pidfile), os.FileMode(0755)); err != nil {
+		return err
+	}
+
+	file, err := atomicfile.New(*pidfile, os.FileMode(0644))
+	if err != nil {
+		return fmt.Errorf("error opening pidfile %s: %s", *pidfile, err)
+	}
+	defer file.Close() // in case we fail before the explicit close
+
+	_, err = fmt.Fprintf(file, "%d", os.Getpid())
+	if err != nil {
+		return err
+	}
+
+	err = file.Close()
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Read the pid from the configured file. It is an error if the pidfile hasn't
+// been configured.
+func Read() (int, error) {
+	if *pidfile == "" {
+		return 0, errNotConfigured
+	}
+
+	d, err := ioutil.ReadFile(*pidfile)
+	if err != nil {
+		return 0, err
+	}
+
+	pid, err := strconv.Atoi(string(bytes.TrimSpace(d)))
+	if err != nil {
+		return 0, fmt.Errorf("error parsing pid from %s: %s", *pidfile, err)
+	}
+
+	return pid, nil
+}