link asymmetry in process ring - process

Question

I'm currently working through the ubiquitous process ring in elixir.
The ring is linked, but in the following fashion:
iex(1)> Ring.Worker.create_ring_of_linked_processes(3)
Ring.Worker.create_ring_of_linked_processes(3)
[%{"links" => [#PID<0.121.0>, #PID<0.120.0>], "pid" => #PID<0.122.0>},
%{"links" => [#PID<0.120.0>, #PID<0.122.0>], "pid" => #PID<0.121.0>},
%{"links" => [#PID<0.121.0>], "pid" => #PID<0.120.0>}]
I've noticed an asymmetry in the links here - should #PID<0.120.0> have the mapping "links" => [#PID<0.121.0>,#PID<0.122.0>] rather than just "links" => [#PID<0.121.0>] ?
The code is as follows:
def loop() do
receive do
{:link, pid} when is_pid(pid) ->
Process.link(pid)
loop()
end
end
def create_ring_of_linked_processes(num_of_processes) do
num_of_processes
|> create_processes
|> link_processes([])
end
def link_processes([pid1, pid2 | rest], linked_processes) do
send(pid1, {:link, pid2})
:timer.sleep(1)
{:links, links} = Process.info(pid1, :links)
link_processes(
[pid2 | rest], [%{"pid" => pid1, "links" => links} | linked_processes]
)
end
def link_processes([pid | []], linked_processes) do
%{"pid" => first_pid, "links" => _} = List.last(linked_processes)
send(pid, {:link, first_pid})
:timer.sleep(1)
{:links, links} = Process.info(pid, :links)
[%{"pid" => pid, "links" => links} | linked_processes]
end
#spec create_processes(integer) :: [pid]
def create_processes(num_of_processes) do
for _ <- 1..num_of_processes, do: spawn(__MODULE__, :loop, [])
end

Answer 1

This is because you're linking the processes at the same time as collecting its :links, but some links for that process are being created after you collect its links.
For example, if you spawn a process a, and then collect its links, it'll be an empty list.
iex(1)> a = spawn(fn -> :timer.sleep(:infinity) end)
#PID<0.82.0>
iex(2)> Process.info(a, :links)
{:links, []}
If you spawn b now and link it to a, b will have [a] in its links and a will have [b].
iex(3)> b = spawn(fn -> Process.link(a); :timer.sleep(:infinity) end)
#PID<0.85.0>
iex(4)> Process.info(b, :links)
{:links, [#PID<0.82.0>]}
iex(5)> Process.info(a, :links)
{:links, [#PID<0.85.0>]}
So, you need to collect the links for each process after all the linking is complete if you want the final links for each process.