bun.sh/src/bun.js/event_loop/DeferredTaskQueue.zig

//! Sometimes, you have work that will be scheduled, cancelled, and rescheduled multiple times
//! The order of that work may not particularly matter.
//!
//! An example of this is when writing to a file or network socket.
//!
//! You want to balance:
//!     1) Writing as much as possible to the file/socket in as few system calls as possible
//!     2) Writing to the file/socket as soon as possible
//!
//! That is a scheduling problem. How do you decide when to write to the file/socket? Developers
//! don't want to remember to call `flush` every time they write to a file/socket, but we don't
//! want them to have to think about buffering or not buffering either.
//!
//! Our answer to this is the DeferredTaskQueue.
//!
//! When you call write() when sending a streaming HTTP response, we don't actually write it immediately
//! by default. Instead, we wait until the end of the microtask queue to write it, unless either:
//!
//! - The buffer is full
//! - The developer calls `flush` manually
//!
//! But that means every time you call .write(), we have to check not only if the buffer is full, but also if
//! it previously had scheduled a write to the file/socket. So we use an ArrayHashMap to keep track of the
//! list of pointers which have a deferred task scheduled.
//!
//! The DeferredTaskQueue is drained after the microtask queue, but before other tasks are executed. This avoids re-entrancy
//! issues with the event loop.

const DeferredTaskQueue = @This();

pub const DeferredRepeatingTask = *const (fn (*anyopaque) bool);

map: std.AutoArrayHashMapUnmanaged(?*anyopaque, DeferredRepeatingTask) = .{},

pub fn postTask(this: *DeferredTaskQueue, ctx: ?*anyopaque, task: DeferredRepeatingTask) bool {
    const existing = this.map.getOrPutValue(bun.default_allocator, ctx, task) catch bun.outOfMemory();
    return existing.found_existing;
}

pub fn unregisterTask(this: *DeferredTaskQueue, ctx: ?*anyopaque) bool {
    return this.map.swapRemove(ctx);
}

pub fn run(this: *DeferredTaskQueue) void {
    var i: usize = 0;
    var last = this.map.count();
    while (i < last) {
        const key = this.map.keys()[i] orelse {
            this.map.swapRemoveAt(i);
            last = this.map.count();
            continue;
        };

        if (!this.map.values()[i](key)) {
            this.map.swapRemoveAt(i);
            last = this.map.count();
        } else {
            i += 1;
        }
    }
}

pub fn deinit(this: *DeferredTaskQueue) void {
    this.map.deinit(bun.default_allocator);
}

const bun = @import("bun");
const std = @import("std");