On Mon, Jul 6, 2020 at 7:26 AM Kenton Varda <[email protected]> wrote:
> Almost. Two issues I can think of:
>
> 1. After each pump completes, you probably want to call shutdownWrite() to
> propagate the EOF.
> 2. `joinPromises()` always waits for all promises to resolve. I've often
> found that this turns out to be the wrong behavior when one of the joined
> promises throws an exception. Usually you want the other tasks canceled in
> that case. I think that might be the case here -- if you get an I/O error
> in one direction, you probably want to kill the whole stream. Then again,
> probably that'll happen anyway in most cases. (Whereas, EOF is not an
> error, so you do want to wait for the other promise in that case.)
>
So more like this?
return stream1.pumpTo(stream2).ignoreResult().then(
[&] {stream2.shutdownWrite()},
[&](kj::Exception&& e){
stream1.shutdownWrite();
stream1.abortRead();
stream2.shutdownWrite();
stream2.abortRead();
})).exclusiveJoin(
stream2.pumpTo(stream1).ignoreResult().then(
[&] {stream1.shutdownWrite()},
[&](kj::Exception&& e){
stream1.shutdownWrite();
stream1.abortRead();
stream2.shutdownWrite();
stream2.abortRead();
}));
> On another note, a warning: I'm likely to change the AsyncOutputStream
> interface significantly in the future, in order to replace
> `shutdownWrite()` with something that returns a promise, and to make it so
> that if you don't explicitly shut down a stream, then it's treated as an
> error. Currently, AsyncOutputStream's destructor implicitly sends a clean
> EOF, but that's the wrong thing to do when the sender terminated
> prematurely due to an exception. So, your code will need some updating when
> that happens.
>
Yeah, as long as it's a strict compilation error if there's changes that
need to be made to make previously correct work correct again, that's fine
(+ maybe documentation in the release notes on how to migrate). Can just
treat that as the cost of updating to a new version of the library. More
challenging would be any implicit behavioural changes that don't change the
API. Those can be harder to catch even with unit tests.
Also while you're at it, does it make sense to have 1 function that does
both shutdownWrite/abortRead so that if both are returning futures, joining
them correctly can be hidden as an implementation detail? I'm finding the
promises stuff to be straightforward for simple cases but anything more
custom is harder to reason about in terms of making sure they compose
correctly - I never feel quite comfortable if I've written the promise code
correctly (especially with all the different types of promises, Canceler &
TaskSet). Hopefully it's easy to integrate C++20 coroutine support to make
things read more linearly again. Not sure when I'll get to use C++20 though.
>
> -Kenton
>
> On Sun, Jul 5, 2020 at 8:13 PM Vitali Lovich <[email protected]> wrote:
>
>> I was wondering what would be the best way to bridge 2 kj::AsyncIoStreams
>> to each other (read to write/write to read) so that they act as
>> pass-through? I'm assuming something like:
>>
>> auto pumped = kj::ArrayBuilder<kj::Promise<void>>(2);
>> pumped.add(stream1.pumpTo(stream2).ignoreResult());
>> pumped.add(stream2.pumpTo(stream1).ignoreResult());
>> return kj::joinPromises(pumped.finish()).ignoreResult();
>>
>> Thanks,
>> Vitali
>>
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>
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