AsyncCacheCell wip

jet · bartelink · Sep 2, 2022 · Sep 1, 2022 · Sep 1, 2022 · Sep 1, 2022
commit 6e4e82c83ef9a12f47c822d8fae7ddd46dd8ca9d
diff --git a/src/Equinox.Core/AsyncCacheCell.fs b/src/Equinox.Core/AsyncCacheCell.fs
@@ -6,43 +6,51 @@ open System.Threading.Tasks
 /// Asynchronous Lazy<'T> used to gate a workflow to ensure at most once execution of a computation.
 type AsyncLazy<'T>(workflow : unit -> Task<'T>) =
 
-    let task = Lazy.Create workflow
+    let workflow = lazy workflow ()
 
     /// Await the outcome of the computation.
     /// NOTE due to `Lazy<T>` semantics, failed attempts will cache any exception; AsyncCacheCell compensates for this
-    member _.Await() = task.Value
+    member _.Await() = workflow.Value
 
-    /// Used to rule out values where the computation yielded an exception or the result has now expired
-    member _.TryAwaitValid(isExpired) : 'T voption =
-        if not task.IsValueCreated then ValueNone else
+    /// Synchronously check whether the value has been computed (and/or remains valid)
+    member x.IsValid(isExpired) =
+        if not workflow.IsValueCreated then false else
 
-        let t = task.Value
+        let t = workflow.Value
+        if t = null || not t.IsCompleted || t.IsFaulted then false else
 
-        // Determines if the last attempt completed, but failed; For TMI see https://stackoverflow.com/a/33946166/11635
-        if t.Status <> TaskStatus.RanToCompletion then ValueNone // net6.0 brings an IsCompletedSuccessfully, but we're still netstandard
-        else match isExpired with
-             | ValueSome check when not (check t.Result) -> ValueNone
-             | _ -> ValueSome t.Result
+        match isExpired with
+        | ValueSome f -> not (f t.Result)
+        | _ -> true
 
-    /// Synchronously check whether the value has been computed (and/or remains valid)
-    member x.IsValid(isExpired) = x.TryAwaitValid isExpired |> ValueOption.isSome
+    /// Used to rule out values where the computation yielded an exception or the result has now expired
+    member _.TryAwaitValid(isExpired) : Task<'T voption> =
+        let t = workflow.Value
+
+        // Determines if the last attempt completed, but failed; For TMI see https://stackoverflow.com/a/33946166/11635
+        if t = null || t.IsFaulted then Task.FromResult ValueNone
+        else task {
+            let! (res : 'T) = t
+            match isExpired with
+            | ValueSome check when not (check res) -> return ValueNone
+            | _ -> return ValueSome res }
 
 /// Generic async lazy caching implementation that admits expiration/recomputation/retry on exception semantics.
 /// If `workflow` fails, all readers entering while the load/refresh is in progress will share the failure
 /// The first caller through the gate triggers a recomputation attempt if the previous attempt ended in failure
 type AsyncCacheCell<'T>(workflow : CancellationToken -> Task<'T>, ?isExpired : 'T -> bool) =
 
     let isExpired = match isExpired with Some x -> ValueSome x | None -> ValueNone
-    let mutable cell = AsyncLazy(fun () -> Task.FromCanceled<_>(CancellationToken.None))
+    let mutable cell = AsyncLazy(fun () -> null)
 
     /// Synchronously check the value remains valid (to short-circuit an Async AwaitValue step where value not required)
     member _.IsValid() = cell.IsValid(isExpired)
     /// Gets or asynchronously recomputes a cached value depending on expiry and availability
-    member _.Await(ct) =
+    member _.Await(ct) = task {
         // First, take a local copy of the current state
         let current = cell
-        match current.TryAwaitValid(isExpired) with
-        | ValueSome res -> Task.FromResult res // ... if it's already / still valid, we're done
+        match! current.TryAwaitValid(isExpired) with
+        | ValueSome res -> return res // ... if it's already / still valid, we're done
         | ValueNone ->
             // Prepare to do the work, with cancellation under out control
             let attemptLoad () = workflow ct
@@ -51,4 +59,4 @@ type AsyncCacheCell<'T>(workflow : CancellationToken -> Task<'T>, ?isExpired : '
             // avoid unnecessary recomputation in cases where competing threads detect expiry;
             // the first write attempt wins, and everybody else reads off that value
             let _ = Interlocked.CompareExchange(&cell, AsyncLazy(dispatch), current)
-            cell.Await()
+            return! cell.Await() }