[MAJOR] concat() unsubscribe does not tear down the active source after the first source completes #71

Package/version: Rocket.jl v1.9.0
Environment: Julia 1.12.6

Summary

ConcatInnerActor.on_complete! has a guard that is always false, so after the first
source completes the subscription for the currently-active source is never stored.
unsubscribe!(concatSubscription) then unsubscribes a stale (completed) source and the
running source keeps emitting.

src/observable/concat.jl:93-104

function on_complete!(actor::ConcatInnerActor)
    cindex = get_current_index(actor)
    if cindex === length(actor.sources)
        complete!(actor.actor)
    else
        set_current_index!(actor, cindex + 1)                     # line 98
        subscription = subscribe!(actor.sources[cindex+1], actor) # line 99
        if get_current_index(actor) === cindex                    # line 100  <- always false
            set_subscription!(actor, subscription)                # line 101
        end
    end
end

current_index is already advanced to cindex + 1 on line 98, so the line-100 comparison
can never be true; set_subscription! (the only update to actor.subscription for the new
active source) is dead code. on_subscribe! (lines 57-63) keeps an analogous, working guard
(get_current_index === 1), confirming line 100 should compare against cindex + 1.

Repro

sub_a = Subject(Int);  sub_b = Subject(Int);  out = Int[]
sub = subscribe!(concat(sub_a, sub_b), lambda(on_next=v->push!(out,v)))
next!(sub_a, 1); complete!(sub_a)   # concat advances to sub_b
next!(sub_b, 2)
unsubscribe!(sub)                   # should detach sub_b
next!(sub_b, 3)                     # still delivered -> leak
sleep(0.1)
println(out)                        # => [1, 2, 3]

Expected vs Actual

Expected: after unsubscribe!, next!(sub_b, 3) is dropped → [1, 2].
Actual: [1, 2, 3] — the active (2nd) source is not unsubscribed. For an unbounded source
(interval, timer, network) this leaks the underlying resource forever and spurious
post-unsubscribe emissions reach the observer.

Suggested fix

Track the newly-set index:

set_current_index!(actor, cindex + 1)
subscription = subscribe!(actor.sources[cindex+1], actor)
if get_current_index(actor) === cindex + 1
    set_subscription!(actor, subscription)
end

Fix PR

See prs/pr-03-concat-teardown-leak.md.