apollo / src / queue.zig

const std = @import("std");
const assert = std.debug.assert;
const atomic = std.atomic;
const Condition = std.Thread.Condition;

/// Thread safe. Fixed size. Blocking push and pop.
pub fn Queue(
    comptime T: type,
    comptime size: usize,
) type {
    return struct {
        buf: [size]T = undefined,

        read_index: usize = 0,
        write_index: usize = 0,

        mutex: std.Thread.Mutex = .{},
        // blocks when the buffer is full
        not_full: Condition = .{},
        // ...or empty
        not_empty: Condition = .{},

        /// Optionally supply an eventfd. Queue will write to this when receiving an event if it was
        /// empty. Can be helpful for applications using poll/epoll and tryPop. Note that any
        /// failure to write to the eventfd will be logged and a panic will occur.
        eventfd: ?std.posix.fd_t = null,

        const Self = @This();

        /// Pop an item from the queue. Blocks until an item is available.
        pub fn pop(self: *Self) T {
            self.mutex.lock();
            defer self.mutex.unlock();
            while (self.isEmptyLH()) {
                self.not_empty.wait(&self.mutex);
            }
            std.debug.assert(!self.isEmptyLH());
            if (self.isFullLH()) {
                // If we are full, wake up a push that might be
                // waiting here.
                self.not_full.signal();
            }

            return self.popLH();
        }

        /// Push an item into the queue. Blocks until an item has been
        /// put in the queue.
        pub fn push(self: *Self, item: T) void {
            self.mutex.lock();
            defer self.mutex.unlock();
            while (self.isFullLH()) {
                self.not_full.wait(&self.mutex);
            }
            std.debug.assert(!self.isFullLH());
            const was_empty = self.isEmptyLH();

            self.buf[self.mask(self.write_index)] = item;
            self.write_index = self.mask2(self.write_index + 1);

            // If we were empty, wake up a pop if it was waiting.
            if (was_empty) {
                self.not_empty.signal();
                if (self.eventfd) |fd| {
                    const val = @as([8]u8, @bitCast(@as(u64, 1)));
                    _ = std.posix.write(fd, &val) catch |err| {
                        std.log.err("writing to eventfd failed: {}", .{err});
                        @panic("eventfd write failure");
                    };
                }
            }
        }

        /// Push an item into the queue. Returns true when the item
        /// was successfully placed in the queue, false if the queue
        /// was full.
        pub fn tryPush(self: *Self, item: T) bool {
            self.mutex.lock();
            if (self.isFullLH()) {
                self.mutex.unlock();
                return false;
            }
            self.mutex.unlock();
            self.push(item);
            return true;
        }

        /// Pop an item from the queue. Returns null when no item is
        /// available.
        pub fn tryPop(self: *Self) ?T {
            self.mutex.lock();
            defer self.mutex.unlock();
            if (self.isEmptyLH()) return null;
            return self.popLH();
        }

        /// Poll the queue. This call blocks until events are in the queue
        pub fn poll(self: *Self) void {
            self.mutex.lock();
            defer self.mutex.unlock();
            while (self.isEmptyLH()) {
                self.not_empty.wait(&self.mutex);
            }
            std.debug.assert(!self.isEmptyLH());
        }

        pub fn lock(self: *Self) void {
            self.mutex.lock();
        }

        pub fn unlock(self: *Self) void {
            self.mutex.unlock();
        }

        /// Used to efficiently drain the queue while the lock is externally held
        pub fn drain(self: *Self) ?T {
            if (self.isEmptyLH()) return null;
            return self.popLH();
        }

        fn isEmptyLH(self: Self) bool {
            return self.write_index == self.read_index;
        }

        fn isFullLH(self: Self) bool {
            return self.mask2(self.write_index + self.buf.len) ==
                self.read_index;
        }

        /// Returns `true` if the queue is empty and `false` otherwise.
        pub fn isEmpty(self: *Self) bool {
            self.mutex.lock();
            defer self.mutex.unlock();
            return self.isEmptyLH();
        }

        /// Returns `true` if the queue is full and `false` otherwise.
        pub fn isFull(self: *Self) bool {
            self.mutex.lock();
            defer self.mutex.unlock();
            return self.isFullLH();
        }

        /// Returns the length
        fn len(self: Self) usize {
            const wrap_offset = 2 * self.buf.len *
                @intFromBool(self.write_index < self.read_index);
            const adjusted_write_index = self.write_index + wrap_offset;
            return adjusted_write_index - self.read_index;
        }

        /// Returns `index` modulo the length of the backing slice.
        fn mask(self: Self, index: usize) usize {
            return index % self.buf.len;
        }

        /// Returns `index` modulo twice the length of the backing slice.
        fn mask2(self: Self, index: usize) usize {
            return index % (2 * self.buf.len);
        }

        fn popLH(self: *Self) T {
            const result = self.buf[self.mask(self.read_index)];
            self.read_index = self.mask2(self.read_index + 1);
            return result;
        }
    };
}

const testing = std.testing;
const cfg = Thread.SpawnConfig{ .allocator = testing.allocator };
test "Queue: simple push / pop" {
    var queue: Queue(u8, 16) = .{};
    queue.push(1);
    queue.push(2);
    const pop = queue.pop();
    try testing.expectEqual(1, pop);
    try testing.expectEqual(2, queue.pop());
}

const Thread = std.Thread;
fn testPushPop(q: *Queue(u8, 2)) !void {
    q.push(3);
    try testing.expectEqual(2, q.pop());
}

test "Fill, wait to push, pop once in another thread" {
    var queue: Queue(u8, 2) = .{};
    queue.push(1);
    queue.push(2);
    const t = try Thread.spawn(cfg, testPushPop, .{&queue});
    try testing.expectEqual(false, queue.tryPush(3));
    try testing.expectEqual(1, queue.pop());
    t.join();
    try testing.expectEqual(3, queue.pop());
    try testing.expectEqual(null, queue.tryPop());
}

fn testPush(q: *Queue(u8, 2)) void {
    q.push(0);
    q.push(1);
    q.push(2);
    q.push(3);
    q.push(4);
}

test "Try to pop, fill from another thread" {
    var queue: Queue(u8, 2) = .{};
    const thread = try Thread.spawn(cfg, testPush, .{&queue});
    for (0..5) |idx| {
        try testing.expectEqual(@as(u8, @intCast(idx)), queue.pop());
    }
    thread.join();
}

fn sleepyPop(q: *Queue(u8, 2)) !void {
    // First we wait for the queue to be full.
    while (!q.isFull())
        try Thread.yield();

    // Then we spuriously wake it up, because that's a thing that can
    // happen.
    q.not_full.signal();
    q.not_empty.signal();

    // Then give the other thread a good chance of waking up. It's not
    // clear that yield guarantees the other thread will be scheduled,
    // so we'll throw a sleep in here just to be sure. The queue is
    // still full and the push in the other thread is still blocked
    // waiting for space.
    try Thread.yield();
    std.time.sleep(std.time.ns_per_s);
    // Finally, let that other thread go.
    try std.testing.expectEqual(1, q.pop());

    // This won't continue until the other thread has had a chance to
    // put at least one item in the queue.
    while (!q.isFull())
        try Thread.yield();
    // But we want to ensure that there's a second push waiting, so
    // here's another sleep.
    std.time.sleep(std.time.ns_per_s / 2);

    // Another spurious wake...
    q.not_full.signal();
    q.not_empty.signal();
    // And another chance for the other thread to see that it's
    // spurious and go back to sleep.
    try Thread.yield();
    std.time.sleep(std.time.ns_per_s / 2);

    // Pop that thing and we're done.
    try std.testing.expectEqual(2, q.pop());
}

test "Fill, block, fill, block" {
    // Fill the queue, block while trying to write another item, have
    // a background thread unblock us, then block while trying to
    // write yet another thing. Have the background thread unblock
    // that too (after some time) then drain the queue. This test
    // fails if the while loop in `push` is turned into an `if`.

    var queue: Queue(u8, 2) = .{};
    const thread = try Thread.spawn(cfg, sleepyPop, .{&queue});
    queue.push(1);
    queue.push(2);
    const now = std.time.milliTimestamp();
    queue.push(3); // This one should block.
    const then = std.time.milliTimestamp();

    // Just to make sure the sleeps are yielding to this thread, make
    // sure it took at least 900ms to do the push.
    try std.testing.expect(then - now > 900);

    // This should block again, waiting for the other thread.
    queue.push(4);

    // And once that push has gone through, the other thread's done.
    thread.join();
    try std.testing.expectEqual(3, queue.pop());
    try std.testing.expectEqual(4, queue.pop());
}

fn sleepyPush(q: *Queue(u8, 1)) !void {
    // Try to ensure the other thread has already started trying to pop.
    try Thread.yield();
    std.time.sleep(std.time.ns_per_s / 2);

    // Spurious wake
    q.not_full.signal();
    q.not_empty.signal();

    try Thread.yield();
    std.time.sleep(std.time.ns_per_s / 2);

    // Stick something in the queue so it can be popped.
    q.push(1);
    // Ensure it's been popped.
    while (!q.isEmpty())
        try Thread.yield();
    // Give the other thread time to block again.
    try Thread.yield();
    std.time.sleep(std.time.ns_per_s / 2);

    // Spurious wake
    q.not_full.signal();
    q.not_empty.signal();

    q.push(2);
}

test "Drain, block, drain, block" {
    // This is like fill/block/fill/block, but on the pop end. This
    // test should fail if the `while` loop in `pop` is turned into an
    // `if`.

    var queue: Queue(u8, 1) = .{};
    const thread = try Thread.spawn(cfg, sleepyPush, .{&queue});
    try std.testing.expectEqual(1, queue.pop());
    try std.testing.expectEqual(2, queue.pop());
    thread.join();
}

fn readerThread(q: *Queue(u8, 1)) !void {
    try testing.expectEqual(1, q.pop());
}

test "2 readers" {
    // 2 threads read, one thread writes
    var queue: Queue(u8, 1) = .{};
    const t1 = try Thread.spawn(cfg, readerThread, .{&queue});
    const t2 = try Thread.spawn(cfg, readerThread, .{&queue});
    try Thread.yield();
    std.time.sleep(std.time.ns_per_s / 2);
    queue.push(1);
    queue.push(1);
    t1.join();
    t2.join();
}

fn writerThread(q: *Queue(u8, 1)) !void {
    q.push(1);
}

test "2 writers" {
    var queue: Queue(u8, 1) = .{};
    const t1 = try Thread.spawn(cfg, writerThread, .{&queue});
    const t2 = try Thread.spawn(cfg, writerThread, .{&queue});

    try testing.expectEqual(1, queue.pop());
    try testing.expectEqual(1, queue.pop());
    t1.join();
    t2.join();
}