What causes "function factory must be set before registering" in databendlabs/databend?

Set the function factory on the builder before calling `register` (e.g. assign the `factory` field or call the builder's factory-setting method).

databendlabs/databend · error

function factory must be set before registering

Error message

function factory must be set before registering

What it means

This panic occurs in `FunctionRegistryBuilder::register` when the builder's optional `factory` field is `None`. The function factory must be installed (via the builder's factory-setting API) before `register` is called, because registration delegates function resolution to the factory. The library intentionally panics via `expect` since this is a programmer/API-usage error, not a runtime condition.

Solutions

Set the function factory on the builder before calling `register` (e.g. assign the `factory` field or call the builder's factory-setting method).
Prefer the existing function-registration macros/helpers in the crate, which wire the factory automatically.
Audit custom builder construction paths for `..Default::default()` that silently reset `factory` to None.

Example fix

// before
let builder = FunctionRegistryBuilder { name, ..Default::default() };
builder.register(registry);
// after
let builder = FunctionRegistryBuilder { name, factory: Some(Box::new(MyFactory)), ..Default::default() };
builder.register(registry);

Defensive patterns

Strategy: type-guard

Validate before calling

assert!(builder.factory.is_some(), "factory must be set before register");

Type guard

fn factory_is_set(b: &FunctionRegistryBuilder) -> bool { b.factory.is_some() }

Try / catch

// Rust panics are not catchable idiomaticly here; guard instead:
if !factory_is_set(&builder) { return Err(ErrorCode::Internal("factory not set")); }

Prevention

Always construct registry builders through the provided macros/helpers.
Avoid `..Default::default()` struct-update syntax that can reset required fields.
Add a debug_assert for factory presence in custom registration paths.

When it happens

Trigger: Calling `FunctionRegistryBuilder::register(name, ...)` on a builder that was never given a function factory — i.e., skipping the factory assignment step (e.g. `with_factory` / the `factory` builder field) before calling `register`.

Common situations: Hand-rolling a registry builder instead of using the standard helper macros; refactoring that moves or deletes the factory setup call; constructing a builder with struct-update syntax (`..Default::default()`) that leaves `factory` as None.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

Related errors

AI-assisted analysis of databendlabs/databend@288d84d76e (2026-09-11). Data as JSON: /api/errors/e238f6b5d4ce185c. Report an issue: GitHub.

Appendix: source

Thrown at src/query/expression/src/function/function_builder.rs:238

            })
            .map(|builder| Arc::new(builder.finish()))
        })));
        self
    }

    pub fn register(self) -> &'a mut FunctionRegistry {
        let FunctionBuilder {
            registry,
            name,
            property,
            aliases,
            additional_cast_rules,
            dynamic_cast_rules,
            factory,
            ..
        } = self;

        let factory = factory.expect("function factory must be set before registering");

        registry.register_function_factory(&name, factory);

        Self::register_common(
            registry,
            &name,
            property,
            aliases,
            additional_cast_rules,
            dynamic_cast_rules,
        );

        registry
    }
}

pub struct InlineFunctionBuilder {
    name: String,

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