Promises resolving the same value are handled incorrectly

[linkvt]

Hi,

it seems that I found a bug with turbo-stream handling promises resolving the same vlaue incorrectly.

I originally opened remix-run/remix#9466 but decided to dig a bit deeper and found the problem in this package.
In short, instead of the value of the second promise (resolving the same value) an empty array is returned like this:

[...{"31":32,"33":34,"35":36},"promiseA",["P",32],"somethingStatic",0,"promiseB",["P",36]]
P32:[1]
P36:[]

Analysis

It seems to me, that flatten is used to flatten the initial array but also responses of resolved promises:

turbo-stream/src/turbo-stream.ts

Line 171 in a72f7a7

const id = flatten.call(encoder, resolved);

The problem is, that on resolving the 2nd promise this check here does not add a new value to the response, as it was already returned with the first promise:

turbo-stream/src/flatten.ts

Lines 24 to 25 in a72f7a7

	const existing = indices.get(input);
	if (existing) return existing;

The consequence is, that an empty array is returned here:

turbo-stream/src/turbo-stream.ts

Lines 177 to 184 in a72f7a7

	const values = encoder.stringified
	.slice(lastSentIndex + 1)
	.join(",");
	controller.enqueue(
	textEncoder.encode(
	`${TYPE_PROMISE}${deferredId}:[${values}]\n`
	)
	);

This causes an error in the unflatten function which checks for an empty erray:

turbo-stream/src/unflatten.ts

Line 35 in a72f7a7

if (!Array.isArray(parsed) || !parsed.length) throw new SyntaxError();

Possible Fix

After looking through the code it seems to me, that we cannot reference previous values when unflattening the value of a promise.
Because of this my quick fix was to send the whole promise again, which is of course not optimal.

I opened a PR with a test case and a fix, see #29

Best, Vincent

[jacob-ebey]

Closing for now. Please re-open if the issue persists. Thanks for the detailed report.