Async pre-deploy/bundle step

[mmkal]

Hi - I'm trying to build a construct similar to NodeJSFunction. We are using webpack with swc rather than esbuild, so we're taking the implementation as a reference rather than using the existing construct directly.

I notice that there's a this.command = ['bash', '-c', bundlingCommand] part, which is eventually passed to spawnSync. Does this mean that if we have to do bundling/preparation which is async (promises, callbacks etc.), we always need to create a subprocess to un-asyncify it? Are there any plans to allow declaring "hooks" which can run asynchronously, in process, similar to serverless framework? Or are we thinking about this the wrong way?

I could imagine an API something like this:

import {core, aws_lambda} from 'monocdk'
import {BuildTask} from '@aws-cdk/build-task'

export const fooLambda = (scope: core.Construct) => {
  const outputPath = 'dist/output.js'
  const buildTask = new BuildTask(scope, 'fooFunctionPackager', {
    build: async () => await magicalWebpackesqueBuild({
      entry: 'src/some/path.ts',
      output: outputPath,
    }),
  })

  const fooFunction = new aws_lambda.Function(scope, 'fooFunction', {
    code: aws_lambda.Code.fromAsset(outputPath),
  })
}

When these resources are created, there could be a part of cdk deploy/cdk synth that runs the build() function for any BuildTask instances under scope, before trying to synthesize or deploy anything.

As a follow-up it could maybe take hashable inputs to tell it when it's safe to skip the build() task, which might be slow.

This could be used for bundling, or fetching deploy-time assets from a remote server, or all sorts of things.