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Don't hide exceptions on the async cancellation-path #3257

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Don't hide exceptions on the async cancellation-path #3257

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7075338
don't hide potentially important exceptions when cancelling the workf…
matthid Jun 24, 2017
e742506
add more tests to check if StartAsTask and RunSynchronously behave th…
matthid Jun 24, 2017
b8a325e
use AsyncTaskMethodBuilder to fix the new tests
matthid Jun 24, 2017
304e5dc
more tests, flatten exceptions and improve information flow on AwaitT…
matthid Jun 25, 2017
a7d97e3
cleanup code
matthid Jun 25, 2017
d5881c9
Switch branches.
matthid Feb 9, 2018
ac0a9c1
some more fixes and PR hints.
matthid Feb 18, 2018
49c26bf
remove FX_NO_ASYNCTASKMETHODBUILDER
matthid Feb 22, 2018
bd08e6e
Merge branch 'master' into fix_3219_exception
matthid Mar 17, 2018
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88 changes: 74 additions & 14 deletions src/fsharp/FSharp.Core/control.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -494,6 +494,25 @@ namespace Microsoft.FSharp.Control
// delayPrim = "bindA (return ()) f"
let delayA f = callA f ()

// protect an exception in an cancellation workflow and adds it to the given OperationCanceledException
// ie creates a new instance, where the new information is added,
// if cexn is a TaskCanceledException the type is preserved as well.
let augmentOperationCancelledException (cexn:OperationCanceledException) (edi:ExceptionDispatchInfo) =
// It's probably ok to not care about the stack of the cexn object, because
// 1. we often don't even collect the stack in the ccont route
// 2. there are no suited APIs to handle this (at best we could add the original instance to the new instance...)
// If we ever need this we probably need to provide our own sub-types of OperationCanceledException
// and TaskCanceledException
let exn = edi.GetAssociatedSourceException()
let collected =
match cexn.InnerException with
| null -> [|exn|]
| :? AggregateException as a -> Array.append (a.Flatten().InnerExceptions |> Seq.toArray) [|exn|]
| inner -> [|inner; exn|]
let aggr = (new AggregateException(collected)).Flatten()
match cexn with
| :? TaskCanceledException -> new TaskCanceledException(cexn.Message, aggr) :> OperationCanceledException
| _ -> new OperationCanceledException(cexn.Message, aggr)
// Call p but augment the normal, exception and cancel continuations with a call to finallyFunction.
// If the finallyFunction raises an exception then call the original exception continuation
// with the new exception. If exception is raised after a cancellation, exception is ignored
Expand All @@ -511,8 +530,8 @@ namespace Microsoft.FSharp.Control
// If an exception is thrown we continue with the previous exception continuation.
let econt exn = protect trampolineHolder args.aux.econt finallyFunction () (fun () -> args.aux.econt exn)
// The cancellation continuation runs the finallyFunction and then runs the previous cancellation continuation.
// If an exception is thrown we continue with the previous cancellation continuation (the exception is lost)
let ccont cexn = protect trampolineHolder (fun _ -> args.aux.ccont cexn) finallyFunction () (fun () -> args.aux.ccont cexn)
// If an exception is thrown we collect/protect it in the OperationCancelledException
let ccont cexn = protect trampolineHolder (augmentOperationCancelledException cexn >> args.aux.ccont) finallyFunction () (fun () -> args.aux.ccont cexn)
invokeA p { args with cont = cont; aux = { args.aux with econt = econt; ccont = ccont } })

// Re-route the exception continuation to call to catchFunction. If catchFunction or the new process fail
Expand All @@ -531,7 +550,7 @@ namespace Microsoft.FSharp.Control
/// Call the finallyFunction if the computation results in a cancellation
let whenCancelledA (finallyFunction : OperationCanceledException -> unit) p =
unprotectedPrimitive (fun ({ aux = aux } as args)->
let ccont exn = protect aux.trampolineHolder (fun _ -> aux.ccont exn) finallyFunction exn (fun _ -> aux.ccont exn)
let ccont exn = protect aux.trampolineHolder (augmentOperationCancelledException exn >> aux.ccont) finallyFunction exn (fun _ -> aux.ccont exn)
invokeA p { args with aux = { aux with ccont = ccont } })

let getCancellationToken() =
Expand Down Expand Up @@ -866,7 +885,11 @@ namespace Microsoft.FSharp.Control
queueAsync
token
(fun res -> resultCell.RegisterResult(Ok(res),reuseThread=true))
(fun edi -> resultCell.RegisterResult(Error(edi),reuseThread=true))
(fun edi ->
let result =
if token.IsCancellationRequested then Canceled(augmentOperationCancelledException (new OperationCanceledException()) edi)
else Error(edi)
resultCell.RegisterResult(result,reuseThread=true))
(fun exn -> resultCell.RegisterResult(Canceled(exn),reuseThread=true))
computation
|> unfake
Expand Down Expand Up @@ -897,7 +920,11 @@ namespace Microsoft.FSharp.Control
token
trampolineHolder
(fun res -> resultCell.RegisterResult(Ok(res),reuseThread=true))
(fun edi -> resultCell.RegisterResult(Error(edi),reuseThread=true))
(fun edi ->
let result =
if token.IsCancellationRequested then Canceled(augmentOperationCancelledException (new OperationCanceledException()) edi)
else Error(edi)
resultCell.RegisterResult(result,reuseThread=true))
(fun exn -> resultCell.RegisterResult(Canceled(exn),reuseThread=true))
computation)
|> unfake
Expand Down Expand Up @@ -939,9 +966,15 @@ namespace Microsoft.FSharp.Control
member __.Proceed = not isStopped
member __.Stop() = isStopped <- true

let StartAsTask (token:CancellationToken, computation : Async<_>,taskCreationOptions) : Task<_> =
let StartAsTask (token:CancellationToken, computation : Async<'a>,taskCreationOptions) : Task<'a> =
#if FX_NO_ASYNCTASKMETHODBUILDER
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Is this an artifact?

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I mean: Does this define still exist?

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This one doesn't exist any more

let taskCreationOptions = defaultArg taskCreationOptions TaskCreationOptions.None
let tcs = new TaskCompletionSource<_>(taskCreationOptions)
#else
// AsyncTaskMethodBuilder allows us to better control the cancellation process by setting custom exception objects.
let _ = defaultArg taskCreationOptions TaskCreationOptions.None
let tcs = System.Runtime.CompilerServices.AsyncTaskMethodBuilder<'a>()
#endif

// The contract:
// a) cancellation signal should always propagate to the computation
Expand All @@ -950,8 +983,15 @@ namespace Microsoft.FSharp.Control
queueAsync
token
(fun r -> tcs.SetResult r |> fake)
(fun edi -> tcs.SetException edi.SourceException |> fake)
(fun _ -> tcs.SetCanceled() |> fake)
(fun edi ->
let wrapper =
if token.IsCancellationRequested then
augmentOperationCancelledException (new TaskCanceledException()) edi :> exn
else
edi.SourceException
tcs.SetException wrapper |> fake)
// We wrap in a TaskCanceledException to maintain backwards compat.
(fun exn -> tcs.SetException (new TaskCanceledException(exn.Message, exn.InnerException)) |> fake)
computation
|> unfake
task
Expand Down Expand Up @@ -1148,29 +1188,49 @@ namespace Microsoft.FSharp.Control
// Contains helpers that will attach continuation to the given task.
// Should be invoked as a part of protectedPrimitive(withResync) call
module TaskHelpers =
let continueWith (task : Task<'T>, args, useCcontForTaskCancellation) =
// This uses a trick to get the underlying OperationCanceledException
let inline getCancelledException (completedTask:Task) (waitWithAwaiter) =
let fallback() = new TaskCanceledException(completedTask) :> OperationCanceledException
// sadly there is no other public api to retrieve it, but to call .GetAwaiter().GetResult().
try waitWithAwaiter()
// should not happen, but just in case...
fallback()
with
| :? OperationCanceledException as o -> o
| other ->
// shouldn't happen, but just in case...
new TaskCanceledException(fallback().Message, other) :> OperationCanceledException

let continueWith (task : Task<'T>, args, useSimpleCcontForTaskCancellationAndLooseException) =

let continuation (completedTask : Task<_>) : unit =
args.aux.trampolineHolder.Protect((fun () ->
if completedTask.IsCanceled then
if useCcontForTaskCancellation
if useSimpleCcontForTaskCancellationAndLooseException
then args.aux.ccont (new OperationCanceledException(args.aux.token))
else args.aux.econt (ExceptionDispatchInfo.Capture(new TaskCanceledException(completedTask)))
else
let cancelledException =
getCancelledException completedTask (fun () -> completedTask.GetAwaiter().GetResult() |> ignore)
args.aux.econt (ExceptionDispatchInfo.Capture(cancelledException))
elif completedTask.IsFaulted then
args.aux.econt (MayLoseStackTrace(completedTask.Exception))
else
args.cont completedTask.Result)) |> unfake

task.ContinueWith(Action<Task<'T>>(continuation)) |> ignore |> fake

let continueWithUnit (task : Task, args, useCcontForTaskCancellation) =
let continueWithUnit (task : Task, args, useSimpleCcontForTaskCancellationAndLooseException) =


let continuation (completedTask : Task) : unit =
args.aux.trampolineHolder.Protect((fun () ->
if completedTask.IsCanceled then
if useCcontForTaskCancellation
if useSimpleCcontForTaskCancellationAndLooseException
then args.aux.ccont (new OperationCanceledException(args.aux.token))
else args.aux.econt (ExceptionDispatchInfo.Capture(new TaskCanceledException(completedTask)))
else
let cancelledException =
getCancelledException completedTask (fun () -> completedTask.GetAwaiter().GetResult())
args.aux.econt (ExceptionDispatchInfo.Capture(cancelledException))
elif completedTask.IsFaulted then
args.aux.econt (MayLoseStackTrace(completedTask.Exception))
else
Expand Down
116 changes: 111 additions & 5 deletions tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncType.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,7 +126,12 @@ type AsyncType() =
)

member private this.WaitASec (t:Task) =
let result = t.Wait(TimeSpan(hours=0,minutes=0,seconds=1))
let result =
try t.Wait(TimeSpan(hours=0,minutes=0,seconds=1))
with :? AggregateException ->
// This throws the "original" exception
t.GetAwaiter().GetResult()
false
Assert.IsTrue(result, "Task did not finish after waiting for a second.")


Expand Down Expand Up @@ -162,17 +167,118 @@ type AsyncType() =
try
let result = t.Wait(300)
Assert.IsFalse (result)
with :? AggregateException -> Assert.Fail "Task should not finish, jet"
with :? AggregateException -> Assert.Fail "Task should not finish, yet"

tcs.SetCanceled()

try
this.WaitASec t
with :? AggregateException as a ->
match a.InnerException with
| :? TaskCanceledException as t -> ()
with :? TaskCanceledException -> ()
Assert.IsTrue (t.IsCompleted, "Task is not completed")
Assert.IsTrue (t.IsCanceled, "Task is not cancelled")

[<Test>]
member this.StartAsTaskCancellationViaException () =
let cts = new CancellationTokenSource()
let tcs = TaskCompletionSource<unit>()
let a = async {
cts.CancelAfter (100)
do! tcs.Task |> Async.AwaitTask }
#if FSCORE_PORTABLE_NEW || coreclr
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Does this still exist?

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cc @KevinRansom

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I'm taking a look through these various old flags now, will send a PR to flush this

let t : Task<unit> =
#else
use t : Task<unit> =
#endif
Async.StartAsTask(a, cancellationToken = cts.Token)

// Should not finish
try
let result = t.Wait(300)
Assert.IsFalse (result)
with :? AggregateException -> Assert.Fail "Task should not finish, yet"

let msg = "Custom non-conforming 3rd-Party-Api throws"
tcs.SetException(Exception msg)

try
this.WaitASec t
with :? TaskCanceledException as t ->
match t.InnerException with
| :? AggregateException as a ->
Assert.AreEqual(1, a.InnerExceptions.Count)
Assert.AreEqual(msg, a.InnerException.Message)
| _ -> reraise()
Assert.IsTrue (t.IsCompleted, "Task is not completed")
Assert.IsTrue (t.IsCanceled, "Task is not cancelled")

[<Test>]
member this.RunSynchronouslyCancellation () =
let cts = new CancellationTokenSource()
let tcs = TaskCompletionSource<unit>()
let a = async {
cts.CancelAfter (100)
do! tcs.Task |> Async.AwaitTask }
#if FSCORE_PORTABLE_NEW || coreclr
let t : Task<unit> =
#else
use t : Task<unit> =
#endif
Task.Run(new Func<unit>(fun () -> Async.RunSynchronously(a, cancellationToken = cts.Token)))

// Should not finish
try
let result = t.Wait(300)
Assert.IsFalse (result)
with :? AggregateException -> Assert.Fail "Task should not finish, yet"

tcs.SetCanceled()

try
this.WaitASec t
with :? OperationCanceledException -> ()

Assert.IsTrue (t.IsCompleted, "Task is not completed")
// We used Task.Run for convenience, it will not notice the cancellation
// -> Cancellation is noticed by RunSynchronously throwing 'OperationCanceledException'
// which is tested above
//Assert.IsTrue (t.IsCanceled, "Task is not cancelled")

[<Test>]
member this.RunSynchronouslyCancellationViaException () =
let cts = new CancellationTokenSource()
let tcs = TaskCompletionSource<unit>()
let a = async {
cts.CancelAfter (100)
do! tcs.Task |> Async.AwaitTask }
#if FSCORE_PORTABLE_NEW || coreclr
let t : Task<unit> =
#else
use t : Task<unit> =
#endif
Task.Run(new Func<unit>(fun () -> Async.RunSynchronously(a, cancellationToken = cts.Token)))

// Should not finish
try
let result = t.Wait(300)
Assert.IsFalse (result)
with :? AggregateException -> Assert.Fail "Task should not finish, yet"

let msg = "Custom non-conforming 3rd-Party-Api throws"
tcs.SetException(Exception msg)

try
this.WaitASec t
with :? OperationCanceledException as t ->
match t.InnerException with
| :? AggregateException as a ->
Assert.AreEqual(1, a.InnerExceptions.Count)
Assert.AreEqual(msg, a.InnerException.Message)
| _ -> reraise()
Assert.IsTrue (t.IsCompleted, "Task is not completed")
// We used Task.Run for convenience, it will not notice the cancellation
// -> Cancellation is noticed by RunSynchronously throwing 'OperationCanceledException'
// which is tested above
//Assert.IsTrue (t.IsCanceled, "Task is not cancelled")

[<Test>]
member this.StartTask () =
Expand Down
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