Add LazyMultiError.

go/src/infra/gae/libs/gae/upstream_errors_test.go

Index: go/src/infra/gae/libs/gae/upstream_errors_test.go
diff --git a/go/src/infra/gae/libs/gae/upstream_errors_test.go b/go/src/infra/gae/libs/gae/upstream_errors_test.go
index a5a2e8276454482ce4eabd22ab326b6ecfafb037..33012db25b73b88a1e6ad6b447fee3abd70f6227 100644
--- a/go/src/infra/gae/libs/gae/upstream_errors_test.go
+++ b/go/src/infra/gae/libs/gae/upstream_errors_test.go
@@ -6,6 +6,7 @@ package gae
 
 import (
 	"errors"
+	"sync"
 	"testing"
 
 	. "github.com/smartystreets/goconvey/convey"
@@ -58,3 +59,79 @@ func TestUpstreamErrors(t *testing.T) {
 		So(SingleError(e), ShouldEqual, e)
 	})
 }
+
+func TestLazyMultiError(t *testing.T) {
+	t.Parallel()
+
+	Convey("Test LazyMultiError", t, func() {
+		lme := LazyMultiError{Size: 10}
+		So(lme.Get(), ShouldEqual, nil)
+
+		e := errors.New("sup")
+		lme.Assign(6, e)
+		So(lme.Get(), ShouldResemble,
+			MultiError{nil, nil, nil, nil, nil, nil, e, nil, nil, nil})
+
+		lme.Assign(2, e)
+		So(lme.Get(), ShouldResemble,
+			MultiError{nil, nil, e, nil, nil, nil, e, nil, nil, nil})
+
+		So(func() { lme.Assign(20, e) }, ShouldPanic)
+
+		Convey("Try to freak out the race detector", func() {
+			lme := LazyMultiError{Size: 64}
+			Convey("all nils", func() {
+				wg := sync.WaitGroup{}
+				for i := 0; i < 64; i++ {
+					wg.Add(1)
+					go func(i int) {
+						lme.Assign(i, nil)
+						wg.Done()
+					}(i)
+				}
+				wg.Wait()
+				So(lme.Get(), ShouldBeNil)
+			})
+			Convey("every other", func() {
+				wow := errors.New("wow")
+				wg := sync.WaitGroup{}
+				for i := 0; i < 64; i++ {
+					wg.Add(1)
+					go func(i int) {
+						e := error(nil)
+						if i&1 == 1 {
+							e = wow
+						}
+						lme.Assign(i, e)
+						wg.Done()
+					}(i)
+				}
+				wg.Wait()
+				me := make(MultiError, 64)
+				for i := range me {
+					if i&1 == 1 {
+						me[i] = wow
+					}
+				}
+				So(lme.Get(), ShouldResemble, me)
+			})
+			Convey("all", func() {
+				wow := errors.New("wow")
+				wg := sync.WaitGroup{}
+				for i := 0; i < 64; i++ {
+					wg.Add(1)
+					go func(i int) {
+						lme.Assign(i, wow)
+						wg.Done()
+					}(i)
+				}
+				wg.Wait()
+				me := make(MultiError, 64)
+				for i := range me {
+					me[i] = wow
+				}
+				So(lme.Get(), ShouldResemble, me)
+			})
+		})
+	})
+}