fix difficulty gauge metric in stratumserver, add simple test coverage (#2930)

Signed-off-by: garyschulte <garyschulte@gmail.com>

ethereum/stratum/build.gradle

@@ -48,4 +48,5 @@ dependencies {
   testImplementation 'junit:junit'
   testImplementation 'org.assertj:assertj-core'
   testImplementation 'org.mockito:mockito-core'
+  testImplementation project(path: ':metrics:core', configuration: 'testSupportArtifacts')
 }

ethereum/stratum/src/main/java/org/hyperledger/besu/ethereum/stratum/StratumServer.java

@@ -18,12 +18,15 @@ import static org.apache.logging.log4j.LogManager.getLogger;
 
 import org.hyperledger.besu.ethereum.blockcreation.MiningCoordinator;
 import org.hyperledger.besu.ethereum.chain.PoWObserver;
+import org.hyperledger.besu.ethereum.mainnet.EthHash;
 import org.hyperledger.besu.ethereum.mainnet.PoWSolution;
 import org.hyperledger.besu.ethereum.mainnet.PoWSolverInputs;
 import org.hyperledger.besu.metrics.BesuMetricCategory;
 import org.hyperledger.besu.plugin.services.MetricsSystem;
 import org.hyperledger.besu.plugin.services.metrics.Counter;
 
+import java.math.BigInteger;
+import java.util.Optional;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicLong;
@@ -45,12 +48,11 @@ import org.apache.tuweni.units.bigints.UInt256;
 public class StratumServer implements PoWObserver {
 
   private static final Logger logger = getLogger();
-  private static final UInt256 DIFFICULTY_1_TARGET = UInt256.valueOf(2).pow(256);
 
   private final Vertx vertx;
   private final int port;
   private final String networkInterface;
-  private final AtomicBoolean started = new AtomicBoolean(false);
+  protected final AtomicBoolean started = new AtomicBoolean(false);
   private final AtomicLong numberOfMiners = new AtomicLong(0);
   private final AtomicDouble currentDifficulty = new AtomicDouble(0.0);
   private final StratumProtocol[] protocols;
@@ -159,7 +161,16 @@ public class StratumServer implements PoWObserver {
     for (StratumProtocol protocol : protocols) {
       protocol.setCurrentWorkTask(poWSolverInputs);
     }
-    UInt256 difficulty = poWSolverInputs.getTarget().divide(DIFFICULTY_1_TARGET);
+
+    // reverse the target calculation to get the difficulty
+    // and ensure we do not get divide by zero:
+    UInt256 difficulty =
+        Optional.of(poWSolverInputs.getTarget().toUnsignedBigInteger())
+            .filter(td -> td.compareTo(BigInteger.ONE) > 0)
+            .map(EthHash.TARGET_UPPER_BOUND::divide)
+            .map(UInt256::valueOf)
+            .orElse(UInt256.MAX_VALUE);
+
     currentDifficulty.set(difficulty.toUnsignedBigInteger().doubleValue());
   }
 

ethereum/stratum/src/test/java/org/hyperledger/besu/ethereum/stratum/StratumServerTest.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright ConsenSys AG.
+ *
+ * Licensed 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.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package org.hyperledger.besu.ethereum.stratum;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.hyperledger.besu.ethereum.blockcreation.PoWMiningCoordinator;
+import org.hyperledger.besu.ethereum.mainnet.EpochCalculator;
+import org.hyperledger.besu.ethereum.mainnet.PoWSolverInputs;
+import org.hyperledger.besu.metrics.ObservableMetricsSystem;
+import org.hyperledger.besu.metrics.StubMetricsSystem;
+
+import java.util.concurrent.CompletableFuture;
+
+import org.apache.tuweni.bytes.Bytes;
+import org.apache.tuweni.units.bigints.UInt256;
+import org.junit.Test;
+import org.mockito.Mockito;
+
+public class StratumServerTest {
+
+  @Test
+  public void runJobSetDifficultyShouldSucceed() {
+    StubMetricsSystem metrics = new StubMetricsSystem();
+    stratumServerWithMocks(metrics).newJob(new PoWSolverInputs(UInt256.MAX_VALUE, Bytes.EMPTY, 1));
+    assertThat(metrics.getGaugeValue("difficulty")).isEqualTo(1d);
+  }
+
+  @Test
+  public void runJobSetZeroDifficultyShouldNotThrow() {
+    StubMetricsSystem metrics = new StubMetricsSystem();
+    stratumServerWithMocks(metrics).newJob(new PoWSolverInputs(UInt256.MIN_VALUE, Bytes.EMPTY, 1));
+
+    assertThat(metrics.getGaugeValue("difficulty"))
+        .isEqualTo(UInt256.MAX_VALUE.toUnsignedBigInteger().doubleValue());
+  }
+
+  private StratumServer stratumServerWithMocks(final ObservableMetricsSystem metrics) {
+    PoWMiningCoordinator mockPoW =
+        Mockito.when(Mockito.mock(PoWMiningCoordinator.class).getEpochCalculator())
+            .thenReturn(new EpochCalculator.DefaultEpochCalculator())
+            .getMock();
+
+    StratumServer ss =
+        new StratumServer(null, mockPoW, 0, "lo", "", metrics) {
+          @Override
+          public CompletableFuture<?> start() {
+            this.started.set(true);
+            return null;
+          }
+        };
+    ss.start();
+    return ss;
+  }
+}