Add Cassandra system keyspace and virtual view support

[Copilot]

Cassandra drivers query system.local, system.peers, and system_schema.* tables during connection setup. Without these, no standard driver can connect to objstore's native protocol.

Changes

• VirtualRows result type — New ResultKind::VirtualRows with VirtualColumn/VirtualRow structs in engine.cppm for query results not backed by btree storage
• System table handlers in engine.cpp:
  • system.local — single row with cluster name, data center, rack, partitioner, protocol version, etc.
  • system.peers / system.peers_v2 — empty (single-node)
  • system_schema.keyspaces / tables / columns — reflect live schema with column kinds and types
• Native protocol serialization — append_result_virtual_rows() in native.cppm serializes virtual rows identically to storage-backed rows
• System keyspace guards — USE system, CREATE KEYSPACE IF NOT EXISTS system handled without touching storage
• REPL/HTTP — VirtualRows case added to both output paths

Example

objstore> SELECT * FROM system.local;
 key   | cluster_name | data_center  | rack  | release_version
 local | objstore     | datacenter1  | rack1 | 4.0.0
(1 rows)

objstore> SELECT * FROM system_schema.columns;
 keyspace_name | table_name | column_name | kind          | type
 test_ks       | users      | id          | partition_key | int
 test_ks       | users      | name        | regular       | text
(2 rows)

System queries are intercepted in SelectFrom before the WHERE/LIMIT assertions, so driver queries like SELECT * FROM system.local WHERE key='local' work without general WHERE execution support.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• lexy.foonathan.net
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Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for Cassandra system keyspace and other virtual views</issue_title>
<issue_description>The objstore database has basic support for the cassandra query language native protocol. To enable existing cassandra drivers to communicate with objstore we need to support system views such as system.local and expand our current excution implementation to support basic queries.

Use the Cassandra java source file https://github.com/apache/cassandra/blob/5519df34cb725904ef567e377657cddd580e7c0f/src/java/org/apache/cassandra/db/SystemKeyspace.java#L4 as a reference for all the cassandra system keyspaces. Implementing new features in the data storage layer (blob and btree) is out of scope.

Testing

You can test using the cassandra driver using test_cql.js and new unit tests. To test with test_cql.js

• Start ./build/objstore/objstore_server <db_path> --native in the background
• Wait for "listening on port 9042" in its output
• Run node extra/node/test_cql.js
• Sending SIGUSR1 to the server PID using objstore_server_shutdown.sh to shut it down cleanly</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add support for Cassandra system keyspace and other virtual views #14

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