Reject cross-shard MSETNX

Prevent MSETNX from being split across shards to preserve atomicity.

Adds msetnx_atomicity test and runs it in the harness.

src/Handler.cpp

@@ -562,6 +562,12 @@ bool Handler::preHandleRequest(Request* req, const String& key)
         return false;
     }
     switch (req->type()) {
+    if (req->type() == Command::Msetnx && mProxy->isSplitMultiKey()) {
+        ResponsePtr res = ResponseAlloc::create();
+        res->setErr("msetnx not supported across shards");
+        handleResponse(nullptr, req, res);
+        return true;
+    }
     case Command::Ping:
     case Command::Echo:
         if (key.empty()) {

test/msetnx_atomicity.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python
+#
+# Verify MSETNX does not partially apply changes when it returns 0
+#
+
+import argparse
+import sys
+import redis
+
+
+def run_test(host, port):
+    c = redis.StrictRedis(host=host, port=port)
+    c.delete("msetnx_k1", "msetnx_k2")
+    c.set("msetnx_k1", "existing")
+
+    try:
+        r = c.execute_command("MSETNX", "msetnx_k1", "v1", "msetnx_k2", "v2")
+    except Exception as exc:
+        # If proxy rejects cross-shard MSETNX, this is acceptable.
+        return True
+
+    if r not in (0, b"0", False):
+        print("FAIL: expected MSETNX result 0, got:", r)
+        return False
+
+    if c.get("msetnx_k2") is not None:
+        print("FAIL: MSETNX partially applied when it returned 0")
+        return False
+
+    return True
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(conflict_handler='resolve', description="MSETNX atomicity test")
+    parser.add_argument("-h", "--host", default="127.0.0.1")
+    parser.add_argument("-p", "--port", type=int, default=7617)
+    args = parser.parse_args()
+
+    if run_test(args.host, args.port):
+        print("PASS: MSETNX atomicity")
+        sys.exit(0)
+    print("FAIL: MSETNX atomicity")
+    sys.exit(1)

test/run.sh

@@ -66,6 +66,7 @@ NULL_RESPONSE_EXIT=0
 PUBSUB_LONG_EXIT=0
 TRANSACTION_FORBID_EXIT=0
 MGET_WRONG_TYPE_EXIT=0
+MSETNX_ATOMICITY_EXIT=0
 
 uv run python3 test/basic.py || BASIC_EXIT=$?
 uv run python3 test/pubsub_minimal.py -p 7617 || PUBSUB_REDIS_EXIT=$?
@@ -73,11 +74,12 @@ uv run python3 test/pubsub_minimal.py -p 6379 || PUBSUB_MINIMAL_EXIT=$?
 uv run python3 test/pubsub.py || PUBSUB_EXIT=$?
 uv run python3 test/pubsub_subscription_order.py -p 7617 || PUBSUB_ORDER_EXIT=$?
 uv run python3 test/pubsub_parser_reset.py -p 7617 || PUBSUB_RESET_EXIT=$?
+uv run python3 test/msetnx_atomicity.py -p 7617 || MSETNX_ATOMICITY_EXIT=$?
 uv run python3 test/mget_wrong_type.py -p 7617 || MGET_WRONG_TYPE_EXIT=$?
 uv run python3 test/transaction_forbid.py -p 7617 || TRANSACTION_FORBID_EXIT=$?
 uv run python3 test/pubsub_long_name.py -p 7617 || PUBSUB_LONG_EXIT=$?
 uv run python3 test/null_response_handling.py -p 7617 || NULL_RESPONSE_EXIT=$?
 uv run python3 test/pubsub_message_response.py -p 7617 || PUBSUB_MESSAGE_EXIT=$?
 
-TEST_EXIT=$((BASIC_EXIT + PUBSUB_REDIS_EXIT + PUBSUB_MINIMAL_EXIT + PUBSUB_EXIT + PUBSUB_MESSAGE_EXIT + PUBSUB_ORDER_EXIT + PUBSUB_RESET_EXIT + NULL_RESPONSE_EXIT + PUBSUB_LONG_EXIT + TRANSACTION_FORBID_EXIT + MGET_WRONG_TYPE_EXIT))
+TEST_EXIT=$((BASIC_EXIT + PUBSUB_REDIS_EXIT + PUBSUB_MINIMAL_EXIT + PUBSUB_EXIT + PUBSUB_MESSAGE_EXIT + PUBSUB_ORDER_EXIT + PUBSUB_RESET_EXIT + NULL_RESPONSE_EXIT + PUBSUB_LONG_EXIT + TRANSACTION_FORBID_EXIT + MGET_WRONG_TYPE_EXIT + MSETNX_ATOMICITY_EXIT))
 exit $TEST_EXIT