From 31fb6f51bc0423b40ea777d0029e1d5754b7bf80 Mon Sep 17 00:00:00 2001
From: Derek Kraan <derek@kraan.dev>
Date: Thu, 15 Sep 2022 11:29:35 +0200
Subject: [PATCH] Add OpentelemetryMonitor to contrib

---
 .../opentelemetry_monitor/.formatter.exs      |  4 ++
 .../opentelemetry_monitor/.gitignore          | 26 +++++++++
 .../opentelemetry_monitor/README.md           | 39 +++++++++++++
 .../lib/opentelemetry_monitor.ex              | 56 +++++++++++++++++++
 instrumentation/opentelemetry_monitor/mix.exs | 44 +++++++++++++++
 .../test/opentelemetry_monitor_test.exs       |  4 ++
 .../test/test_helper.exs                      |  1 +
 7 files changed, 174 insertions(+)
 create mode 100644 instrumentation/opentelemetry_monitor/.formatter.exs
 create mode 100644 instrumentation/opentelemetry_monitor/.gitignore
 create mode 100644 instrumentation/opentelemetry_monitor/README.md
 create mode 100644 instrumentation/opentelemetry_monitor/lib/opentelemetry_monitor.ex
 create mode 100644 instrumentation/opentelemetry_monitor/mix.exs
 create mode 100644 instrumentation/opentelemetry_monitor/test/opentelemetry_monitor_test.exs
 create mode 100644 instrumentation/opentelemetry_monitor/test/test_helper.exs

diff --git a/instrumentation/opentelemetry_monitor/.formatter.exs b/instrumentation/opentelemetry_monitor/.formatter.exs
new file mode 100644
index 00000000..d2cda26e
--- /dev/null
+++ b/instrumentation/opentelemetry_monitor/.formatter.exs
@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]
diff --git a/instrumentation/opentelemetry_monitor/.gitignore b/instrumentation/opentelemetry_monitor/.gitignore
new file mode 100644
index 00000000..37920e07
--- /dev/null
+++ b/instrumentation/opentelemetry_monitor/.gitignore
@@ -0,0 +1,26 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+opentelemetry_monitor-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/
diff --git a/instrumentation/opentelemetry_monitor/README.md b/instrumentation/opentelemetry_monitor/README.md
new file mode 100644
index 00000000..75c8c93d
--- /dev/null
+++ b/instrumentation/opentelemetry_monitor/README.md
@@ -0,0 +1,39 @@
+# OpentelemetryMonitor
+
+This library makes it possible to monitor a process, and close its spans when it has died.
+
+Without this, a crashed process can result in missing spans.
+
+To use, add the process to your supervision tree:
+
+```elixir
+children = [
+  OpentelemetryMonitor
+]
+```
+
+Then, call `OpentelemetryMonitor.monitor(span_ctx)` as appropriate.
+
+Example:
+
+```elixir
+OpentelemetryMonitor.monitor(OpenTelemetry.Tracer.current_span_ctx())
+```
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `opentelemetry_monitor` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:opentelemetry_monitor, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/opentelemetry_monitor>.
+
diff --git a/instrumentation/opentelemetry_monitor/lib/opentelemetry_monitor.ex b/instrumentation/opentelemetry_monitor/lib/opentelemetry_monitor.ex
new file mode 100644
index 00000000..4029b295
--- /dev/null
+++ b/instrumentation/opentelemetry_monitor/lib/opentelemetry_monitor.ex
@@ -0,0 +1,56 @@
+defmodule OpentelemetryMonitor do
+  use GenServer
+
+  def start_link(_arg) do
+    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
+  end
+
+  def init(nil) do
+    _table_id = :ets.new(__MODULE__, [:bag, :public, {:write_concurrency, true}, :named_table])
+    {:ok, nil}
+  end
+
+  def handle_call({:monitor, pid}, _from, state) do
+    Process.monitor(pid)
+    {:reply, :ok, state}
+  end
+
+  def handle_info({:DOWN, _ref, :process, pid, :normal}, state) do
+    :ets.take(__MODULE__, pid)
+    |> Enum.each(fn {_pid, ctx} ->
+      _span_ctx = OpenTelemetry.Tracer.set_current_span(ctx)
+      _ = OpenTelemetry.Tracer.end_span()
+    end)
+
+    {:noreply, state}
+  end
+
+  def handle_info({:DOWN, _ref, :process, pid, {:shutdown, _}}, state) do
+    :ets.take(__MODULE__, pid)
+    |> Enum.each(fn {_pid, ctx} ->
+      _span_ctx = OpenTelemetry.Tracer.set_current_span(ctx)
+      _ = OpenTelemetry.Tracer.end_span()
+    end)
+
+    {:noreply, state}
+  end
+
+  def handle_info({:DOWN, _ref, :process, pid, reason}, state) do
+    :ets.take(__MODULE__, pid)
+    |> Enum.each(fn {_pid, ctx} ->
+      _span_ctx = OpenTelemetry.Tracer.set_current_span(ctx)
+      _ = OpenTelemetry.Tracer.add_event("Process died", [{"reason", inspect(reason)}])
+      _ = OpenTelemetry.Tracer.end_span()
+    end)
+
+    {:noreply, state}
+  end
+
+  def monitor(span_ctx) do
+    if Application.fetch_env!(:opentelemetry, :processors) != [] do
+      # monitor first, because the monitor is necessary to clean the ets table.
+      :ok = GenServer.call(__MODULE__, {:monitor, self()})
+      true = :ets.insert(__MODULE__, {self(), span_ctx})
+    end
+  end
+end
diff --git a/instrumentation/opentelemetry_monitor/mix.exs b/instrumentation/opentelemetry_monitor/mix.exs
new file mode 100644
index 00000000..47bd2e5e
--- /dev/null
+++ b/instrumentation/opentelemetry_monitor/mix.exs
@@ -0,0 +1,44 @@
+defmodule OpentelemetryMonitor.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :opentelemetry_monitor,
+      version: "0.1.0",
+      elixir: "~> 1.10",
+      start_permanent: Mix.env() == :prod,
+      package: package(),
+      deps: deps(),
+      source_url:
+        "https://github.com/open-telemetry/opentelemetry-erlang-contrib/tree/main/instrumentation/opentelemetry_monitor"
+    ]
+  end
+
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  defp package() do
+    [
+      description: "Enables closing otel spans of processes that die",
+      licenses: ["Apache-2.0"],
+      links: %{
+        "GitHub" =>
+          "https://github.com/open-telemetry/opentelemetry-erlang-contrib/instrumentation/opentelemetry_monitor",
+        "OpenTelemetry Erlang" => "https://github.com/open-telemetry/opentelemetry-erlang",
+        "OpenTelemetry Erlang Contrib" =>
+          "https://github.com/open-telemetry/opentelemetry-erlang-contrib",
+        "OpenTelemetry.io" => "https://opentelemetry.io"
+      }
+    ]
+  end
+
+  defp deps do
+    [
+      {:ex_doc, "~> 0.28", only: :dev, runtime: false},
+      {:opentelemetry_api, "~> 1.0"}
+    ]
+  end
+end
diff --git a/instrumentation/opentelemetry_monitor/test/opentelemetry_monitor_test.exs b/instrumentation/opentelemetry_monitor/test/opentelemetry_monitor_test.exs
new file mode 100644
index 00000000..f1a1027c
--- /dev/null
+++ b/instrumentation/opentelemetry_monitor/test/opentelemetry_monitor_test.exs
@@ -0,0 +1,4 @@
+defmodule OpentelemetryMonitorTest do
+  use ExUnit.Case
+  doctest OpentelemetryMonitor
+end
diff --git a/instrumentation/opentelemetry_monitor/test/test_helper.exs b/instrumentation/opentelemetry_monitor/test/test_helper.exs
new file mode 100644
index 00000000..869559e7
--- /dev/null
+++ b/instrumentation/opentelemetry_monitor/test/test_helper.exs
@@ -0,0 +1 @@
+ExUnit.start()