encoding/json: avoid supurious synctest deadlock detection

Use a sync.OnceValue rather than a sync.WaitGroup to
coordinate access to encoderCache entries.

The OnceValue better expresses the intent of the code
(we want to initialize the cache entry only once).

However, the motivation for this change is to avoid
testing/synctest incorrectly reporting a deadlock
when multiple bubbles call Marshal at the same time.
Goroutines blocked on WaitGroup.Wait are "durably blocked",
causing confusion when a goroutine in one bubble Waits
for a goroutine in a different bubble. Goroutines blocked
on OnceValue are not durably blocked, avoiding the problem.

Fixes #73733
For #67434

Change-Id: I81cddda80af67cf5c280fd4327620bc37e7a6fe6
Reviewed-on: https://go-review.googlesource.com/c/go/+/673335
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/encoding/json/v2_encode_test.go b/src/encoding/json/v2_encode_test.go
index 16e8d01218..11c5218649 100644
--- a/src/encoding/json/v2_encode_test.go
+++ b/src/encoding/json/v2_encode_test.go
@@ -16,7 +16,9 @@ import (
 	"regexp"
 	"runtime/debug"
 	"strconv"
+	"sync"
 	"testing"
+	"testing/synctest"
 	"time"
 )
 
@@ -1408,3 +1410,21 @@ func TestIssue63379(t *testing.T) {
 		}
 	}
 }
+
+// Issue #73733: encoding/json used a WaitGroup to coordinate access to cache entries.
+// Since WaitGroup.Wait is durably blocking, this caused apparent deadlocks when
+// multiple bubbles called json.Marshal at the same time.
+func TestSynctestMarshal(t *testing.T) {
+	var wg sync.WaitGroup
+	for range 5 {
+		wg.Go(func() {
+			synctest.Test(t, func(t *testing.T) {
+				_, err := Marshal([]string{})
+				if err != nil {
+					t.Errorf("Marshal: %v", err)
+				}
+			})
+		})
+	}
+	wg.Wait()
+}