fix(server): prevent graph clear API from clearing meta table in MySQL backend storage

hugegraph-server/hugegraph-mysql/src/main/java/org/apache/hugegraph/backend/store/mysql/MysqlStore.java

@@ -223,6 +223,7 @@ public void truncate() {
         this.checkOpened();
 
         this.truncateTables();
+        this.init();
         LOG.debug("Store truncated: {}", this.store);
     }
 

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/unit/mysql/BaseMysqlUnitTest.java

@@ -0,0 +1,63 @@
+/*
+ *
+ *  * Licensed to the Apache Software Foundation (ASF) under one or more
+ *  * contributor license agreements.  See the NOTICE file distributed with
+ *  * this work for additional information regarding copyright ownership.
+ *  * The ASF licenses this file to You under the Apache License, Version 2.0
+ *  * (the "License"); you may not use this file except in compliance with
+ *  * the License.  You may obtain a copy of the License at
+ *  *
+ *  *     http://www.apache.org/licenses/LICENSE-2.0
+ *  *
+ *  * Unless required by applicable law or agreed to in writing, software
+ *  * distributed under the License is distributed on an "AS IS" BASIS,
+ *  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  * See the License for the specific language governing permissions and
+ *  * limitations under the License.
+ *
+ */
+
+package org.apache.hugegraph.unit.mysql;
+
+import org.apache.hugegraph.backend.store.BackendStore;
+import org.apache.hugegraph.backend.store.hbase.HbaseStoreProvider;
+import org.apache.hugegraph.backend.store.mysql.MysqlStoreProvider;
+import org.apache.hugegraph.config.HugeConfig;
+import org.apache.hugegraph.unit.BaseUnitTest;
+import org.apache.hugegraph.unit.FakeObjects;
+import org.junit.After;
+import org.junit.Before;
+
+public class BaseMysqlUnitTest extends BaseUnitTest {
+    protected BackendStore store;
+
+    protected HugeConfig config;
+    protected MysqlStoreProvider provider;
+
+    @Before
+    public void setup() {
+        this.config = FakeObjects.newConfig();
+
+        this.provider = new MysqlStoreProvider();
+
+        this.store = this.provider.loadSystemStore(config);
+    }
+
+    @After
+    public void down(){
+        if (this.store != null) {
+            try {
+                this.store.close();
+            } catch (Exception e) {
+                // pass
+            }
+        }
+        if (this.provider != null) {
+            try {
+                this.provider.close();
+            } catch (Exception e) {
+                // pass
+            }
+        }
+    }
+}

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/unit/mysql/MysqlTest.java

@@ -0,0 +1,57 @@
+/*
+ *
+ *  * Licensed to the Apache Software Foundation (ASF) under one or more
+ *  * contributor license agreements.  See the NOTICE file distributed with
+ *  * this work for additional information regarding copyright ownership.
+ *  * The ASF licenses this file to You under the Apache License, Version 2.0
+ *  * (the "License"); you may not use this file except in compliance with
+ *  * the License.  You may obtain a copy of the License at
+ *  *
+ *  *     http://www.apache.org/licenses/LICENSE-2.0
+ *  *
+ *  * Unless required by applicable law or agreed to in writing, software
+ *  * distributed under the License is distributed on an "AS IS" BASIS,
+ *  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  * See the License for the specific language governing permissions and
+ *  * limitations under the License.
+ *
+ */
+
+package org.apache.hugegraph.unit.mysql;
+
+import org.apache.hugegraph.testutil.Assert;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+public class MysqlTest extends BaseMysqlUnitTest{
+    private static final String GRAPH_NAME = "test_graph";
+
+    @Before
+    public void setUp(){
+        this.provider.open(GRAPH_NAME);
+        this.provider.init();
+    }
+
+    @After
+    public void teardown(){
+        if (this.store != null) {
+            this.store.close();
+        }
+        if (this.provider != null) {
+            this.provider.close();
+        }
+    }
+
+    @Test
+    public void testMysqlMetaVersion(){
+        // init store
+        this.store.init();
+        String beforeVersion = this.store.storedVersion();
+        this.store.truncate();
+        String afterInitVersion = this.store.storedVersion();
+        Assert.assertNotNull(beforeVersion);
+        Assert.assertNotNull(afterInitVersion);
+        Assert.assertEquals(beforeVersion, afterInitVersion);
+    }
+}