feat(api):fix a concurrency issue for toString in BlockCapsule (#5657)

chainbase/src/main/java/org/tron/core/capsule/BlockCapsule.java

@@ -56,7 +56,6 @@ public class BlockCapsule implements ProtoCapsule<Block> {
 
   private Block block;
   private List<TransactionCapsule> transactions = new ArrayList<>();
-  private StringBuilder toStringBuff = new StringBuilder();
   private boolean isSwitch;
   @Getter
   @Setter
@@ -314,7 +313,7 @@ public boolean hasWitnessSignature() {
 
   @Override
   public String toString() {
-    toStringBuff.setLength(0);
+    StringBuilder toStringBuff = new StringBuilder();
 
     toStringBuff.append("BlockCapsule \n[ ");
     toStringBuff.append("hash=").append(getBlockId()).append("\n");

framework/src/test/java/org/tron/core/capsule/BlockCapsuleTest.java

@@ -2,7 +2,9 @@
 
 import com.google.protobuf.ByteString;
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.List;
 import lombok.extern.slf4j.Slf4j;
 import org.junit.AfterClass;
 import org.junit.Assert;
@@ -146,4 +148,20 @@ public void testGetTimeStamp() {
     Assert.assertEquals(1234L, blockCapsule0.getTimeStamp());
   }
 
+  @Test
+  public void testConcurrentToString() throws InterruptedException {
+    List<Thread> threadList = new ArrayList<>();
+    int n = 10;
+    for (int i = 0; i < n; i++) {
+      threadList.add(new Thread(() -> blockCapsule0.toString()));
+    }
+    for (int i = 0; i < n; i++) {
+      threadList.get(i).start();
+    }
+    for (int i = 0; i < n; i++) {
+      threadList.get(i).join();
+    }
+    Assert.assertTrue(true);
+  }
+
 }