What causes "Failed to register now_ms function: {}" in databendlabs/databend?

Inspect the inner mlua error; recreate the Lua instance for memory/stack errors.

Failed to register now_ms function

Error message

Failed to register now_ms function: {}

What it means

After creation, the `now_ms` closure is stored into the `metactl` table with `Table::set`. If mlua fails to push or store that function value, the error is surfaced as `Failed to register now_ms function` and setup aborts before scripts execute.

Solutions

Inspect the inner mlua error; recreate the Lua instance for memory/stack errors.
Ensure `now_ms_fn` and `metactl_table` come from the same `Lua` instance.
Raise or remove custom Lua memory limits if the inner error is a memory error.
If it reproduces on a clean state, collect the inner error and report/upgrade mlua.

Defensive patterns

Strategy: try-catch

Try / catch

if let Err(e) = setup_lua_environment(&lua) {
    if e.to_string().contains("Failed to register now_ms function") {
        // recreate the Lua environment before retrying the script
    }
}

Prevention

Avoid reusing a Lua state after a fatal error; rebuild it.
Do not set restrictive MemoryLimits on the runtime.
Create the value and the target table in the same Lua instance.
Always inspect the wrapped inner mlua error.

When it happens

Trigger: `metactl_table.set("now_ms", now_ms_fn)` returning an mlua::Error during `setup_lua_environment`, i.e. during any Lua-script run or its unit tests.

Common situations: Lua stack/memory exhaustion in the shared Lua instance, typically caused by an earlier script that exhausted resources or corrupted the state.

Related errors

AI-assisted analysis of databendlabs/databend@288d84d76e (2026-09-11). Data as JSON: /api/errors/2f4764886dea2951. Report an issue: GitHub.

Appendix: source

Thrown at src/meta/control/src/lua_support.rs:474

            Ok(())
        })
        .map_err(|e| anyhow::anyhow!("Failed to create sleep function: {}", e))?;

    metactl_table
        .set("sleep", sleep_fn)
        .map_err(|e| anyhow::anyhow!("Failed to register sleep function: {}", e))?;

    // Register a monotonic millisecond clock for timing and benchmarks. The
    // epoch is this environment's setup time; only differences between calls are
    // meaningful. `Instant` guarantees the value never goes backwards.
    let start = std::time::Instant::now();
    let now_ms_fn = lua
        .create_function(move |_lua, ()| Ok(start.elapsed().as_secs_f64() * 1000.0))
        .map_err(|e| anyhow::anyhow!("Failed to create now_ms function: {}", e))?;

    metactl_table
        .set("now_ms", now_ms_fn)
        .map_err(|e| anyhow::anyhow!("Failed to register now_ms function: {}", e))?;

    // Register the Zipf load generator. The module returns the `ZipfGenerator`
    // table, exposed as `metactl.ZipfGenerator` so any script can call
    // `metactl.ZipfGenerator:new(num_keys, alpha)` directly.
    let zipf_generator = lua
        .load(ZIPF_LOAD_GENERATOR)
        .eval::<Table>()
        .map_err(|e| anyhow::anyhow!("Failed to load zipf_load_generator: {}", e))?;

    metactl_table
        .set("ZipfGenerator", zipf_generator)
        .map_err(|e| anyhow::anyhow!("Failed to register ZipfGenerator: {}", e))?;

    // Set the metactl table as a global
    lua.globals()
        .set("metactl", metactl_table)
        .map_err(|e| anyhow::anyhow!("Failed to register metactl namespace: {}", e))?;

View on GitHub (pinned to 288d84d76e)