metrixwire

defmodule MetrixWire.Instrument.SourceLocation do
  @moduledoc """
  Best-effort `file:line` of the application frame that issued a span, so the
  dashboard can point at the line that ran a query / HTTP call.

  Walks the current process stacktrace and returns the first frame that belongs to
  the host application — skipping the SDK itself, the standard library, and known
  instrumentation/driver dependencies (Ecto, Phoenix, Plug, Finch, Tesla, Postgrex,
  DBConnection, …). Returns nil rather than throwing.
  """

  # Skip frames whose source file lives in the SDK, a dependency, or a known
  # framework/driver. Matches both absolute (`/deps/...`) and dep-relative paths.
  @skip_file ~r{(metrixwire/|/deps/|telemetry|/ecto|/phoenix|/plug|/finch|/tesla|db_connection|postgrex|bandit|thousand_island|cowboy)}

  # Modules that are never application code. Their frames are always skipped.
  @skip_prefixes ~w(
    Elixir.Process Elixir.GenServer Elixir.Task Elixir.Enum Elixir.Stream
    Elixir.Kernel Elixir.Ecto Elixir.Phoenix Elixir.Plug Elixir.Finch
    Elixir.Tesla Elixir.Postgrex Elixir.DBConnection Elixir.Bandit
    Elixir.MetrixWire Elixir.ThousandIsland
  )

  @doc "The nearest application caller frame as \"file.ex:42\", or nil."
  def nearest do
    if MetrixWire.capture_source?() do
      {:current_stacktrace, stack} = Process.info(self(), :current_stacktrace)
      find_frame(stack)
    end
  rescue
    _ -> nil
  end

  defp find_frame(stack) do
    Enum.find_value(stack, fn
      {mod, _fun, _arity, meta} ->
        file = meta[:file]
        line = meta[:line]

        cond do
          is_nil(file) or is_nil(line) -> nil
          skip_module?(mod) -> nil
          stdlib_file?(to_string(file)) -> nil
          Regex.match?(@skip_file, to_string(file)) -> nil
          true -> "#{Path.basename(to_string(file))}:#{line}"
        end

      _ ->
        nil
    end)
  rescue
    _ -> nil
  end

  defp skip_module?(mod) do
    name = Atom.to_string(mod)

    # Erlang modules (`:elixir_compiler`, `:gen_server`, …) are never app code.
    not String.starts_with?(name, "Elixir.") or
      Enum.any?(@skip_prefixes, &String.starts_with?(name, &1))
  rescue
    _ -> false
  end

  # Elixir/Erlang standard-library sources are stored as bare relative paths like
  # "lib/process.ex" or "src/elixir_compiler.erl" (no leading directory). Real app
  # frames carry a directory (e.g. "lib/native/scenarios.ex").
  defp stdlib_file?(file) do
    Regex.match?(~r{^lib/[^/]+\.ex$}, file) or Regex.match?(~r{^src/[^/]+\.erl$}, file)
  rescue
    _ -> false
  end
end