[@aws-cdk/aws-events] Schedule cannot be created from CfnParameter

[alopix]

When trying to use a parameter for the duration of a events Schedule, on synth, an error occurs: "Unable to perform time unit conversion on un-resolved token -1.8881545897087682e+289."

Specifying the duration for e.g. a Lambda timeout works, but not if the Duration is used in a Schedule.rate.

Reproduction Steps

import { Stack, App, Duration, CfnParameter } from '@aws-cdk/core';
import { Rule, Schedule } from '@aws-cdk/aws-events';

const invokeScheduleInMinutes = new CfnParameter(this, 'invokeScheduleInMinutes', {
      type: 'Number',
      description: 'The minutes between scheduled lambda invocations.'
});

new Rule(this, 'Rule', {
      ruleName: `cron`,
      schedule: Schedule.rate(Duration.minutes(invokeScheduleInMinutes.valueAsNumber))
});

Error Log

…/node_modules/@aws-cdk/core/lib/duration.ts:289
    throw new Error(`Unable to perform time unit conversion on un-resolved token ${amount}.`);
          ^
Error: Unable to perform time unit conversion on un-resolved token -1.8881545897087682e+289.
    at convert (…/node_modules/@aws-cdk/core/lib/duration.ts:289:11)
    at Duration.toSeconds (…node_modules/@aws-cdk/core/lib/duration.ts:127:12)
    at Function.rate (…/node_modules/@aws-cdk/aws-events/lib/schedule.ts:20:18)
    at new AppStack (…/infrastructure/stacks/src/AppStack.ts:47:26)
    at Object.<anonymous> (…/infrastructure/stacks/src/app.ts:21:1)
    at Module._compile (internal/modules/cjs/loader.js:1158:30)
    at Module.m._compile (…/node_modules/ts-node/src/index.ts:858:23)
    at Module._extensions..js (internal/modules/cjs/loader.js:1178:10)
    at Object.require.extensions.<computed> [as .ts] (…/node_modules/ts-node/src/index.ts:861:12)
    at Module.load (internal/modules/cjs/loader.js:1002:32)

Environment

• CLI Version : 1.56.0
• Framework Version: 1.56.0
• Node.js Version: v12.16.1
• OS : macOS
• Language (Version): TypeScript 3.9.7

Other

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This is 🐛 Bug Report

[rix0rrr]

As a workaround, use Schedule.expression() for now.

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For fixing this:

Duration should should have an isUnresolved() method, and Schedule.rate should just render the number in the given unit if the duration is unresolved.

Same for other rate() methods in the library.