Add replace rule function (vektah#338)

Signed-off-by: Steve Coffman <steve@khanacademy.org>

Author: StevenACoffman

validator/validator.go

@@ -17,12 +17,15 @@ type Rule struct {
 
 var specifiedRules []Rule
 
-// AddRule adds rule to the rule set.
-// f is called once each time `Validate` is executed.
+// AddRule adds a rule to the rule set.
+// ruleFunc is called once each time `Validate` is executed.
 func AddRule(name string, ruleFunc RuleFunc) {
 	specifiedRules = append(specifiedRules, Rule{Name: name, RuleFunc: ruleFunc})
 }
 
+// RemoveRule removes an existing rule from the rule set
+// if one of the same name exists.
+// The rule set is global, so it is not safe for concurrent changes
 func RemoveRule(name string) {
 	var result []Rule // nolint:prealloc // using initialized with len(rules) produces a race condition
 	for _, r := range specifiedRules {
@@ -34,6 +37,28 @@ func RemoveRule(name string) {
 	specifiedRules = result
 }
 
+// ReplaceRule replaces an existing rule from the rule set
+// if one of the same name exists.
+// If no match is found, it will add a new rule to the rule set.
+// The rule set is global, so it is not safe for concurrent changes
+func ReplaceRule(name string, ruleFunc RuleFunc) {
+	var found bool
+	var result []Rule // nolint:prealloc // using initialized with len(rules) produces a race condition
+	for _, r := range specifiedRules {
+		if r.Name == name {
+			found = true
+			result = append(result, Rule{Name: name, RuleFunc: ruleFunc})
+			continue
+		}
+		result = append(result, r)
+	}
+	if !found {
+		specifiedRules = append(specifiedRules, Rule{Name: name, RuleFunc: ruleFunc})
+		return
+	}
+	specifiedRules = result
+}
+
 func Validate(schema *Schema, doc *QueryDocument, rules ...Rule) gqlerror.List {
 	if rules == nil {
 		rules = specifiedRules