[BUGFIX] fix lob remote query memory usage

src/storage/lob/ob_lob_manager.cpp

@@ -3687,6 +3687,9 @@ int ObLobQueryRemoteReader::open(ObLobAccessParam& param, common::ObDataBuffer &
   int ret = OB_SUCCESS;
   char *buf = NULL;
   int64_t buf_len = ObLobQueryArg::OB_LOB_QUERY_BUFFER_LEN;
+  if (GET_MIN_CLUSTER_VERSION() < CLUSTER_VERSION_4_3_0_0) {
+    buf_len *= ObLobQueryArg::OB_LOB_QUERY_OLD_LEN_REFACTOR; // compat with old vesion
+  }
   if (NULL == (buf = reinterpret_cast<char*>(param.allocator_->alloc(buf_len)))) {
     ret = OB_ALLOCATE_MEMORY_FAILED;
     LOG_WARN("failed to alloc buf", K(ret));
@@ -3694,6 +3697,7 @@ int ObLobQueryRemoteReader::open(ObLobAccessParam& param, common::ObDataBuffer &
     ret = OB_ALLOCATE_MEMORY_FAILED;
     LOG_WARN("failed to set rpc buffer", K(ret));
   } else if (NULL == (buf = reinterpret_cast<char*>(param.allocator_->alloc(buf_len)))) {
+    ret = OB_ALLOCATE_MEMORY_FAILED;
     LOG_WARN("failed to alloc buf", K(ret));
   } else {
     data_buffer_.assign_buffer(buf, buf_len);

src/storage/lob/ob_lob_rpc_struct.h

@@ -53,7 +53,8 @@ public:
   ~ObLobQueryArg();
   TO_STRING_KV(K_(tenant_id), K_(offset), K_(len), K_(cs_type), K_(qtype), K_(scan_backward), K_(lob_locator));
 public:
-  static const int64_t OB_LOB_QUERY_BUFFER_LEN = 2*1024L*1024L;
+  static const int64_t OB_LOB_QUERY_BUFFER_LEN = 256*1024L;
+  static const int64_t OB_LOB_QUERY_OLD_LEN_REFACTOR = 8;
   uint64_t tenant_id_;
   uint64_t offset_; // char offset
   uint64_t len_; // char len