ddl: implement backoff strategy for async event notification in tests (…

…pingcap#58386)

close pingcap#58344, close pingcap#58421

pkg/ddl/ddl.go

@@ -594,7 +594,19 @@ func asyncNotifyEvent(jobCtx *jobContext, e *notifier.SchemaChangeEvent, job *mo
 	if intest.InTest {
 		ch := jobCtx.oldDDLCtx.ddlEventCh
 		if ch != nil {
-			ch <- e
+		forLoop:
+			// Try sending the event to the channel with a backoff strategy to avoid blocking indefinitely.
+			// Since most unit tests don't consume events, we make a few attempts and then give up rather
+			// than blocking the DDL job forever on a full channel.
+			for i := 0; i < 10; i++ {
+				select {
+				case ch <- e:
+					break forLoop
+				default:
+					time.Sleep(time.Microsecond * 10)
+				}
+			}
+			logutil.DDLLogger().Warn("fail to notify DDL event", zap.Stringer("event", e))
 		}
 	}