fix(server): Fix SCAN deadlock (#3235)

src/server/generic_family.cc

@@ -543,10 +543,16 @@ uint64_t ScanGeneric(uint64_t cursor, const ScanOpts& scan_opts, StringVec* keys
   DbContext db_cntx{.db_index = cntx->conn_state.db_index, .time_now_ms = GetCurrentTimeMs()};
 
   do {
-    ess->Await(sid, [&] {
+    auto cb = [&] {
       OpArgs op_args{EngineShard::tlocal(), 0, db_cntx};
       OpScan(op_args, scan_opts, &cursor, keys);
-    });
+    };
+
+    // Avoid deadlocking, if called from shard queue script
+    if (EngineShard::tlocal() && EngineShard::tlocal()->shard_id() == sid)
+      cb();
+    else
+      ess->Await(sid, cb);
 
     if (cursor == 0) {
       ++sid;

src/server/multi_test.cc

@@ -396,13 +396,13 @@ TEST_F(MultiTest, Eval) {
   resp = Run({"eval", "return redis.call('get', 'foo')", "0"});
   EXPECT_THAT(resp, "42");
 
-  Run({"script", "flush"});
+  Run({"script", "flush"});  // Reset global flag due to lua_allow_undeclared_auto_correct effect
 
   resp = Run({"eval", "return redis.call('get', 'foo')", "1", "bar"});
   EXPECT_THAT(resp, ErrArg("undeclared"));
   ASSERT_FALSE(service_->IsLocked(0, "foo"));
 
-  Run({"script", "flush"});
+  Run({"script", "flush"});  // Reset global flag from autocorrect
 
   resp = Run({"eval", "return redis.call('get', 'foo')", "1", "foo"});
   EXPECT_THAT(resp, "42");
@@ -459,6 +459,11 @@ TEST_F(MultiTest, Eval) {
                  "1", "foo"}),
             "42");
   fb.Join();
+
+  // Call multi-shard command scan from single shard mode
+  resp = Run({"eval", "return redis.call('scan', '0'); ", "1", "key"});
+  EXPECT_EQ(resp.GetVec()[0], "0");
+  EXPECT_EQ(resp.GetVec()[1].type, RespExpr::Type::ARRAY);
 }
 #endif