Removing `caller_pid` from log entries break some integrations

[Zach Daniel]

In this commit: 8d09248#diff-152ab0727a1d450c0fe74d178e80d1f0

LogEntry had its caller_pid field removed. However, spandex_ecto (which has its own problems but we're going to resolve those), relies on that field being set to tie parent process trace data with the metrics in the log entry. It also seems like this information is not present in the telemetry information. Any suggestions on how we might go about getting query data into traces in a similar format? We don't want to have to wrap all calls to a repo in traces/spans.

[José Valim]

The caller_pid is going to be self() in the majority of the cases, except on multi-preloads, which use separate queries. So you can just use self(). For the remaining cases, then elixir-lang/elixir#7995 will be the way to go, which should work generally and not only within Ecto.

[Zach Daniel]

Thanks, I look forward to seeing that make it in. That would be useful for our tracing library in many different ways.

[Ievgen Pyrogov]

@josevalim what would be a recommended way of having log metadata in the multi-preload queries now that Elixir 1.8 is out? Would you saw developers should handle this themselves or?

So far, this is what I arrived at to extra metadata from the caller process – would you say this is normal approach to have request_id during parallel-preloading:

require Logger

Logger.metadata(request_id: "03872725-6411-4746-acb8-047f10aa0072")

telemetry_fn = fn a,b,c,d ->
  case Process.info(self())[:dictionary][:"$callers"] do
    [caller_process] ->
      {:dictionary, stuff} = Process.info(caller_process, :dictionary)
      {true, log_metadata} = stuff[:logger_metadata]
      Logger.info("We're in a task process, preloading some assoc, request_id is #{log_metadata[:request_id]}")

    _ ->
      Logger.info("We've in a caller process itself")
  end
end

:telemetry.attach("my-handler", [:my_app, :repo, :query], telemetry_fn, nil)

# Test
post = Repo.insert!(%Post{author_id: 1})
Repo.insert!(%Comment{post_id: post.id, state: "approved"})
Repo.insert!(%Comment{post_id: post.id, state: "disapproved"})
Repo.insert!(%Comment{post_id: post.id, state: "pending"})
Repo.preload(post, [:comments, :author])

[José Valim]

Exactly. Just replace [caller_process] -> by [caller_process | _] ->.