Rust client should support comma-separated bootstrap servers

[luoyuxia]

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• I searched in the issues and found nothing similar.

Please describe the bug 🐞

The Rust client currently appears to support only one bootstrap endpoint, even though the public configuration is named bootstrap_servers / bootstrap.servers. A comma-separated value such as 127.0.0.1:9123,127.0.0.1:9124 should be accepted so users can configure multiple bootstrap nodes for high availability.

Current behavior:

1. Set the Rust client config to a comma-separated bootstrap list, for example:

let mut config = Config::default();
config.bootstrap_servers = "127.0.0.1:9123,127.0.0.1:9124".to_string();
let conn = FlussConnection::create(config).await?;

2. The client passes the whole string through Metadata::new / Metadata::init_cluster.
3. Metadata::parse_bootstrap calls to_socket_addrs() on the complete string, so the comma-separated value is treated as a single socket address and fails to resolve.

Expected behavior:

The client should parse bootstrap_servers as a comma-separated list of host:port entries, trim whitespace, validate each endpoint, and use one reachable bootstrap endpoint to initialize metadata. Reinitialization should also keep using the configured bootstrap list instead of assuming a single address.

Impact:

Users cannot configure multiple bootstrap nodes/coordinators for HA or failover. This also makes the plural config name misleading.

Solution

Update the client bootstrap parsing to return a list of socket addresses rather than a single SocketAddr. During metadata initialization and reinitialization, try the parsed endpoints in a deterministic order, falling back to the next endpoint if one cannot be resolved or connected.

Suggested test coverage:

• comma-separated IPv4/hostname entries, e.g. 127.0.0.1:9123,localhost:9124;
• whitespace around entries;
• IPv6 entries such as [::1]:9123;
• invalid empty entries and invalid ports;
• reinit_cluster uses the configured bootstrap list.

Are you willing to submit a PR?

• I'm willing to submit a PR!