RATIS-2734. Fix NPE in RaftServerImpl.watchAsync for a write with a replication level

[smengcl]

Generated-by: Claude Code (Opus 5.5)

What changes were proposed in this pull request?

A write sent with a replication level other than MAJORITY, for example client.async().send(message, ReplicationLevel.ALL_COMMITTED), never succeeds. After the write is applied, RaftServerImpl.waitForReplication builds an internal watch request without a message, and the dummy request check that RATIS-2497 added to RaftServerImpl.watchAsync calls request.getMessage().getContent() on it. The leader throws a NullPointerException, and the client keeps retrying according to its retry policy.

RaftServerImpl already has a null safe version of that check, isDummyRead, used on the read path. This change uses it in watchAsync as well and renames it to isDummy.

RaftAsyncTests.testWriteAsyncCustomReplicationLevel did not catch this because it never waited for the futures returned by send. The client was closed before any reply arrived, every future completed with AlreadyClosedException, and the callback only logged it. The test now waits for each reply and fails on an exception.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-2734

How was this patch tested?

testWriteAsyncCustomReplicationLevel with the stricter assertions fails without the fix and passes with it in TestRaftAsyncWithGrpc, TestRaftAsyncWithGrpcTracing and TestRaftAsyncWithNetty. TestLinearizableReadWithGrpc passes with the renamed check.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The focused fix addresses the NPE and the updated test reliably exercises the affected path.

0 open findings

What changed in this PR

Fixes the null-message NPE in custom-replication writes and strengthens regression coverage.

Changes:

• Reuses the null-safe dummy-request check in watch and read paths.
• Waits for and validates every asynchronous test reply.

File	Description
RaftServerImpl.java	Safely handles internal watch requests without messages.
RaftAsyncTests.java	Ensures custom-replication writes complete successfully.

🧠 Review effort: Balanced

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[amaliujia]

Thanks!

[amaliujia]

        if (exception != null) {
            LOG.error("Failed to send message " + s, exception);
            // reply should be null in case of exception
            Assertions.assertNull(reply);
            return;
          }

This code has been dropped. Is it because the NPE is gone?

[szetszwo]

@smengcl , thanks for working on this! We need to update it for RATIS-2724. Otherwise, the change looks good.