Add utulity to expand tilde in file path

Relates-To: #460
Change-Id: I564887643388d5db32e13b3f432f3d0f1b6428e7

pkg/util/errors.go

@@ -0,0 +1,26 @@
+/*
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+     https://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+*/
+
+package util
+
+import "fmt"
+
+// ErrCantExpandTildePath returned when malformed path is passed to ExpandTilde function
+type ErrCantExpandTildePath struct {
+	Path string
+}
+
+func (e *ErrCantExpandTildePath) Error() string {
+	return fmt.Sprintf("cannot expand user-specific home dir: %s", e.Path)
+}

pkg/util/homedir.go

@@ -14,7 +14,10 @@
 
 package util
 
-import "os"
+import (
+	"os"
+	"path/filepath"
+)
 
 // UserHomeDir is a utility function that wraps os.UserHomeDir and returns no
 // errors. If the user has no home directory, the returned value will be the
@@ -26,3 +29,23 @@ func UserHomeDir() string {
 	}
 	return homeDir
 }
+
+// ExpandTilde expands the path to include the home directory if the path
+// is prefixed with `~`. If it isn't prefixed with `~`, the path is
+// returned as-is.
+// Original source code: https://github.com/mitchellh/go-homedir/blob/master/homedir.go#L55-L77
+func ExpandTilde(path string) (string, error) {
+	if len(path) == 0 {
+		return path, nil
+	}
+
+	if path[0] != '~' {
+		return path, nil
+	}
+
+	if len(path) > 1 && path[1] != '/' {
+		return "", &ErrCantExpandTildePath{}
+	}
+
+	return filepath.Join(UserHomeDir(), path[1:]), nil
+}

pkg/util/homedir_test.go

@@ -16,6 +16,8 @@ package util_test
 
 import (
 	"os"
+	"path"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -41,3 +43,35 @@ func setHome(path string) (resetHome func()) {
 		os.Setenv("HOME", oldHome)
 	}
 }
+
+func TestExpandTilde(t *testing.T) {
+	t.Run("success home path", func(t *testing.T) {
+		airshipDir := ".airship"
+		dir := filepath.Join("~", airshipDir)
+		expandedDir, err := util.ExpandTilde(dir)
+		assert.NoError(t, err)
+		homedir := path.Join(util.UserHomeDir(), airshipDir)
+		assert.Equal(t, homedir, expandedDir)
+	})
+
+	t.Run("success nothing to expand", func(t *testing.T) {
+		dir := "/home/ubuntu/.airship"
+		expandedDir, err := util.ExpandTilde(dir)
+		assert.NoError(t, err)
+		assert.Equal(t, dir, expandedDir)
+	})
+
+	t.Run("error malformed path", func(t *testing.T) {
+		malformedDir := "~.home/ubuntu/.airship"
+		expandedDir, err := util.ExpandTilde(malformedDir)
+		assert.Error(t, err)
+		assert.Equal(t, "", expandedDir)
+	})
+
+	t.Run("success empty path", func(t *testing.T) {
+		emptyDir := ""
+		expandedDir, err := util.ExpandTilde(emptyDir)
+		assert.NoError(t, err)
+		assert.Equal(t, emptyDir, expandedDir)
+	})
+}