Limit concurrency on structure_pid calls

[jbodah]

We're on a fork of an earlier version of wobserver, but I thought I'd bring this issue up in the core repo as an issue.

We've been seeing consistent timeouts from this parallel_map call: https://github.com/shinyscorpion/wobserver/blob/master/lib/wobserver/util/application.ex#L97. We've also noticed that the BEAM load spikes pretty hard whenever we navigate to the wobserver page for one of our apps that has many processes (~100 Ranch acceptors under a single supervisor). I'm guessing this has to do with all of the core :erlang calls getting process info

Here's a stacktrace from our version:

14:40:38.116 [error] Task #PID<0.1046.0> started from #PID<0.1033.0> terminating
** (stop) exited in: Task.await(%Task{owner: #PID<0.1046.0>, pid: #PID<0.1056.0>, ref: #Reference<0.0.1.97714>}, 5000)
    ** (EXIT) time out
    (elixir) lib/task.ex:416: Task.await/2
    (elixir) lib/enum.ex:1229: Enum."-map/2-lists^map/1-0-"/2
    (wobserver) lib/wobserver/util/application.ex:101: Wobserver.Util.Application.structure_pid/1
    (elixir) lib/task/supervised.ex:85: Task.Supervised.do_apply/2
    (elixir) lib/task/supervised.ex:36: Task.Supervised.reply/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Function: #Function<1.105204156/0 in Wobserver.Util.Helper.parallel_map/2>
    Args: []
14:40:38.116 [error] Ranch protocol #PID<0.1024.0> (:cowboy_protocol) of listener TaskRunner.Endpoint.HTTP terminated
** (exit) exited in: Task.await(%Task{owner: #PID<0.1046.0>, pid: #PID<0.1056.0>, ref: #Reference<0.0.1.97714>}, 5000)
    ** (EXIT) time out

When this time out happens the entire wobserver page fails to load which has made it unusable in app without changes.

I'd suggest using a pool to limit concurrency or chunking the processes

[jbodah]

Actually on further investigation, this is probably failing because we have a long-running message handler in one of our processes. Will report back when we get more info

[jbodah]

Disregard, this was our own dumb fault for having a Supervisor.Spec.supervisor that should've been a worker