ConcurrencyUnavailable on single-threaded

Description

ConcurrencyUnavailable on single-threaded.

• On a single-threaded Io backend, mailbox.receive_future returns error.ConcurrencyUnavailable.
• No concurrent task can be spawned to service the future.
• Synchronous mailbox.receive still works — it needs no concurrency.

Diagram

 mailbox (single-threaded io)
 │
 receive_future ──► error.ConcurrencyUnavailable
 (no concurrent task can be spawned on single-threaded backend)
 │
 mailbox.receive (synchronous) still works

Source

pub fn concurrencyunavailable_on_single_threaded(allocator: std.mem.Allocator, io: std.Io) !void {
    const mbh: MailboxHandle = try mailbox.new(io, allocator);
    defer {
        var rem: std.DoublyLinkedList = mailbox.close(mbh);
        items.freeList(&rem, allocator);
        mailbox.destroy(mbh, allocator);
    }

    try testFutureUnavailable(mbh);
    try testSynchronousReceive(mbh, allocator);
}

fn testFutureUnavailable(mbh: MailboxHandle) !void {
    if (mailbox.receive_future(mbh, null)) |_| {
        return error.FutureSingleThreadedFailed;
    } else |_| {}
    std.log.info("receive_future: ConcurrencyUnavailable on single-threaded backend as expected", .{});
}

fn testSynchronousReceive(mbh: MailboxHandle, alloc: std.mem.Allocator) !void {
    var slot: Slot = null;
    defer items.Event.EventPolyHelper.destroy(alloc, &slot);
    try items.Event.EventPolyHelper.create(alloc, &slot);
    items.Event.EventPolyHelper.mustIdentifySlotAs(&slot).code = 1;
    try mailbox.send(mbh, &slot);

    var received: Slot = null;
    defer items.freeSlot(&received, alloc);
    try mailbox.receive(mbh, &received, null);
    std.log.info("synchronous receive still works: code={d}", .{items.Event.EventPolyHelper.mustIdentifySlotAs(&received).code});
}

const items = @import("../items/items.zig");
const matryoshka = @import("matryoshka");
const std = @import("std");
const mailbox = matryoshka.mailbox;
const polynode = matryoshka.polynode;
const Slot = polynode.Slot;
const MailboxHandle = mailbox.MailboxHandle;