RATIS-2499. Allow the LogAppender restart when LogAppenderDaemon exception

ratis-server/src/main/java/org/apache/ratis/server/leader/LogAppenderBase.java

@@ -125,7 +125,11 @@ public void start() {
   @Override
   public boolean isRunning() {
     return daemon.isWorking()
-        && server.getInfo().isAlive()
+        && isLeaderAlive();
+  }
+
+  private boolean isLeaderAlive() {
+    return server.getInfo().isAlive()
         && server.getInfo().isLeader()
         && getRaftLog().isOpened();
   }
@@ -136,8 +140,12 @@ public CompletableFuture<LifeCycle.State> stopAsync() {
   }
 
   void restart() {
-    if (!isRunning()) {
-      LOG.warn("{} is not running: skipping restart", this);
+    if (daemon.isClosingOrClosed()) {
+      LOG.warn("{}: daemon is closing or closed, skipping restart", this);
+      return;
+    }
+    if (!isLeaderAlive()) {
+      LOG.warn("{}: leader is not ready, skipping restart", this);
       return;
     }
     getLeaderState().restart(this);

ratis-server/src/main/java/org/apache/ratis/server/leader/LogAppenderDaemon.java

@@ -58,6 +58,10 @@ public boolean isWorking() {
     return !LifeCycle.States.CLOSING_OR_CLOSED_OR_EXCEPTION.contains(lifeCycle.getCurrentState());
   }
 
+  public boolean isClosingOrClosed() {
+    return LifeCycle.States.CLOSING_OR_CLOSED.contains(lifeCycle.getCurrentState());
+  }
+
   public void tryToStart() {
     if (lifeCycle.compareAndTransition(NEW, STARTING)) {
       daemon.start();

ratis-server/src/test/java/org/apache/ratis/LogAppenderTests.java

@@ -171,6 +171,7 @@ public void testFollowerHeartbeatMetric() throws IOException, InterruptedExcepti
         assertTrue(t.getTimer().getCount() > 0L);
       }
     }
+    cluster.shutdown();
   }
 
   void runTest(CLUSTER cluster) throws Exception {

ratis-test/src/test/java/org/apache/ratis/grpc/TestLogAppenderWithGrpc.java

@@ -18,7 +18,10 @@
 package org.apache.ratis.grpc;
 
 import org.apache.ratis.LogAppenderTests;
+import org.apache.ratis.grpc.server.GrpcServicesImpl;
 import org.apache.ratis.proto.RaftProtos;
+import org.apache.ratis.protocol.RaftGroupId;
+import org.apache.ratis.protocol.RaftPeerId;
 import org.apache.ratis.server.impl.MiniRaftCluster;
 import org.apache.ratis.RaftTestUtil;
 import org.apache.ratis.client.RaftClient;
@@ -29,11 +32,14 @@
 import org.apache.ratis.server.RaftServerConfigKeys;
 import org.apache.ratis.server.leader.FollowerInfo;
 import org.apache.ratis.server.impl.RaftServerTestUtil;
+import org.apache.ratis.server.leader.LogAppender;
 import org.apache.ratis.statemachine.impl.SimpleStateMachine4Testing;
 import org.apache.ratis.statemachine.StateMachine;
+import org.apache.ratis.util.CodeInjectionForTesting;
 import org.apache.ratis.util.JavaUtils;
 import org.apache.ratis.util.Slf4jUtils;
 import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 import org.slf4j.event.Level;
@@ -42,7 +48,10 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Set;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Collectors;
 
 import static org.apache.ratis.RaftTestUtil.waitForLeader;
 
@@ -148,4 +157,58 @@ private void runTestRestartLogAppender(MiniRaftClusterWithGrpc cluster) throws E
       Assertions.assertTrue(newleaderMetrics.getRegistry().counter(counter).getCount() >= 1L);
     }
   }
+
+  @Test
+  public void testLogAppenderAutoRestartOnException() throws Exception {
+    runWithNewCluster(3, this::runTestAutoRestartOnException);
+  }
+
+  private void runTestAutoRestartOnException(MiniRaftClusterWithGrpc cluster) throws Exception {
+    final RaftServer.Division leader = waitForLeader(cluster);
+    final RaftPeerId leaderId = leader.getId();
+
+    try (RaftClient client = cluster.createClient(leaderId)) {
+      for (int i = 0; i < 5; i++) {
+        Assertions.assertTrue(client.io().send(new RaftTestUtil.SimpleMessage("init-" + i)).isSuccess());
+      }
+    }
+
+    final Set<LogAppender> before = RaftServerTestUtil.getLogAppenders(leader).collect(Collectors.toSet());
+    Assertions.assertEquals(2, before.size());
+
+    // Inject a one-time IllegalStateException into the leader's AppendEntries send path.
+    // This causes the LogAppenderDaemon to enter EXCEPTION state and call restart().
+    final RaftGroupId groupId = cluster.getGroupId();
+    final AtomicInteger failCount = new AtomicInteger(0);
+    try {
+      CodeInjectionForTesting.put(GrpcServicesImpl.GRPC_SEND_SERVER_REQUEST, (localId, remoteId, args) -> {
+        if (leaderId.equals(localId)
+                && args.length > 0 && args[0] instanceof RaftProtos.AppendEntriesRequestProto) {
+          final RaftProtos.AppendEntriesRequestProto proto = (RaftProtos.AppendEntriesRequestProto) args[0];
+          if (RaftGroupId.valueOf(proto.getServerRequest().getRaftGroupId().getId()).equals(groupId)
+                  && failCount.getAndIncrement() < 1) {
+            throw new IllegalStateException("Injected failure for restart test");
+          }
+        }
+        return false;
+      });
+
+      JavaUtils.attempt(() -> {
+        final Set<LogAppender> current = RaftServerTestUtil.getLogAppenders(leader)
+                .collect(Collectors.toSet());
+        Assertions.assertEquals(2, current.size());
+        Assertions.assertTrue(current.stream().anyMatch(a -> !before.contains(a)),
+                "Expected at least one new LogAppender instance after daemon exception restart");
+      }, 30, ONE_SECOND, "LogAppender auto-restart after exception", LOG);
+    } finally {
+      CodeInjectionForTesting.remove(GrpcServicesImpl.GRPC_SEND_SERVER_REQUEST);
+    }
+
+    try (RaftClient client = cluster.createClient(leaderId)) {
+      for (int i = 0; i < 5; i++) {
+        Assertions.assertTrue(
+                client.io().send(new RaftTestUtil.SimpleMessage("after-restart-" + i)).isSuccess());
+      }
+    }
+  }
 }