escape filenames that have invisible characters while allowing other languages

Fixes #712

src/croc/croc.go

@@ -1209,8 +1209,7 @@ func (c *Client) processMessageFileInfo(m message.Message) (done bool, err error
 		if strings.Contains(c.FilesToTransfer[i].FolderRemote, ".ssh") {
 			return true, fmt.Errorf("invalid path detected: '%s'", fi.FolderRemote)
 		}
-		// Issue #595 - disallow filenames with anything but 0-9a-zA-Z.-_. and / characters
-
+		// Issue #595 - disallow filenames with invisible characters
 		if !utils.ValidFileName(path.Join(c.FilesToTransfer[i].FolderRemote, fi.Name)) {
 			return true, fmt.Errorf("invalid filename detected: '%s'", fi.Name)
 		}

src/utils/utils.go

@@ -21,6 +21,7 @@ import (
 	"path/filepath"
 	"strings"
 	"time"
+	"unicode"
 
 	"github.com/cespare/xxhash"
 	"github.com/kalafut/imohash"
@@ -485,16 +486,21 @@ func UnzipDirectory(destination string, source string) error {
 }
 
 // ValidFileName checks if a filename is valid
-// and returns true only if it all of the characters are either
-// 0-9, a-z, A-Z, ., _, -, space, or /
+// by making sure it has no invisible characters
 func ValidFileName(fname string) bool {
-	for _, r := range fname {
-		if !((r >= '0' && r <= '9') ||
-			(r >= 'a' && r <= 'z') ||
-			(r >= 'A' && r <= 'Z') ||
-			r == '.' || r == '_' || r == '-' || r == ' ' || r == '/') {
-			return false
+	clean1 := strings.Map(func(r rune) rune {
+		if unicode.IsGraphic(r) {
+			return r
 		}
-	}
-	return true
+		return -1
+	}, fname)
+
+	clean2 := strings.Map(func(r rune) rune {
+		if unicode.IsPrint(r) {
+			return r
+		}
+		return -1
+	}, fname)
+
+	return (fname == clean1) && (fname == clean2)
 }

src/utils/utils_test.go

@@ -231,3 +231,10 @@ func TestFindOpenPorts(t *testing.T) {
 func TestIsLocalIP(t *testing.T) {
 	assert.True(t, IsLocalIP("192.168.0.14:9009"))
 }
+
+func TestValidFileName(t *testing.T) {
+	// contains regular characters
+	assert.True(t, ValidFileName("中文.csl"))
+	// contains invisible character
+	assert.False(t, ValidFileName("D中文.cslouglas​"))
+}