AsyncExceptionBadness.fs

open System
open System.Diagnostics
open System.IO
open System.Runtime.ExceptionServices

module ThirdPartyLibrary =
    // Represents some low-level async "primitive" which is used in many many places and which may fail. This type of
    // method/function is not typically what you're interested in when examining stacktraces.
    // Example: a database query function from a thid-party library
    let doWorkAsync crash = ExtraTopLevelOperators.async {
        do! Async.Sleep 100
        if crash then
            raise (IOException("Oh no!"))
    }

// Helper function to improve stacktraces. This is the recommended way to augment (not replace) exception stacktraces
// with the caller's stack frame information
// See: https://stackoverflow.com/a/17091351/1231925
let inline rethrowAsync (computation: Async<'a>) = async {
    try
        return! computation
    with e ->
        let s = StackTrace()
        Debug.WriteLine $"StackTrace: %O{s}"
        Debug.WriteLine $"Rethrowing: %s{e.StackTrace}"
        Debug.WriteLine ""
        ExceptionDispatchInfo.Throw e
        return Unchecked.defaultof<_> // unreachable, but the compiler doesn't realize that
}

// Application function. We need this to show up in stack traces
let doLoopAsync (n: int) = async {
    for n in n .. -1 .. 0 do
        printfn "n = %d" n
        let crash = n = 6
        do! rethrowAsync (ThirdPartyLibrary.doWorkAsync crash)
}

// Higher level application function. We also need this to show up in stacktraces
let mainAsync () = async {
    let n = 10
    printfn "Starting with iterations: %d" n
    do! rethrowAsync (doLoopAsync n)
    printfn "Done."
}

[<EntryPoint>]
let main _ =
    Async.RunSynchronously (mainAsync ())
    
    // Even when using the suggested workarounds to recapture stack trace information by rethrowing, the stacktrace we
    // get is completely useless:
    //
    // Unhandled exception. System.IO.IOException: Oh no!                                                                                                                                                                        
    //    at Program.ThirdPartyLibrary.doWorkAsync@12-1.Invoke(Unit _arg1) in C:\Users\jwostenberg\Documents\Code\FSharpSandbox\ConsoleApp2\Program.fs:line 13                                                                   
    //    at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvokeNoHijackCheck[a,b](AsyncActivation`1 ctxt, b result1, FSharpFunc`2 userCode) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 525                             
    //    at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 112                                                                                           
    // --- End of stack trace from previous location ---                                                                                                                                                                         
    //    at Microsoft.FSharp.Control.AsyncResult`1.Commit() in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 453                                                                                                                 
    //    at Microsoft.FSharp.Control.AsyncPrimitives.QueueAsyncAndWaitForResultSynchronously[a](CancellationToken token, FSharpAsync`1 computation, FSharpOption`1 timeout) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 1133
    //    at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronously[T](CancellationToken cancellationToken, FSharpAsync`1 computation, FSharpOption`1 timeout) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 1160           
    //    at Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T](FSharpAsync`1 computation, FSharpOption`1 timeout, FSharpOption`1 cancellationToken) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 1504                  
    //    at Program.main(String[] _arg1) in C:\Users\jwostenberg\Documents\Code\FSharpSandbox\ConsoleApp2\Program.fs:line 49   
    //
    // Question of the day: what blew up? ¯\_(ツ)_/¯ Everything useful (doLoopAsync and mainAsync) still got lost, so
    // there's no way to know. It's especially confounding since the rethrow helper is *supposed to work*, and indeed
    // the debug print line shows that it appears to! I'm even fairly suspicious that this used to work at some point.
    //
    //
    // Bonus points -- take this snippet:
    
    // Async.RunSynchronously (async {
    //     try return! mainAsync ()
    //     with e -> eprintfn "%O" e
    // })
    
    // Somehow, this actually does show the complete stacktrace:
    // 
    // System.IO.IOException: Oh no!                                                                                                                          
    //    at Program.ThirdPartyLibrary.doWorkAsync@12-1.Invoke(Unit _arg1) in C:\Users\jwostenberg\Documents\Code\FSharpSandbox\ConsoleApp2\Program.fs:line 13
    //    at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvokeNoHijackCheck[a,b](AsyncActivation`1 ctxt, b result1, FSharpFunc`2 userCode) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 525             
    //    at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 112                                                                           
    // --- End of stack trace from previous location ---                                                                                                                                                         
    //    at Program.doLoopAsync@36-4.Invoke(Exception _arg1)                                                                                                                                                    
    //    at Program.doLoopAsync@36-7.Invoke(Exception exn)                                                                                                                                                      
    //    at Microsoft.FSharp.Control.AsyncPrimitives.CallFilterThenInvoke[T](AsyncActivation`1 ctxt, FSharpFunc`2 filterFunction, ExceptionDispatchInfo edi) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 543
    //    at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 112                                                                           
    // --- End of stack trace from previous location ---                                                                                                                                                         
    //    at Program.mainAsync@43-3.Invoke(Exception _arg1)                                                                                                                                                      
    //    at Program.mainAsync@43-6.Invoke(Exception exn)                                                                                                                                                        
    //    at Microsoft.FSharp.Control.AsyncPrimitives.CallFilterThenInvoke[T](AsyncActivation`1 ctxt, FSharpFunc`2 filterFunction, ExceptionDispatchInfo edi) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 543
    //    at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in D:\a\_work\1\s\src\FSharp.Core\async.fs:line 112

    
    0