[cipd] Move ValidateRoot to common package

cipd/client/cipd/common/common.go

Index: cipd/client/cipd/common/common.go
diff --git a/cipd/client/cipd/common/common.go b/cipd/client/cipd/common/common.go
index 14764f2705bb0e3e716dc602e229cf51be76803d..7930c5452bb06a83b7ac154b7c599e12f07eb05b 100644
--- a/cipd/client/cipd/common/common.go
+++ b/cipd/client/cipd/common/common.go
@@ -7,6 +7,7 @@ package common
 
 import (
 	"fmt"
+	"path"
 	"regexp"
 	"runtime"
 	"strings"
@@ -119,6 +120,30 @@ func ValidateInstanceVersion(v string) error {
 	return fmt.Errorf("bad version (not an instance ID, a ref or a tag): %q", v)
 }
 
+// ValidateRoot returns an error if the string can't be used as an ensure-file
+// root.
+func ValidateRoot(root string) error {
+	if root == "" { // empty is fine
+		return nil
+	}
+	if strings.Contains(root, "\\") {
+		return fmt.Errorf(`bad root path: backslashes not allowed (use "/"): %q`, root)
+	}
+	if strings.Contains(root, ":") {
+		return fmt.Errorf(`bad root path: colons are not allowed: %q`, root)
+	}
+	if cleaned := path.Clean(root); cleaned != root {
+		return fmt.Errorf("bad root path: %q (should be %q)", root, cleaned)
+	}
+	if strings.HasPrefix(root, "./") || strings.HasPrefix(root, "../") || root == "." {
+		return fmt.Errorf(`bad root path: invalid ".": %q`, root)
+	}
+	if strings.HasPrefix(root, "/") {
+		return fmt.Errorf("bad root path: absolute paths not allowed: %q", root)
+	}
+	return nil
+}
+
 // GetInstanceTagKey returns key portion of the instance tag or empty string.
 func GetInstanceTagKey(t string) string {
 	chunks := strings.SplitN(t, ":", 2)