join: fix ELOOP error path

When returning an error, we should use the original path in the error
message. Previously we would append the unresolved remaining path to the
root, which is not a real path.

Fixes: 322b993 ("Ditch pkg/errors, use native error (un)wrapping")
Signed-off-by: Aleksa Sarai <[email protected]>

join.go

@@ -53,20 +53,21 @@ func SecureJoinVFS(root, unsafePath string, vfs VFS) (string, error) {
 
 	unsafePath = filepath.FromSlash(unsafePath)
 	var (
-		currentPath string
-		linksWalked int
+		currentPath   string
+		remainingPath = unsafePath
+		linksWalked   int
 	)
-	for unsafePath != "" {
-		if v := filepath.VolumeName(unsafePath); v != "" {
-			unsafePath = unsafePath[len(v):]
+	for remainingPath != "" {
+		if v := filepath.VolumeName(remainingPath); v != "" {
+			remainingPath = remainingPath[len(v):]
 		}
 
 		// Get the next path component.
 		var part string
-		if i := strings.IndexRune(unsafePath, filepath.Separator); i == -1 {
-			part, unsafePath = unsafePath, ""
+		if i := strings.IndexRune(remainingPath, filepath.Separator); i == -1 {
+			part, remainingPath = remainingPath, ""
 		} else {
-			part, unsafePath = unsafePath[:i], unsafePath[i+1:]
+			part, remainingPath = remainingPath[:i], remainingPath[i+1:]
 		}
 
 		// Apply the component lexically to the path we are building.
@@ -103,7 +104,7 @@ func SecureJoinVFS(root, unsafePath string, vfs VFS) (string, error) {
 		if err != nil {
 			return "", err
 		}
-		unsafePath = dest + string(filepath.Separator) + unsafePath
+		remainingPath = dest + string(filepath.Separator) + remainingPath
 		// Absolute symlinks reset any work we've already done.
 		if filepath.IsAbs(dest) {
 			currentPath = ""