Swift Combine - Does the zip operator retain all values that it hasn't had a chance to publish? - memory-leaks

Question

Here's the code I'm wondering about:
final class Foo {
var subscriptions = Set<AnyCancellable>()
init () {
Timer
.publish(every: 2, on: .main, in: .default)
.autoconnect()
.zip(Timer.publish(every: 3, on: .main, in: .default).autoconnect())
.sink {
print($0)
}
.store(in: &subscriptions)
}
}
This is the output it produces:
(2020-12-08 15:45:41 +0000, 2020-12-08 15:45:42 +0000)
(2020-12-08 15:45:43 +0000, 2020-12-08 15:45:45 +0000)
(2020-12-08 15:45:45 +0000, 2020-12-08 15:45:48 +0000)
(2020-12-08 15:45:47 +0000, 2020-12-08 15:45:51 +0000)
Would this code eventually crash from memory shortage? It seems like the zip operator is storing every value that it receives but can't yet publish.

Answer 1

zip does not limit its upstream buffer size. You can prove it like this:
import Combine
let ticket = (0 ... .max).publisher
.zip(Empty<Int, Never>(completeImmediately: false))
.sink { print($0) }
The (0 ... .max) publisher will try to publish 263 values synchronously (that, is, before returning control to the Zip subscriber). Run this and watch the memory gauge in Xcode's Debug navigator. It will climb steadily. You probably want to kill it after a few seconds, because it will eventually use up an awful lot of memory and make your Mac unpleasant to use before finally crashing.
If you run it in Instruments for a few seconds, you'll see that all of the allocations happen in this call stack, indicating that Zip internally uses a plain old Array to buffer the incoming values.
66.07 MB 99.8% 174 main
64.00 MB 96.7% 45 Publisher<>.sink(receiveValue:)
64.00 MB 96.7% 42 Publisher.subscribe<A>(_:)
64.00 MB 96.7% 41 Publishers.Zip.receive<A>(subscriber:)
64.00 MB 96.7% 12 Publisher.subscribe<A>(_:)
64.00 MB 96.7% 2 Empty.receive<A>(subscriber:)
64.00 MB 96.7% 2 AbstractZip.Side.receive(subscription:)
64.00 MB 96.7% 2 AbstractZip.receive(subscription:index:)
64.00 MB 96.7% 2 AbstractZip.resolvePendingDemandAndUnlock()
64.00 MB 96.7% 2 protocol witness for Subscription.request(_:) in conformance Publishers.Sequence<A, B>.Inner<A1, B1, C1>
64.00 MB 96.7% 2 Publishers.Sequence.Inner.request(_:)
64.00 MB 96.7% 1 AbstractZip.Side.receive(_:)
64.00 MB 96.7% 1 AbstractZip.receive(_:index:)
64.00 MB 96.7% 1 specialized Array._copyToNewBuffer(oldCount:)
64.00 MB 96.7% 1 specialized _ArrayBufferProtocol._forceCreateUniqueMutableBufferImpl(countForBuffer:minNewCapacity:requiredCapacity:)
64.00 MB 96.7% 1 swift_allocObject
64.00 MB 96.7% 1 swift_slowAlloc
64.00 MB 96.7% 1 malloc
64.00 MB 96.7% 1 malloc_zone_malloc