#if !EQUINOX_CORE

// NOT PUBLIC in Equinox.CosmosStore library - used internally to ensure the stored proc init is checked on first append once per container per process

// PUBLIC in Equinox.Core (it can also be used independent of Equinox)

internal

TaskCell<'T>(startWorkflow : System.Func<CancellationToken, Task<'T>>, [<O; D null>]?isExpired: System.Func<'T, bool>) =

let isValid = match isExpired with Some f -> not << f.Invoke | None -> fun _ -> true

let mutable cell = LazyTask<'T>.Empty

/// Synchronously check the value remains valid (to enable short-circuiting an Await step where value not required)

member _.IsValid() =

cell.TryCompleted() |> ValueOption.exists isValid

/// Gets or asynchronously recomputes value depending on whether value passes the validity check

member _.Await(ct: CancellationToken) = task {

// Each concurrent execution takes a copy of the cell, and attempts to reuse the value; later used to ensure only one triggers the workflow

let current = cell

match! current.TryAwaitValid() with

| ValueSome res when isValid res -> return res

| _ ->

// Prepare a new instance, with cancellation under our control (it won't start until the first Await on the LazyTask triggers it though)

let newInstance = LazyTask<'T>(fun () -> startWorkflow.Invoke ct)

// If there are concurrent executions, the first through the gate wins; everybody else awaits the instance the winner wrote

let _ = Interlocked.CompareExchange(&cell, newInstance, current)

return! cell.Await() }

member x.Await() = Async.call x.Await