async computation doesn't catch OperationCancelledException - f#

Question

I'm trying to make an asynchronous web request to a URL that will return if the request takes too long. I'm using the F# asynchronous workflow and the System.Net.Http library to do this.
However, I am unable to catch the Task/OperationCancelledExceptions that are raised by the System.Net.Http library in the async workflow. Instead, the exception is raised at the Async.RunSynchronously method, as you can see in this stack trace:
> System.OperationCanceledException: The operation was canceled. at
> Microsoft.FSharp.Control.AsyncBuilderImpl.commit[a](Result`1 res)
> at
> Microsoft.FSharp.Control.CancellationTokenOps.RunSynchronously[a](CancellationToken
> token, FSharpAsync`1 computation, FSharpOption`1 timeout) at
> Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T](FSharpAsync`1
> computation, FSharpOption`1 timeout, FSharpOption`1 cancellationToken)
> at <StartupCode$FSI_0004>.$FSI_0004.main#()
The code:
#r "System.Net.Http"
open System.Net.Http
open System
let readGoogle () = async {
try
let request = new HttpRequestMessage(HttpMethod.Get, "https://google.co.uk")
let client = new HttpClient()
client.Timeout <- TimeSpan.FromSeconds(0.01) //intentionally low to always fail in this example
let! response = client.SendAsync(request, HttpCompletionOption.ResponseContentRead) |> Async.AwaitTask
return Some response
with
| ex ->
//is never called
printfn "TIMED OUT"
return None
}
//exception is raised here
readGoogle ()
|> Async.RunSynchronously
|> ignore

Answer 1

Cancellation was always different from the error. In your case you can override default behavior of AwaitTask that invokes "cancel continuation" if task is cancelled and handle it differently:
let readGoogle () = async {
try
let request = new HttpRequestMessage(HttpMethod.Get, "https://google.co.uk")
let client = new HttpClient()
client.Timeout <- TimeSpan.FromSeconds(0.01) //intentionally low to always fail in this example
return! (
let t = client.SendAsync(request, HttpCompletionOption.ResponseContentRead)
Async.FromContinuations(fun (s, e, _) ->
t.ContinueWith(fun (t: Task<_>) ->
// if task is cancelled treat it as timeout and process on success path
if t.IsCanceled then s(None)
elif t.IsFaulted then e(t.Exception)
else s(Some t.Result)
)
|> ignore
)
)
with
| ex ->
//is never called
printfn "TIMED OUT"
return None
}