Guard against null server responses

Convert unexpected null responses into DeliverRequestFail instead of crashing.

Adds null_response_handling test and runs it in the harness.

src/Handler.cpp

@@ -772,6 +772,11 @@ void Handler::directResponse(Request* req, Response::GenericCode code, ConnectCo
 }
 
 void Handler::handleResponse(ConnectConnection* s, Request* req, Response* res)
+    ResponsePtr fallback;
+    if (!res) {
+        fallback = ResponseAlloc::create(Response::DeliverRequestFail);
+        res = fallback;
+    }
 {
     FuncCallTimer();
     SegmentStr<Const::MaxKeyLen> key(req->key());

test/null_response_handling.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python
+#
+# Exercise server write mismatch handling to ensure proxy stays alive
+#
+
+import argparse
+import socket
+import sys
+import time
+import redis
+
+
+def run_test(host, port):
+    # Send a malformed pipelined request and close quickly.
+    try:
+        sock = socket.create_connection((host, port), timeout=1.0)
+        sock.sendall(b"*2\r\n$4\r\nping\r\n$4\r\nping\r\n")
+        sock.close()
+    except Exception as exc:
+        print("WARN: socket setup failed:", exc)
+
+    # Ensure the proxy still accepts connections.
+    try:
+        c = redis.StrictRedis(host=host, port=port)
+        if c.ping() is not True:
+            print("FAIL: ping did not return True")
+            return False
+    except Exception as exc:
+        print("FAIL: ping after malformed request:", exc)
+        return False
+
+    return True
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(conflict_handler='resolve', description="Null response handling 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: null response handling")
+        sys.exit(0)
+    print("FAIL: null response handling")
+    sys.exit(1)

test/run.sh

@@ -62,6 +62,7 @@ PUBSUB_EXIT=0
 PUBSUB_MESSAGE_EXIT=0
 PUBSUB_ORDER_EXIT=0
 PUBSUB_RESET_EXIT=0
+NULL_RESPONSE_EXIT=0
 
 uv run python3 test/basic.py || BASIC_EXIT=$?
 uv run python3 test/pubsub_minimal.py -p 7617 || PUBSUB_REDIS_EXIT=$?
@@ -69,7 +70,8 @@ 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/null_response_handling.py -p 7617 || NULL_RESPONSE_EXIT=$?
 uv run python3 test/pubsub_message_response.py -p 7617 || PUBSUB_MESSAGE_EXIT=$?
 
-TEST_EXIT=$((BASIC_EXIT + PUBSUB_REDIS_EXIT + PUBSUB_MINIMAL_EXIT + PUBSUB_EXIT + PUBSUB_MESSAGE_EXIT + PUBSUB_ORDER_EXIT + PUBSUB_RESET_EXIT))
+TEST_EXIT=$((BASIC_EXIT + PUBSUB_REDIS_EXIT + PUBSUB_MINIMAL_EXIT + PUBSUB_EXIT + PUBSUB_MESSAGE_EXIT + PUBSUB_ORDER_EXIT + PUBSUB_RESET_EXIT + NULL_RESPONSE_EXIT))
 exit $TEST_EXIT