bug: syslog plugin crashes with nil-index error instead of returning an error when logger init fails

Current Behavior

In apisix/plugins/syslog/init.lua, send_syslog_data() fetches the logger instance from the lrucache and correctly checks whether it failed:

https://github.com/apache/apisix/blob/master/apisix/plugins/syslog/init.lua#L45-L78

local function send_syslog_data(conf, log_message, api_ctx)
    local err_msg
    local res = true

    core.log.info("sending a batch logs to ", conf.host, ":", conf.port)

    -- fetch it from lrucache
    local logger, err = core.lrucache.plugin_ctx(
        lrucache, api_ctx, nil, logger_socket.new, logger_socket, {
            host = conf.host,
            port = conf.port,
            flush_limit = conf.flush_limit,
            drop_limit = conf.drop_limit,
            timeout = conf.timeout,
            sock_type = conf.sock_type,
            pool_size = conf.pool_size,
            tls = conf.tls,
        }
    )

    if not logger then
        res = false
        err_msg = "failed when initiating the sys logger processor".. err
    end

    -- reuse the logger object
    local ok, err = logger:log(log_message)
    ...

When logger_socket.new(...) fails (e.g. bad host/port, socket/resource errors), logger is nil and the if not logger then branch correctly sets res = false and builds err_msg. But there is no return in that branch, so execution falls through to the very next line, logger:log(log_message), which indexes a nil value.

This throws an uncaught Lua runtime error ("attempt to index a nil value") inside the log-phase batch processor callback, instead of returning the already-built (false, err_msg) back to the caller like every other error branch in this codebase does (e.g. the same pattern in apisix/plugins/lago.lua's send_http_data, or the if not ok then branch a few lines below in this same function).

Expected Behavior

When the logger fails to initialize, send_syslog_data should return early with res, err_msg (i.e. false, err_msg), the same way the function already does for the logger:log() failure case just below it, instead of crashing on a nil index.

Suggested fix — add a return res, err_msg right after err_msg is set in the if not logger then block:

    if not logger then
        res = false
        err_msg = "failed when initiating the sys logger processor".. err
        return res, err_msg
    end

This is a small, single-file, single-function fix and should be a good first issue for a new contributor.

Error Logs

attempt to index a nil value (local 'logger')

(Uncaught Lua runtime error thrown from logger:log(log_message) in apisix/plugins/syslog/init.lua, surfaced in the APISIX error log during the log phase.)

Steps to Reproduce

1. Run APISIX via the Docker image (or locally built).
2. Create a Route with the Admin API and enable the syslog plugin on it, pointing host/port at a syslog endpoint.
3. Force core.lrucache.plugin_ctx(...) / logger_socket.new(...) to fail for that route (e.g. point host/port at an address the socket cannot connect to, or otherwise make resty.logger.socket's new() return nil, err).
4. Send a request through the route and observe an unhandled "attempt to index a nil value" error in the log phase instead of a clean, logged (false, err_msg) failure.

Environment

• APISIX version (run apisix version): master branch (code unchanged for a long time; present in released versions too)
• Operating system (run uname -a): not relevant — logic bug, reproducible on any OS
• OpenResty / Nginx version (run openresty -V or nginx -V): not relevant — pure Lua control-flow bug
• etcd version, if relevant: N/A
• APISIX Dashboard version, if relevant: N/A
• Plugin runner version, for issues related to plugin runners: N/A
• LuaRocks version, for installation issues: N/A