helper/validation: Add ValidateListUniqueStrings

Add the ValidateListUniqueStrings function, which is a ValidateFunc that
ensures a list has no duplicate items in it. It's useful for when a list
is needed over a set because order matters, yet the items still need to
be unique.

helper/validation/validation.go

@@ -144,3 +144,17 @@ func ValidateJsonString(v interface{}, k string) (ws []string, errors []error) {
 	}
 	return
 }
+
+// ValidateListUniqueStrings is a ValidateFunc that ensures a list has no
+// duplicate items in it. It's useful for when a list is needed over a set
+// because order matters, yet the items still need to be unique.
+func ValidateListUniqueStrings(v interface{}, k string) (ws []string, errors []error) {
+	for n1, v1 := range v.([]interface{}) {
+		for n2, v2 := range v.([]interface{}) {
+			if v1.(string) == v2.(string) && n1 != n2 {
+				errors = append(errors, fmt.Errorf("%q: duplicate entry - %s", k, v1.(string)))
+			}
+		}
+	}
+	return
+}

helper/validation/validation_test.go

@@ -166,6 +166,25 @@ func TestValidateJsonString(t *testing.T) {
 	}
 }
 
+func TestValidateListUniqueStrings(t *testing.T) {
+	runTestCases(t, []testCase{
+		{
+			val: []interface{}{"foo", "bar"},
+			f:   ValidateListUniqueStrings,
+		},
+		{
+			val:         []interface{}{"foo", "bar", "foo"},
+			f:           ValidateListUniqueStrings,
+			expectedErr: regexp.MustCompile("duplicate entry - foo"),
+		},
+		{
+			val:         []interface{}{"foo", "bar", "foo", "baz", "bar"},
+			f:           ValidateListUniqueStrings,
+			expectedErr: regexp.MustCompile("duplicate entry - (?:foo|bar)"),
+		},
+	})
+}
+
 func runTestCases(t *testing.T, cases []testCase) {
 	matchErr := func(errs []error, r *regexp.Regexp) bool {
 		// err must match one provided
@@ -185,6 +204,10 @@ func runTestCases(t *testing.T, cases []testCase) {
 			continue
 		}
 
+		if len(errs) != 0 && tc.expectedErr == nil {
+			t.Fatalf("expected test case %d to produce no errors, got %v", i, errs)
+		}
+
 		if !matchErr(errs, tc.expectedErr) {
 			t.Fatalf("expected test case %d to produce error matching \"%s\", got %v", i, tc.expectedErr, errs)
 		}