void/log: add logger.child(fields) to bind fields to every line #596

Summary

Apps usually tag every log line from a module with the same fields (component, queue, job), so the lines can be filtered in Workers Logs and with void project logs --filter. void/log has no way to bind those fields, so each call repeats them, or each app writes its own wrapper. The obvious wrapper breaks the logger's guarantee that it never throws.

The wrapper apps write today

import { logger as baseLogger } from 'void/log';

type Fields = Record<string, unknown>;

export const logger = {
  ...baseLogger,
  child: (bound: Fields) => ({
    error: (msg: string, fields?: Fields) => baseLogger.error(msg, { ...bound, ...fields }),
    warn: (msg: string, fields?: Fields) => baseLogger.warn(msg, { ...bound, ...fields }),
    info: (msg: string, fields?: Fields) => baseLogger.info(msg, { ...bound, ...fields }),
  }),
};

The spread { ...bound, ...fields } runs before format(). That means it runs outside the try that makes logging safe. A field whose getter throws makes the base logger fall back to its safe line, but the wrapper throws the error into the caller instead (void@1.2.1; format() on main has the same try):

const fields = Object.defineProperty({}, 'boom', {
  enumerable: true,
  get() {
    throw new Error('getter');
  },
});

logger.info('base.ok', fields);
// {"msg":"base.ok","error":"log: fields not serializable"}

logger.child({ component: 'x' }).info('child.call', fields);
// throws Error: getter

A log call inside a catch or a queue consumer can then throw where it was meant to only record. Supporting this in void/log means each app no longer copies the wrapper, and the merge stays inside the guarantee.

Proposal

import { logger } from 'void/log';

const log = logger.child({ component: 'queue', queue: 'tasks' });

log.warn('task.retry', { taskId, attempt: 2 });
// {"msg":"task.retry","component":"queue","queue":"tasks","taskId":"…","attempt":2}

• logger.child(fields) returns a logger with error, warn, info and child.
• Lines keep the { msg, ...fields } shape, so Cloudflare Tail capture and platform log filtering don't change.
• Fields passed in a call override bound fields with the same name.
• child() can be nested: log.child({ taskId }) adds to the fields already bound.
• Bound fields get the same handling as call fields. msg, toJSON, __proto__ and constructor are skipped, functions are dropped, and Error values are serialized (fix(log): keep Error details in logger fields #583).
• Logging never throws, including when bound or call fields have throwing getters.

Implementation

In packages/void/src/runtime/log.ts, build each logger from a bound object and have format(message, bound, fields) copy both inside the existing try. Bound fields are copied first, then call fields, using the same key filters. logger stays the root, with no bound fields, so existing calls behave the same.

Tests in packages/void/test/unit/runtime-log.test.ts:

• bound fields appear on every level
• call fields override bound fields
• nested child() merges fields
• reserved keys and functions in bound fields are skipped
• a throwing getter in bound or call fields falls back to the safe line instead of throwing

Docs: add logger.child(fields) to the methods line under Logging in docs/reference/api/resources.md, with a one-line example.