client: utility functions for getting detail of v2 auth errors

Current v2 auth API doesn't propagate its error code. This commit adds
utility functions for parsing error messages and getting detail of v2
auth errors.

Fixes etcd-io#5894

client/util.go

@@ -14,10 +14,40 @@
 
 package client
 
+import (
+	"regexp"
+)
+
+var (
+	roleNotFoundRegExp *regexp.Regexp
+	userNotFoundRegExp *regexp.Regexp
+)
+
+func init() {
+	roleNotFoundRegExp = regexp.MustCompile("auth: Role .* does not exist.")
+	userNotFoundRegExp = regexp.MustCompile("auth: User .* does not exist.")
+}
+
 // IsKeyNotFound returns true if the error code is ErrorCodeKeyNotFound.
 func IsKeyNotFound(err error) bool {
 	if cErr, ok := err.(Error); ok {
 		return cErr.Code == ErrorCodeKeyNotFound
 	}
 	return false
 }
+
+// IsRoleNotFound returns true if the error means role not found of v2 API.
+func IsRoleNotFound(err error) bool {
+	if ae, ok := err.(authError); ok {
+		return roleNotFoundRegExp.MatchString(ae.Message)
+	}
+	return false
+}
+
+// IsUserNotFound returns true if the error means user not found of v2 API.
+func IsUserNotFound(err error) bool {
+	if ae, ok := err.(authError); ok {
+		return userNotFoundRegExp.MatchString(ae.Message)
+	}
+	return false
+}