Keep connections open on transaction forbid

Return the error for forbidden commands in transactions without closing the client.

Adds transaction_forbid test and runs it in the harness.

src/Handler.cpp

@@ -554,7 +554,6 @@ bool Handler::preHandleRequest(Request* req, const String& key)
                         req->cmd());
                 res->setErr(buf);
                 handleResponse(nullptr, req, res);
-                addPostEvent(c, Multiplexor::ErrorEvent);
                 return true;
             }
         default:

test/run.sh

@@ -64,6 +64,7 @@ PUBSUB_ORDER_EXIT=0
 PUBSUB_RESET_EXIT=0
 NULL_RESPONSE_EXIT=0
 PUBSUB_LONG_EXIT=0
+TRANSACTION_FORBID_EXIT=0
 
 uv run python3 test/basic.py || BASIC_EXIT=$?
 uv run python3 test/pubsub_minimal.py -p 7617 || PUBSUB_REDIS_EXIT=$?
@@ -71,9 +72,10 @@ 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/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=$((BASIC_EXIT + PUBSUB_REDIS_EXIT + PUBSUB_MINIMAL_EXIT + PUBSUB_EXIT + PUBSUB_MESSAGE_EXIT + PUBSUB_ORDER_EXIT + PUBSUB_RESET_EXIT + NULL_RESPONSE_EXIT + PUBSUB_LONG_EXIT))
+TEST_EXIT=$((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))
 exit $TEST_EXIT

test/transaction_forbid.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python
+#
+# Verify forbidden command in transaction returns error without closing connection
+#
+
+import argparse
+import sys
+import redis
+
+
+def run_test(host, port):
+    c = redis.StrictRedis(host=host, port=port)
+    try:
+        r = c.execute_command("MULTI")
+        if r not in (b"OK", "OK"):
+            print("FAIL: MULTI response:", r)
+            return False
+    except Exception as exc:
+        print("FAIL: MULTI error:", exc)
+        return False
+
+    try:
+        c.execute_command("SELECT", "0")
+        print("FAIL: SELECT should be forbidden in transaction")
+        return False
+    except Exception:
+        pass
+
+    try:
+        r = c.execute_command("PING")
+        if r not in (b"PONG", "PONG", True):
+            print("FAIL: PING after error:", r)
+            return False
+    except Exception as exc:
+        print("FAIL: PING after error exception:", exc)
+        return False
+
+    try:
+        c.execute_command("DISCARD")
+    except Exception:
+        pass
+
+    return True
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(conflict_handler='resolve', description="Transaction forbid 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: transaction forbid")
+        sys.exit(0)
+    print("FAIL: transaction forbid")
+    sys.exit(1)