putLocked

    fn putLocked(q: *TypeErasedQueue, io: Io, elements: []const u8, min: usize, uncancelable: bool) (QueueClosedError || Cancelable)!usize {
        // A closed queue cannot be added to, even if there is space in the buffer.
        if (q.closed) return error.Closed;

        // Getters have first priority on the data, and only when the getters
        // queue is empty do we start populating the buffer.

        // The number of elements we add immediately, before possibly blocking.
        var n: usize = 0;

        while (q.getters.popFirst()) |getter_node| {
            const getter: *Get = @alignCast(@fieldParentPtr("node", getter_node));
            const copy_len = @min(getter.remaining.len, elements.len - n);
            assert(copy_len > 0);
            @memcpy(getter.remaining[0..copy_len], elements[n..][0..copy_len]);
            getter.remaining = getter.remaining[copy_len..];
            getter.needed -|= copy_len;
            n += copy_len;
            if (getter.needed == 0) {
                getter.condition.signal(io);
            } else {
                assert(n == elements.len); // we didn't have enough elements for the getter
                q.getters.prepend(getter_node);
            }
            if (n == elements.len) return elements.len;
        }

        while (q.puttableSlice()) |slice| {
            const copy_len = @min(slice.len, elements.len - n);
            assert(copy_len > 0);
            @memcpy(slice[0..copy_len], elements[n..][0..copy_len]);
            q.len += copy_len;
            n += copy_len;
            if (n == elements.len) return elements.len;
        }

        // Don't block if we hit the min.
        if (n >= min) return n;

        var pending: Put = .{
            .remaining = elements[n..],
            .needed = min - n,
            .condition = .init,
            .node = .{},
        };
        q.putters.append(&pending.node);
        defer if (pending.needed > 0) q.putters.remove(&pending.node);

        while (pending.needed > 0 and !q.closed) {
            if (uncancelable) {
                pending.condition.waitUncancelable(io, &q.mutex);
                continue;
            }
            pending.condition.wait(io, &q.mutex) catch |err| switch (err) {
                error.Canceled => if (pending.remaining.len == elements.len) {
                    // Canceled while waiting, and appended no elements.
                    return error.Canceled;
                } else {
                    // Canceled while waiting, but appended some elements, so report those first.
                    io.recancel();
                    return elements.len - pending.remaining.len;
                },
            };
        }
        if (pending.remaining.len == elements.len) {
            // The queue was closed while we were waiting. We appended no elements.
            assert(q.closed);
            return error.Closed;
        }
        return elements.len - pending.remaining.len;
    }