Port a bug fix for GetPathRelativeTo over from Terragrunt (#76)

Original bug fix: gruntwork-io/terragrunt#100

files/paths.go

@@ -1,11 +1,13 @@
 package files
 
 import (
+	"io/ioutil"
 	"path/filepath"
 	"regexp"
-	"io/ioutil"
-	"github.com/gruntwork-io/go-commons/errors"
+
 	"github.com/mattn/go-zglob"
+
+	"github.com/gruntwork-io/go-commons/errors"
 )
 
 // Return the canonical version of the given path, relative to the given base path. That is, if the given path is a
@@ -68,6 +70,13 @@ func Grep(regex *regexp.Regexp, glob string) (bool, error) {
 
 // Return the relative path you would have to take to get from basePath to path
 func GetPathRelativeTo(path string, basePath string) (string, error) {
+	if path == "" {
+		path = "."
+	}
+	if basePath == "" {
+		basePath = "."
+	}
+
 	inputFolderAbs, err := filepath.Abs(basePath)
 	if err != nil {
 		return "", errors.WithStackTrace(err)
@@ -83,6 +92,5 @@ func GetPathRelativeTo(path string, basePath string) (string, error) {
 		return "", errors.WithStackTrace(err)
 	}
 
-	return relPath, nil
+	return filepath.ToSlash(relPath), nil
 }
-

files/paths_test.go

@@ -1,20 +1,23 @@
 package files
 
 import (
+	"regexp"
 	"testing"
+
 	"github.com/stretchr/testify/assert"
-	"regexp"
 )
 
 func TestGetPathRelativeTo(t *testing.T) {
 	t.Parallel()
 
 	testCases := []struct {
-		path	 string
+		path     string
 		basePath string
 		expected string
 	}{
 		{"", "", "."},
+		{"", "./child", ".."},
+		{"./child", "", "child"},
 		{"/root", "/root", "."},
 		{"/root", "/root/child", ".."},
 		{"/root", "/root/child/sub-child/sub-sub-child", "../../.."},
@@ -35,7 +38,7 @@ func TestCanonicalPath(t *testing.T) {
 	t.Parallel()
 
 	testCases := []struct {
-		path	 string
+		path     string
 		basePath string
 		expected string
 	}{
@@ -97,4 +100,4 @@ func TestGrepDoesNotFindInvalidText(t *testing.T) {
 	found, err := Grep(regex, "../fixtures/files/**/*")
 	assert.Nil(t, err, "Unexpected error: %v", err)
 	assert.False(t, found)
-}
+}