Proposal: a way to check async frame liveness

[fengb]

There are a few instances where it can be ambiguous whether an async function has completed:

1. Error handling — the example uses errdefer if (!awaited_download_frame) { but this can be converted to be errdefer if (!@frameState(download_frame).awaited) {
2. Generators — liveness is an indicator of completion, but there's currently not a way for the execution context to check

It'd be nice if we could check the "state" of the async function. Braindump:

const FrameState = struct {
    running: bool, // not sure if we should expose "suspended" as it sounds like a concurrent problem
    awaited: bool,
};

@frameState(f: anyframe) FrameState

[andrewrk]

Here's what that "async/await typical usage" example would look like with this (I took the liberty of making it a property of a @Frame(func) instance):

fn asyncAwaitTypicalUsage(allocator: *Allocator) !void {
    var download_frame = async fetchUrl(allocator, "https://example.com/");
    errdefer if (download_frame.pending) {
        if (await download_frame) |x| allocator.free(x) else |_| {}
    };

    var file_frame = async readFile(allocator, "something.txt");
    errdefer if (file_frame.pending) {
        if (await file_frame) |x| allocator.free(x) else |_| {}
    };

    const download_text = try await download_frame;
    defer allocator.free(download_text);

    const file_text = try await file_frame;
    defer allocator.free(file_text);

    expect(std.mem.eql(u8, "expected download text", download_text));
    expect(std.mem.eql(u8, "expected file text", file_text));
}

    running: bool, // not sure if we should expose "suspended" as it sounds like a concurrent problem

Indeed, this is problematic. Even the awaited field wouldn't be exposed directly, as it would require an atomic operation. Implementing this would mean adding a caller-managed flag to the frame. Just a convenience so the caller doesn't have to introduce their own flag to manage the state.

I'll create a sister proposal for cancel. I haven't given up hope on that yet. If that panned out I think this proposal would be unneeded. The goal of the cancel proposal would be:

fn asyncAwaitTypicalUsage(allocator: *Allocator) !void {
    var download_frame = async fetchUrl(allocator, "https://example.com/");
    errdefer cancel download_frame;

    var file_frame = async readFile(allocator, "something.txt");
    errdefer cancel file_frame;

    const download_text = try await download_frame;
    defer allocator.free(download_text);

    const file_text = try await file_frame;
    defer allocator.free(file_text);

    expect(std.mem.eql(u8, "expected download text", download_text));
    expect(std.mem.eql(u8, "expected file text", file_text));
}

The main problem to solve with that proposal is making it work with non-async functions. (related: #782).

[tecanec]

An important scenario that doesn't get enough attention from Zig's async, I think, is when time management is important, such as when making a game engine.
As a use case for this proposal, let's say that your game engine is loading a file. You want to do something to that file if it's already loaded, but if it isn't, you'd rather try again later than risk wasting valuable milliseconds by waiting like you would by using await directly. There are ways to do this currently, but none of them are elegant enough that I wouldn't consider a better solution worthwhile.

[lithdew]

After playing around with async/await in other languages and looking into how they're implemented, I definitely think allowing for a way to check an async frames liveness will help simplify a lot of async-related code.

By enabling the ability to check if an async frame is completed, it's trivial to implement operations such as 'await until either frame A or B is completed', or 'await until all frames {A, B, C, ...} are completed'.

Here's an example of how they would be implemented:

const std = @import("std");

// A hack to check if a frame is completed. I don't know if
// this actually works for all kinds of async frames.

pub fn completed(frame: anytype) bool {
    const bytes = std.mem.asBytes(frame);
    return std.mem.allEqual(u8, bytes[8..][0..16], std.math.maxInt(u8));
}

fn ReturnTypeOf(comptime function: anytype) type {
    return @typeInfo(@TypeOf(function)).Fn.return_type.?;
}

fn suspended() void {
    suspend;
}

const All = struct {
    state: @Frame(suspended),
    counter: usize,

    pub fn init() All {
        var self: All = undefined;
        self.state = async suspended();
        self.counter = 0;
        return self;
    }

    pub fn run(self: *All, comptime function: anytype, args: anytype) ReturnTypeOf(function) {
        // 'self.counter' could be made thread-safe.

        self.counter += 1;

        const result = @call(.{}, function, args);

        self.counter -= 1;
        if (self.counter == 0 and !completed(&self.state)) {
            resume self.state;
        }

        return result;
    }

    pub fn wait(self: *All) void {
        await self.state;
    }
};

const Either = struct {
    state: @Frame(suspended),

    pub fn init() Either {
        var self: Either = undefined;
        self.state = async suspended();
        return self;
    }

    pub fn run(self: *Either, comptime function: anytype, args: anytype) ReturnTypeOf(function) {
        const result = @call(.{}, function, args);
        if (!completed(&self.state)) {
            resume self.state;
        }
        return result;
    }

    pub fn wait(self: *Either) void {
        await self.state;
    }
};

test "Either" {
    var either = Either.init();
    var F1 = async either.run(fn_1, .{});
    var F2 = async either.run(fn_2, .{});
    either.wait();

    if (completed(&F1)) {
        // ... handle case of F1
        const result = nosuspend await F1;
    }
}

test "All" {
    var all = All.init();
    var F1 = async all.run(fn_1, .{});
    var F2 = async all.run(fn_2, .{});
    var F3 = async all.run(fn_3, .{});
    all.wait();

    // use results from F1, F2, or F3 here
}

Allowing for checking if an async frame is completed also allows e.g. polling an I/O notifier like epoll/kqueue/IOCP until an async frame is fully completed (where the I/O notifier itself drives the completion of the async frame).

pub fn main() !void {
    var frame: @Frame(httpRequest) = async httpRequest(....);
    while (!completed(&frame)) {
        io_notifier.poll();
    }
    const response = try nosuspend await frame;
}

While these operations can be done right now with Zig as is, it requires unnecessary state to be stored which is already maintained in the header of an async frame.

[daurnimator]

By enabling the ability to check if an async frame is completed, it's trivial to implement operations such as 'await until either frame A or B is completed', or 'await until all frames {A, B, C, ...} are completed'.

async/await are "low level" syntax; I think you're looking for std.event.Batch