batchCompleteBlockingWindows

fn batchCompleteBlockingWindows(
    b: *Io.Batch,
    operation_userdata: *WindowsBatchOperationUserdata,
    result: Io.Operation.Result,
) void {
    const erased_userdata = operation_userdata.toErased();
    const pending: *Io.Operation.Storage.Pending = @fieldParentPtr("userdata", erased_userdata);
    switch (pending.node.prev) {
        .none => b.pending.head = pending.node.next,
        else => |prev_index| b.storage[prev_index.toIndex()].pending.node.next = pending.node.next,
    }
    switch (pending.node.next) {
        .none => b.pending.tail = pending.node.prev,
        else => |next_index| b.storage[next_index.toIndex()].pending.node.prev = pending.node.prev,
    }
    const storage: *Io.Operation.Storage = @fieldParentPtr("pending", pending);
    const index: Io.Operation.OptionalIndex = .fromIndex(storage - b.storage.ptr);
    switch (b.completed.tail) {
        .none => b.completed.head = index,
        else => |tail_index| b.storage[tail_index.toIndex()].completion.node.next = index,
    }
    b.completed.tail = index;
    storage.* = .{ .completion = .{ .node = .{ .next = .none }, .result = result } };
}