kubens: Add facility to store state file

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

cmd/kubens/statefile.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+
+	"github.com/ahmetb/kubectx/internal/cmdutil"
+)
+
+var defaultDir = filepath.Join(cmdutil.HomeDir(), "kubens")
+
+type NSFile struct {
+	dir string
+	ctx string
+}
+
+func NewNSFile(ctx string) NSFile { return NSFile{dir: defaultDir, ctx: ctx} }
+
+func (f NSFile) path() string { return filepath.Join(f.dir, f.ctx) }
+
+// Load reads the previous namespace setting, or returns empty if not exists.
+func (f NSFile) Load() (string, error) {
+	b, err := ioutil.ReadFile(f.path())
+	if err != nil {
+		if os.IsNotExist(err) {
+			return "", nil
+		}
+		return "", err
+	}
+	return string(bytes.TrimSpace(b)), nil
+}
+
+// Save stores the previous namespace information in the file.
+func (f NSFile) Save(value string) error {
+	d := filepath.Dir(f.path())
+	if err := os.MkdirAll(d, 0755); err != nil {
+		return err
+	}
+	return ioutil.WriteFile(f.path(), []byte(value), 0644)
+}

cmd/kubens/statefile_test.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestNSFile(t *testing.T) {
+	td, err := ioutil.TempDir(os.TempDir(), "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(td)
+
+	f := NewNSFile("foo")
+	f.dir = td
+	v, err := f.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v != "" {
+		t.Fatalf("Load() expected empty; got=%v", err)
+	}
+
+	err = f.Save("bar")
+	if err != nil {
+		t.Fatalf("Save() err=%v", err)
+	}
+
+	v, err = f.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if expected := "bar"; v != expected {
+		t.Fatalf("Load()=%q; expected=%q", v, expected)
+	}
+}