Hi Kenton,
Sorry to bring this up from a long time ago, but if it's possible I think
the behavior of joinPromises should be well-defined. For my use case, which
I share below, it's preferable for all promises to complete before an
exception is propagated, but I understand the reasons to go the other way.
Here's my use case:
For each successful promise in the array I would like to call an "undo"
promise in case one of the others fail. So I can write code similar to this:
kj::Vector<kj::Promise<void>> vec;
std::shared_ptr<Cleanup> cleanup; // This is like an "async guard" to undo
successful promises in case one fails
for (p in promises) {
vec.add(p.then()[cleanup] { cleanup->add(undo(p)); });
}
kj::joinPromises(vec.releaseAsArray()).catch_([cleanup](){
cleanup->Go(); // Calls all added undo promises
});
If an exception is called after all promises complete (successfully or
unsuccessfully) - I believe this code is correct.
However, if a single failure propagates immediately - this code is
incorrect, as one promise can be halfway to successful completion and the
cleanup won't be called for it when it completes.
Thanks,
Amit
On Saturday, November 14, 2015 at 1:00:08 AM UTC+2, Kenton Varda wrote:
>
> Yes, an exception from any one promise becomes an exception from the
> combined promise.
>
> I think joinPromises() still waits for all to complete before propagating
> the exception. Arguably it should cancel all the other promises as soon as
> one resolves to an exception.
>
> -Kenton
>
> On Fri, Nov 13, 2015 at 2:26 PM, Nathan Hourt <[email protected]
> <javascript:>> wrote:
>
>> If I use kj::joinPromises to convert my Array<Promise<T>> to
>> Promise<Array<T>>, and one of the promises breaks, what happens to the
>> joined promise? Does it break and thereby throw away all of the resolved
>> promises?
>>
>> Thanks!
>>
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