C#•3y ago

JsonSerializer.DeserializeAsync() randomly hangs indefinitely [Answered]

private static async Task<T> TryReadDataFromFile<T>(string basePath) where T : class
{
    var filepath = GetFilePath<T>(basePath);
    if (!File.Exists(filepath)) return null;
    await using var filestream = File.Open(filepath, FileMode.Open);
    var res = await JsonSerializer.DeserializeAsync<T>(filestream); // This line
    await filestream.DisposeAsync();
    return res;
}

private static async Task<T> TryReadDataFromFile<T>(string basePath) where T : class
{
    var filepath = GetFilePath<T>(basePath);
    if (!File.Exists(filepath)) return null;
    await using var filestream = File.Open(filepath, FileMode.Open);
    var res = await JsonSerializer.DeserializeAsync<T>(filestream); // This line
    await filestream.DisposeAsync();
    return res;
}

System.Text.Json's DeserializeAsync seems to (entirely at random) decide to hang indefinitely, throws no exceptions or anything in the process which is odd, it's always the same file in AppData that causes it to hang.

45 Replies

DoomboxOP•3y ago

I know that it's not necessary to dispose of the filestream explicitly but I was trying anything to make sure it wasn't locking the file for whatever reason

Up•3y ago

unrelated, but you do not need to manually call filestream.DisposeAsync(). that is what that await using automatically does for you

DoomboxOP•3y ago

^^ 😉

Up•3y ago

yeah I had already typed it out ;p

Trinitek•3y ago

what happens when you read the file in, and then deserialize it

Up•3y ago

also what explicit type are you passing it

DoomboxOP•3y ago

would have to move to synchronous, if that's the move then I'll just make the whole thing synchronous and wrap it in a Task.Run I guess, as for the object it's just a POCO

[Filename("Global.cfg")]
public class GlobalConfig : ConfigBase
{
    public double Width { get; set; }
    public double Height { get; set; }
    public double Left { get; set; }
    public double Top { get; set; }
    public WindowState State { get; set; }
    public bool LightTheme { get; set; }
    public bool Use24HourClock { get; set; }

    public GlobalConfig()
    {
        Width = 1100;
        Height = 800;
        Left = SystemParameters.PrimaryScreenWidth / 2 - Width / 2;
        Top = SystemParameters.PrimaryScreenHeight / 2 - Height / 2;
        State = WindowState.Normal;
        LightTheme = false;
        Use24HourClock = true;
    }
}

[Filename("Global.cfg")]
public class GlobalConfig : ConfigBase
{
    public double Width { get; set; }
    public double Height { get; set; }
    public double Left { get; set; }
    public double Top { get; set; }
    public WindowState State { get; set; }
    public bool LightTheme { get; set; }
    public bool Use24HourClock { get; set; }

    public GlobalConfig()
    {
        Width = 1100;
        Height = 800;
        Left = SystemParameters.PrimaryScreenWidth / 2 - Width / 2;
        Top = SystemParameters.PrimaryScreenHeight / 2 - Height / 2;
        State = WindowState.Normal;
        LightTheme = false;
        Use24HourClock = true;
    }
}

nothing too wild

Up•3y ago

would have to move to synchronous

no, what for

DoomboxOP•3y ago

the async method doesn't have an overload for strings, only for streams

Trinitek•3y ago

MemoryStream

DoomboxOP•3y ago

that's true, didn't think of that

Trinitek•3y ago

if it locks up when you read it into a MemoryStream, it might be a file problem

Up•3y ago

~~also please replace that enum with a string. makes it a lot more user-friendly than magic numbers in the config~~

DoomboxOP•3y ago

await using var fs = File.Open(filepath, FileMode.Open, FileAccess.Read);
using var ms = new MemoryStream();
await fs.CopyToAsync(ms);

await using var fs = File.Open(filepath, FileMode.Open, FileAccess.Read);
using var ms = new MemoryStream();
await fs.CopyToAsync(ms);

yeah CopyToAsync into a memorystream also causes a lockup I switched file access just to be sure it wasn't trying to write for whatever reason as well, also a meaningless change but yeah for some reason I'm having (seemingly entirely at random) a file issue

Up•3y ago

what if you use File.OpenRead() instead of Open()? as in await using var stream = File.OpenRead(path);

DoomboxOP•3y ago

private static async Task WriteDataToFile<T>(string basePath, T data, bool writeIndented = false) where T : class
{
    var filepath = GetFilePath<T>(basePath);
    await using var filestream = File.Create(filepath);
    var res = JsonSerializer.SerializeAsync(filestream, data, writeIndented ? IndentedJsonOptions : DefaultJsonOptions);
    await filestream.DisposeAsync();
    await res;
}

private static async Task WriteDataToFile<T>(string basePath, T data, bool writeIndented = false) where T : class
{
    var filepath = GetFilePath<T>(basePath);
    await using var filestream = File.Create(filepath);
    var res = JsonSerializer.SerializeAsync(filestream, data, writeIndented ? IndentedJsonOptions : DefaultJsonOptions);
    await filestream.DisposeAsync();
    await res;
}

interesting, I'm also experiencing a lockup on filestream.DisposeAsync() here

Trinitek•3y ago

async deadlock? what kind of app is this

DoomboxOP•3y ago

WPF

Trinitek•3y ago

add ConfigureAwait(false) after all your calls

DoomboxOP•3y ago

uh this seems to have moved the deadlock somewhere else entirely, interesting do I have to ConfigureAwait(false) everything?

Trinitek•3y ago

pretty much, yes

Unknown User•3y ago

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Trinitek•3y ago

ConfigureAwait(TRUE) is the default, which will try to run the continuation on the original thread

DoomboxOP•3y ago

I'm only using a single GetAwaiter().GetResult() call so I just need to make sure that the entire chain that comes from that has ConfigureAwait(false) then?

D.Mentia•3y ago

I have never had to use ConfigureAwait, I find it a bit sus. It'd just be moving the deadlock to another thread

DoomboxOP•3y ago

everything else is awaited it's really annoying that there's no way to really debug deadlocks, finding them when large parts of my codebase is async is a nightmare

D.Mentia•3y ago

oh, well, I've never used GetAwaiter .GetResult either... why do you need that? And also, not an answer to your question at all but... what's wrong with just File.ReadAllTextAsync 😛

Unknown User•3y ago

Message Not Public

DoomboxOP•3y ago

registering the factory in the container for my global config which everything relies on has to be synchronous so I'm stuck with it unfortunately, I could maybe rethink my architecture and make that async but it'd be a big headache

Trinitek•3y ago

okay, so the real reason why you're deadlocking is because the async library functions you're calling into are also using ConfigureAwait(false) which means the continuations can run on different threads. When you do GetResult() directly, you could potentially be doing that on the non-UI thread, which results in your deadlock the solution is... (drum roll please)

DoomboxOP•3y ago

as for this, System.Text.Json's async methods are for streams only, no strings

Up•3y ago

also the fact that you now have to read the entire file at once, rather than letting the serializer read it as-needed

Unknown User•3y ago

Message Not Public

Trinitek•3y ago

https://github.com/davidfowl/AspNetCoreDiagnosticScenarios/blob/master/AsyncGuidance.md#avoid-using-taskresult-and-taskwait

GitHub

AspNetCoreDiagnosticScenarios/AsyncGuidance.md at master · davidfow...

This repository has examples of broken patterns in ASP.NET Core applications - AspNetCoreDiagnosticScenarios/AsyncGuidance.md at master · davidfowl/AspNetCoreDiagnosticScenarios

Trinitek•3y ago

read that part about deadlocks

D.Mentia•3y ago

lol, that's always the answer. Can we just remove Task.Result in the next version 😉

DoomboxOP•3y ago

yeah I think I'll rethink my architecture a bit and allow for the global config to be loaded later on, that way I can use async properly and avoid constructors

Trinitek•3y ago

either that^, or cheat and throw it into a Task.Run()

D.Mentia•3y ago

It's ugly but I would sort of recommend just, an async Initialize function where you need it

DoomboxOP•3y ago

yeah that's (more or less) what I'm doing for a lot of other stuff already but the global config is a bit sketchier, a fair bit of code is reliant on it being initialised when it's constructed, but I can just rework everything so it's more tolerant handy to know that GetAwaiter().GetResult() can cause deadlocks though, was never really that aware of the inner workings, cheers all

Trinitek•3y ago

async is nasty business

Unknown User•3y ago

Message Not Public

D.Mentia•3y ago

https://stackoverflow.com/questions/8145479/can-constructors-be-async the top answer here looks like a pretty neat solution, though IDK if it can be registered with DI like that

DoomboxOP•3y ago

yeah the IoC container isn't hugely fond of basically any of the TPL, I'll just have to write it so the ViewModels can live without the config until I'm in an async context

Accord•3y ago

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JsonSerializer.DeserializeAsync() randomly hangs indefinitely [Answered]