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Add support for async lambda in RegisterTableFunction #226
Description
(Discussed in Discord. Formalizing as an Issue so someone can work on it. Is a nice-to-have.)
The following code using an async lambda within RegisterTableFunction does not compile under v1.1.3:
void TestTableFn()
{
using var duckDBConnection = new DuckDBConnection("Data Source=:memory:");
duckDBConnection.Open();
#pragma warning disable
duckDBConnection.RegisterTableFunction<string, int>("my_people",
async (parameters) =>
{
string startingName = parameters[0].GetValue<string>();
int startingAge = parameters[1].GetValue<int>();
var result = await GetRandomPersons(startingName, startingAge).ConfigureAwait(false);
// needs to be:
//var result = GetRandomPersons(startingName, startingAge).ConfigureAwait(false).GetAwaiter().GetResult();
return new TableFunction([new ColumnInfo("PersonName", typeof(string)), new ColumnInfo("PersonAge", typeof(int))], result);
},
(item, writers, rowIndex) =>
{
var person = item as Person;
writers[0].WriteValue(person?.Name, rowIndex);
writers[1].WriteValue(person?.Age, rowIndex);
});
#pragma warning restore
using var cmd = duckDBConnection.CreateCommand();
cmd.CommandText = "SELECT * FROM my_people('Alice', 25);";
var reader = cmd.ExecuteReader();
}
public record Person(string Name, int Age);
public static async Task<List<Person>> GetRandomPersons(string startingName, int startingAge)
{
Random random = new Random();
var data = new List<Person>();
for (int i = 0; i < 10; i++)
{
data.Add(new Person($"{startingName}{i + 1:D2}", startingAge + random.Next(1, 50)));
await Task.Delay(100); // Simulate asynchronous operation
}
return data;
}The error is
CS4010 Cannot convert async lambda expression
to delegate type 'Func<IReadOnlyList<IDuckDBValueReader>, TableFunction>'.
An async lambda expression may return void, Task or Task<T>,
none of which are convertible to 'Func<IReadOnlyList<IDuckDBValueReader>, TableFunction>'.
Giorgi mentioned it is a limitation of the C api within which the callback is executed and maybe that is a hard limit. But perhaps someone can find an implementation that allows for non-blocking awaits.
Activity
Giorgi commentedon Nov 13, 2024
Fixing compilation errors is easy: https://github.com/Giorgi/DuckDB.NET/tree/Table-Function-Async-Callback
It doesn't work at runtime though. DuckDB throws this error:
The reason it's happening is that
Bindcallback is nowasync voidso it returns control back to the native code when it hits first await:DuckDB.NET/DuckDB.NET.Data/DuckDBConnection.TableFunction.cs
Line 122 in 3902db8
One workaround that I can think of (but I don't like it) is making the Bind callback void and using the same
GetAwaiter().GetResult()mechanism to invoke theresultCallback.This feels like hiding the problem under the rug and creating a false impression that theresultCallbackis truly async.mldisibio commentedon Nov 17, 2024
I haven't looked at the source code yet. I keep forgetting this is interop with
C, notC++.In the meantime I wrote a demo of how to use the legacy
Asynchronous Programming Model (APM)(preasync) to invoke an otherwiseTask Asynchronous Pattern(async/await) signature.Gist: asyncTvhDemo
So this is at the consuming side rather than your api side. I'm sure you remember how we were all told not to block on asynchronous calls when
async/awaitfirst came out. David Fowler(But if you did, they said
.GetAwaiter().GetResult()was the safest pattern.)That said, and I am no internals expert, but I think the
IAsyncResultinterface is designed so that afterBeginInvokeis called, the threads are managed in such a way that the the main thread is not blocked, whereas I think.GetAwaiter().GetResult()is pure blocking. Maybe it's no different, dunno. In any case, the demo is one possible pattern for consumers to invoke async table-value-functions until the api can be sorted out.VitaliyMF commentedon Nov 25, 2024
Adding my 2 cents, I've also trying to utilize
RegisterTableFunctionand an interface of my data sources is also async. By the nature of DuckDB library I guess true async handlers are not possible.In fact my case is even more complicated because data reader is pushed to a callback, smth like
To use this API with synchronous
RegisterTableFunction- which is divided on 2 synchronous handlers - I'll have to run an additional (concurrent) task to simulate async readHandler that communicates with these 2 TVF handlers using severalTaskCompletionSourceinstances. It seems this Frankenstein even works ^_^Giorgi commentedon Nov 25, 2024
I have come to the same conclusion.