optimize the display of specific statement results inner 1042 (#2718)

* optimize the display of specific statement results inner 1042

* Reuse method

src/main/java/com/actiontech/dble/server/handler/SelectHandler.java

@@ -9,6 +9,7 @@ import com.actiontech.dble.route.parser.util.ParseUtil;
 import com.actiontech.dble.server.parser.ServerParse;
 import com.actiontech.dble.server.parser.ServerParseSelect;
 import com.actiontech.dble.server.response.*;
+import com.actiontech.dble.services.manager.response.SelectMaxAllowedPacket;
 import com.actiontech.dble.services.mysqlsharding.ShardingService;
 
 
@@ -107,6 +108,9 @@ public final class SelectHandler {
             case ServerParseSelect.ROW_COUNT:
                 SelectRowCount.response(service);
                 break;
+            case ServerParseSelect.MAX_ALLOWED_PACKET:
+                SelectMaxAllowedPacket.execute(service);
+                break;
             default:
                 service.execute(stmt, ServerParse.SELECT);
         }

src/main/java/com/actiontech/dble/server/parser/ServerParseSelect.java

@@ -32,6 +32,7 @@ public final class ServerParseSelect {
     public static final int SESSION_TRANSACTION_ISOLATION = 13;
     public static final int SESSION_TRANSACTION_READ_ONLY = 14;
     public static final int ROW_COUNT = 15;
+    public static final int MAX_ALLOWED_PACKET = 16;
 
     private static final char[] TRACE_STR = "TRACE".toCharArray();
     private static final char[] VERSION_COMMENT_STR = "VERSION_COMMENT".toCharArray();
@@ -492,6 +493,9 @@ public final class ServerParseSelect {
                 case 't':
                 case 'T':
                     return traceCheck(stmt, offset);
+                case 'm':
+                case 'M':
+                    return maxCheck(stmt, offset);
                 default:
                     return OTHER;
             }
@@ -499,6 +503,17 @@ public final class ServerParseSelect {
         return OTHER;
     }
 
+    // select @@max_allowed_packet;
+    private static int maxCheck(String stmt, int offset) {
+        if (stmt.length() > offset + 17) {
+            String suffix = stmt.substring(offset).toUpperCase();
+            if (suffix.startsWith("MAX_ALLOWED_PACKET") && (stmt.length() == offset + 18 || ParseUtil.isEOF(stmt, offset + 18))) {
+                return MAX_ALLOWED_PACKET;
+            }
+        }
+        return OTHER;
+    }
+
     /**
      * SELECT DATABASE()
      */

src/main/java/com/actiontech/dble/services/manager/response/SelectMaxAllowedPacket.java

@@ -7,12 +7,16 @@ package com.actiontech.dble.services.manager.response;
 
 import com.actiontech.dble.backend.mysql.PacketUtil;
 import com.actiontech.dble.config.Fields;
+import com.actiontech.dble.config.model.SystemConfig;
 import com.actiontech.dble.net.mysql.*;
-import com.actiontech.dble.services.manager.ManagerService;
+import com.actiontech.dble.net.service.AbstractService;
 import com.actiontech.dble.util.LongUtil;
 
 import java.nio.ByteBuffer;
 
+/**
+ * @author yuanlinzhu
+ */
 public final class SelectMaxAllowedPacket {
     private SelectMaxAllowedPacket() {
     }
@@ -34,7 +38,7 @@ public final class SelectMaxAllowedPacket {
         EOF.setPacketId(++packetId);
     }
 
-    public static void execute(ManagerService service) {
+    public static void execute(AbstractService service) {
         ByteBuffer buffer = service.allocate();
 
         // write header
@@ -52,7 +56,7 @@ public final class SelectMaxAllowedPacket {
         byte packetId = EOF.getPacketId();
         RowDataPacket row = new RowDataPacket(FIELD_COUNT);
         row.setPacketId(++packetId);
-        row.add(LongUtil.toBytes(1048576));
+        row.add(LongUtil.toBytes(SystemConfig.getInstance().getMaxPacketSize()));
         buffer = row.write(buffer, service, true);
 
         // write last eof