This code is running on Mono (5.4.1.7).
I'm using F#'s Agents to handle a lot of data processing in my web application, and one of these messages is Shutdown. When a posted Shutdown message is processed, the agent cleans up some things and stops its message loop. This works fine and dandy, but blows up in my face if I try to perform a Shutdown from Finalize(). I've managed to reproduce this:
open System
open System.Threading
type ConsoleMessage =
| Clear
| Println of string
// Reply back (with unit) so that calling code is able to wait for the agent to clean up (for code dependent on the
// agent's resources definitely being released and such)
| Shutdown of AsyncReplyChannel<unit>
type ConsoleAgent() =
let mutable disposed = false
let mutable stopped = false
let agent = MailboxProcessor.Start(fun agent ->
let rec messageLoop () = async {
let! message = agent.Receive ()
match message with
| Clear -> System.Console.Clear ()
| Println str -> printfn "%s" str
| Shutdown rc ->
// Cleanup goes here
printfn "Shutting Down"
stopped <- true
rc.Reply ()
System.Threading.Thread.Sleep 100
if not stopped then
return! messageLoop () }
messageLoop ())
member this.Post msg = agent.Post msg
member this.PostAndAsyncReply msg = agent.PostAndAsyncReply msg
member this.Dispose disposing =
printfn "Disposing (disposing = %b)" disposing
if not disposed then
Async.RunSynchronously (agent.PostAndAsyncReply Shutdown)
disposed <- true
override this.Finalize () =
this.Dispose false
interface IDisposable with
member this.Dispose () =
this.Dispose true
module Main =
[<EntryPoint>]
let main args =
let console = new ConsoleAgent()
console.Post (Println "Print 1")
console.Post (Println "Print 2")
Thread.Sleep 1000
0
Of course in the real application they have nothing to do with console printing.
Here's the stacktrace I get:
Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
at System.Runtime.Remoting.Contexts.SynchronizationAttribute.EnterContext () [0x00000] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.Runtime.Remoting.Contexts/SynchronizationAttribute.cs:184
at System.Threading.WaitHandle.WaitOneNative (System.Runtime.InteropServices.SafeHandle waitableSafeHandle, System.UInt32 millisecondsTimeout, System.Boolean hasThreadAffinity, System.Boolean exitContext) [0x0002d] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.Threading/WaitHandle.cs:111
at System.Threading.WaitHandle.InternalWaitOne (System.Runtime.InteropServices.SafeHandle waitableSafeHandle, System.Int64 millisecondsTimeout, System.Boolean hasThreadAffinity, System.Boolean exitContext) [0x00014] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/threading/waithandle.cs:250
at System.Threading.WaitHandle.WaitOne (System.Int64 timeout, System.Boolean exitContext) [0x00000] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/threading/waithandle.cs:239
at System.Threading.WaitHandle.WaitOne (System.Int32 millisecondsTimeout, System.Boolean exitContext) [0x00019] in /Users/builder/data/lanes/4992/mono-mac-sdk/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/threading/waithandle.cs:206
at Microsoft.FSharp.Control.AsyncImpl+ResultCell`1[T].TryWaitForResultSynchronously (Microsoft.FSharp.Core.FSharpOption`1[T] timeout) [0x0002a] in <5a7d678a904cf4daa74503838a677d5a>:0
at Microsoft.FSharp.Control.CancellationTokenOps.RunSynchronouslyInCurrentThread[a] (System.Threading.CancellationToken token, Microsoft.FSharp.Control.FSharpAsync`1[T] computation) [0x0001c] in <5a7d678a904cf4daa74503838a677d5a>:0
at Microsoft.FSharp.Control.CancellationTokenOps.RunSynchronously[a] (System.Threading.CancellationToken token, Microsoft.FSharp.Control.FSharpAsync`1[T] computation, Microsoft.FSharp.Core.FSharpOption`1[T] timeout) [0x00013] in <5a7d678a904cf4daa74503838a677d5a>:0
at Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T] (Microsoft.FSharp.Control.FSharpAsync`1[T] computation, Microsoft.FSharp.Core.FSharpOption`1[T] timeout, Microsoft.FSharp.Core.FSharpOption`1[T] cancellationToken) [0x00070] in <5a7d678a904cf4daa74503838a677d5a>:0
at Program+ConsoleAgent.Dispose (System.Boolean disposing) [0x00027] in /Users/jwostenberg/Code/FSharp/Sandbox/Sandbox/Program.fs:38
at Program+ConsoleAgent.Finalize () [0x00000] in /Users/jwostenberg/Code/FSharp/Sandbox/Sandbox/Program.fs:42
What's more, this doesn't happen if the object is disposed properly via the dispose pattern (e.g. change let console = new ConsoleAgent() to use console = new ConsoleAgent()). I can't really do this in my own code without bending over backwards because I don't have direct references to these agents (there are many of them running at a time), but shouldn't I be able to let them dispose via the garbage collector anyways?
Is this my fault, F#'s fault, or Mono's fault? For now, I've wrapped the relevant part of the Dispose() method in a try/catch that just logs the exception, but this feels really dirty.
The argument "disposing" of the method Dispose is here for reason. It differentiates between managed and unmanaged applications of Dispose. In brief, Dispose(true) means that this call is explicit (using statement or F#'s use). It is basically continuation of "normal" .NET programming.
Dispose(false) means that finalisation is happening. This means that any referenced .NET objects can be either alive, disposed, or finalised. So your code is need to care about unmanaged resources only and do not try to call or use in any other way managed objects.
It is important, that Dispose() is not called automatically, while finaliser is. Making the example correct requires two changes:
Code:
member this.Dispose disposing =
if disposing && not disposed then
Async.RunSynchronously (agent.PostAndAsyncReply Shutdown)
disposed <- true
module Main =
[<EntryPoint>]
let main args =
use console = new ConsoleAgent()
Thread.Sleep 1000
0