Fix test instability caused by MVCC slip with lfence command when async commands are enabled

src/server.cpp

@@ -1177,6 +1177,10 @@ struct redisCommand redisCommandTable[] = {
 
     {"failover",failoverCommand,-1,
      "admin no-script ok-stale",
+     0,NULL,0,0,0,0,0,0},
+
+    {"lfence", lfenceCommand,1,
+     "read-only random ok-stale",
      0,NULL,0,0,0,0,0,0}
 };
 
@@ -4661,6 +4665,11 @@ void rejectCommand(client *c, robj *reply, int severity = ERR_CRITICAL) {
     }
 }
 
+void lfenceCommand(client *c) {
+    c->mvccCheckpoint = getMvccTstamp();
+    addReply(c, shared.ok);
+}
+
 void rejectCommandFormat(client *c, const char *fmt, ...) {
     if (c->cmd) c->cmd->rejected_calls++;
     flagTransaction(c);

src/server.h

@@ -3784,6 +3784,7 @@ void hrenameCommand(client *c);
 void stralgoCommand(client *c);
 void resetCommand(client *c);
 void failoverCommand(client *c);
+void lfenceCommand(client *c);
 
 
 int FBrokenLinkToMaster();

tests/support/util.tcl

@@ -110,6 +110,7 @@ proc wait_for_ofs_sync {r1 r2} {
     } else {
         fail "replica didn't sync in time"
     }
+    $r2 lfence
 }
 
 proc wait_done_loading r {