{#await} in SSR should suppress rejecting promises

[Conduitry]

Describe the bug

(This came out of sveltejs/kit#2520 but is a separate matter from how SvelteKit ought to deal with user code that creates unhandled rejected promises.)

Node 16 terminates the process immediately when it detects that a dangling promise has rejected and no callbacks have been attached to handle that. SSR code for an {#await} block ignores the promise resolution (and just renders the waiting-for-promise content), which means that Node thinks the promise has not been handled appropriately. In Node 14, this results in a noisy warning, and in Node 16 this results in the program ending with an error.

Since we specifically do not care about the promise resolution/rejection in SSR in {#await}s (and since there's nothing we can do about this while SSR itself is synchronous), the generated SSR code for an {#await} block should contain explicit (dummy) handlers for the promise, to appease Node.

Reproduction

{#await Promise.reject('foo')}
	bar
{/await}

Compiling this in SSR mode and running it in Node 16 results in the process crashing, because you are generating a promise that rejects but it is then never handled.

Logs

[UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "foo".] {
  code: 'ERR_UNHANDLED_REJECTION'
}

System Info

Node: 16.10.0

Severity

annoyance

[Conduitry]

In 3.43.1, passing a rejecting promise to an {#await} no longer crashes the process in Node 16.