Async.Parallel stack overflow on cancellation of ~2000 uncompleted computations

[bartelink]

When Async.Parallel is handling cancellation (via the token or a computation throwing) with e.g. 2000 items outstanding, the current implementation as of FSharp.Core 6.0.4 is such that the non-tail-recursive impl can blow the stack.

Repro steps

let [<Fact>] ``Async.Parallel blows stack when cancelling many`` () =
    let gen (i : int) = async {
        if i <> 0 then do! Async.Sleep i
        else return failwith (string i) }
    let count = 1800
    let comps = Seq.init count gen
    let result = Async.Parallel(comps, 16) |> Async.Catch |> Async.RunSynchronously
    test <@ match result with
            | Choice2Of2 e -> int e.Message < count
            | x -> failwithf "unexpected %A" x @>

Known workarounds

batch the work with Seq.ChunkBySize 1500, add an outer Async.Parallel call (distributing the degreeOfParallelism over the constituent calls, or using the same value but internally constraining it with Async.Throttle), Array.concat the results.

Related information

(1800 is the approx threshold with SDK 6.0.202 on MacOS x86 with Rider test runner)

Problem appears to be at

fsharp/src/FSharp.Core/async.fs

Line 1473 in 0006b73

worker trampolineHolder |> unfake

[dsyme]

@bartelink I've prepped a fix for this

[bartelink]

Excellent, thanks - nice to see it's [appears to be] a [relatively] simple coding slip rather than an actual oversight.
Certainly glad I didn't venture a fix without taking the time it would take to grok the idioms at play!
Added minor comments you're probably in the middle of invalidating