Move supervisor to it's own package

It allows to keep main namespace cleaner

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

supervisor/io.go

@@ -0,0 +1,66 @@
+package supervisor
+
+import (
+	"io"
+	"os"
+)
+
+type ioConfig struct {
+	StdoutPath string
+	StderrPath string
+	StdinPath  string
+
+	Stdin  io.WriteCloser
+	Stdout io.ReadCloser
+	Stderr io.ReadCloser
+}
+
+func newCopier(i *ioConfig) (*copier, error) {
+	l := &copier{
+		config: i,
+	}
+	if i.StdinPath != "" {
+		f, err := os.OpenFile(i.StdinPath, os.O_RDONLY, 0)
+		if err != nil {
+			return nil, err
+		}
+		l.closers = append(l.closers, f)
+		go func() {
+			io.Copy(i.Stdin, f)
+			i.Stdin.Close()
+		}()
+	}
+	if i.StdoutPath != "" {
+		f, err := os.OpenFile(i.StdoutPath, os.O_RDWR, 0)
+		if err != nil {
+			return nil, err
+		}
+		l.closers = append(l.closers, f)
+		go io.Copy(f, i.Stdout)
+	}
+	if i.StderrPath != "" {
+		f, err := os.OpenFile(i.StderrPath, os.O_RDWR, 0)
+		if err != nil {
+			return nil, err
+		}
+		l.closers = append(l.closers, f)
+		go io.Copy(f, i.Stderr)
+	}
+	return l, nil
+}
+
+type copier struct {
+	config  *ioConfig
+	closers []io.Closer
+}
+
+func (l *copier) Close() (err error) {
+	for _, c := range append(l.closers, l.config.Stdin, l.config.Stdout, l.config.Stderr) {
+		if c != nil {
+			if cerr := c.Close(); err == nil {
+				err = cerr
+			}
+		}
+	}
+	return err
+}