feat: basic error handling/capturing

lib/tower.ex

@@ -3,16 +3,32 @@ defmodule Tower do
   Documentation for `Tower`.
   """
 
-  @doc """
-  Hello world.
+  @default_reporters [Tower.EphemeralReporter]
 
-  ## Examples
+  def attach do
+    :ok = Tower.LoggerHandler.attach()
+  end
 
-      iex> Tower.hello()
-      :world
+  def detach do
+    :ok = Tower.LoggerHandler.detach()
+  end
 
-  """
-  def hello do
-    :world
+  def report_exception(exception, stacktrace, meta \\ %{})
+      when is_exception(exception) and is_list(stacktrace) do
+    reporters()
+    |> Enum.each(fn reporter ->
+      reporter.report_exception(exception, stacktrace, meta)
+    end)
+  end
+
+  def report(type, reason, stacktrace, meta \\ %{}) when is_atom(type) and is_list(stacktrace) do
+    reporters()
+    |> Enum.each(fn reporter ->
+      reporter.report(type, reason, stacktrace, meta)
+    end)
+  end
+
+  def reporters do
+    Application.get_env(:tower, :reporters, @default_reporters)
   end
 end

lib/tower/ephemeral_reporter.ex

@@ -0,0 +1,46 @@
+defmodule Tower.EphemeralReporter do
+  use Agent
+
+  def start_link(_opts) do
+    Agent.start_link(fn -> [] end, name: __MODULE__)
+  end
+
+  def report_exception(exception, stacktrace, meta \\ %{})
+      when is_exception(exception) and is_list(stacktrace) do
+    Agent.update(
+      __MODULE__,
+      fn errors ->
+        [
+          %{
+            time: Map.get(meta, :time, :logger.timestamp()),
+            type: exception.__struct__,
+            reason: Exception.message(exception),
+            stacktrace: stacktrace
+          }
+          | errors
+        ]
+      end
+    )
+  end
+
+  def report(type, reason, stacktrace, meta \\ %{}) when is_atom(type) and is_list(stacktrace) do
+    Agent.update(
+      __MODULE__,
+      fn errors ->
+        [
+          %{
+            time: Map.get(meta, :time, :logger.timestamp()),
+            type: type,
+            reason: reason,
+            stacktrace: stacktrace
+          }
+          | errors
+        ]
+      end
+    )
+  end
+
+  def errors do
+    Agent.get(__MODULE__, & &1)
+  end
+end

lib/tower/logger_handler.ex

@@ -0,0 +1,56 @@
+defmodule Tower.LoggerHandler do
+  @default_level :error
+  @handler_id :tower
+
+  def attach do
+    :logger.add_handler(@handler_id, __MODULE__, %{level: @default_level})
+  end
+
+  def detach do
+    :logger.remove_handler(@handler_id)
+  end
+
+  # :logger callbacks
+
+  def adding_handler(config) do
+    IO.puts("[Tower.LoggerHandler] ADDING config=#{inspect(config)}")
+
+    {:ok, config}
+  end
+
+  def removing_handler(config) do
+    IO.puts("[Tower.LoggerHandler] REMOVING config=#{inspect(config)}")
+
+    :ok
+  end
+
+  def log(%{level: :error, meta: %{crash_reason: {exception, stacktrace}} = meta}, _config)
+      when is_exception(exception) and is_list(stacktrace) do
+    IO.puts("[Tower.LoggerHandler] EXCEPTION #{inspect(exception)}")
+
+    Tower.report_exception(exception, stacktrace, meta)
+  end
+
+  def log(
+        %{level: :error, meta: %{crash_reason: {{:nocatch, reason}, stacktrace}} = meta},
+        _config
+      )
+      when is_list(stacktrace) do
+    IO.puts("[Tower.LoggerHandler] NOCATCH #{inspect(reason)}")
+
+    Tower.report(:nocatch, reason, stacktrace, meta)
+  end
+
+  def log(%{level: :error, meta: %{crash_reason: {exit_reason, stacktrace}} = meta}, _config)
+      when is_list(stacktrace) do
+    IO.puts("[Tower.LoggerHandler] EXIT #{inspect(exit_reason)}")
+
+    Tower.report(:exit, exit_reason, stacktrace, meta)
+  end
+
+  def log(log_event, _config) do
+    IO.puts(
+      "[Tower.LoggerHandler] UNHANDLED LOG EVENT log_event=#{inspect(log_event, pretty: true)}"
+    )
+  end
+end

test/tower_test.exs

@@ -2,7 +2,161 @@ defmodule TowerTest do
   use ExUnit.Case
   doctest Tower
 
-  test "greets the world" do
-    assert Tower.hello() == :world
+  setup do
+    Tower.attach()
+
+    on_exit(fn ->
+      Tower.detach()
+    end)
+  end
+
+  test "starts with 0 exceptions" do
+    Tower.EphemeralReporter.start_link([])
+
+    assert [] = Tower.EphemeralReporter.errors()
+  end
+
+  test "reports arithmetic error" do
+    Tower.EphemeralReporter.start_link([])
+
+    in_unlinked_process(fn ->
+      1 / 0
+    end)
+
+    assert(
+      [
+        %{
+          time: _,
+          type: ArithmeticError,
+          reason: "bad argument in arithmetic expression",
+          stacktrace: stacktrace
+        }
+      ] = Tower.EphemeralReporter.errors()
+    )
+
+    assert is_list(stacktrace)
+  end
+
+  test "reports a raise" do
+    Tower.EphemeralReporter.start_link([])
+
+    in_unlinked_process(fn ->
+      raise "error inside process"
+    end)
+
+    assert(
+      [
+        %{
+          time: _,
+          type: RuntimeError,
+          reason: "error inside process",
+          stacktrace: stacktrace
+        }
+      ] = Tower.EphemeralReporter.errors()
+    )
+
+    assert is_list(stacktrace)
+  end
+
+  test "reports a thrown string" do
+    Tower.EphemeralReporter.start_link([])
+
+    in_unlinked_process(fn ->
+      throw("error")
+    end)
+
+    assert(
+      [
+        %{
+          time: _,
+          type: :nocatch,
+          reason: "error",
+          stacktrace: stacktrace
+        }
+      ] = Tower.EphemeralReporter.errors()
+    )
+
+    assert is_list(stacktrace)
+  end
+
+  test "reports a thrown non-string" do
+    Tower.EphemeralReporter.start_link([])
+
+    in_unlinked_process(fn ->
+      throw(something: "here")
+    end)
+
+    assert(
+      [
+        %{
+          time: _,
+          type: :nocatch,
+          reason: [something: "here"],
+          stacktrace: stacktrace
+        }
+      ] = Tower.EphemeralReporter.errors()
+    )
+
+    assert is_list(stacktrace)
+  end
+
+  test "doesn't report an normal exit" do
+    Tower.EphemeralReporter.start_link([])
+
+    in_unlinked_process(fn ->
+      exit(:normal)
+    end)
+
+    assert [] = Tower.EphemeralReporter.errors()
+  end
+
+  test "reports an abnormal exit" do
+    Tower.EphemeralReporter.start_link([])
+
+    in_unlinked_process(fn ->
+      exit(:abnormal)
+    end)
+
+    assert(
+      [
+        %{
+          time: _,
+          type: :exit,
+          reason: :abnormal,
+          stacktrace: stacktrace
+        }
+      ] = Tower.EphemeralReporter.errors()
+    )
+
+    assert is_list(stacktrace)
+  end
+
+  test "reports a kill exit" do
+    Tower.EphemeralReporter.start_link([])
+
+    in_unlinked_process(fn ->
+      exit(:kill)
+    end)
+
+    assert(
+      [
+        %{
+          time: _,
+          type: :exit,
+          reason: :kill,
+          stacktrace: stacktrace
+        }
+      ] = Tower.EphemeralReporter.errors()
+    )
+
+    assert is_list(stacktrace)
+  end
+
+  defp in_unlinked_process(fun) when is_function(fun, 0) do
+    {:ok, pid} = Task.Supervisor.start_link()
+
+    pid
+    |> Task.Supervisor.async_nolink(fun)
+    |> Task.yield()
   end
 end