How do I gracefully trap an exit with Task.async_stream?

Questions & Help Questions
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I’m trying to convert a directory of Markdown files into HTML, and I’m trying to do it concurrently with Task.async_stream. Here is the code that I have:

defmodule SomeModule do
  defp write_articles_html do
    Process.flag(:trap_exit, true)

    html_conversion_results =
      Article.files
        |> Task.async_stream(fn markdown_file ->
             write_article_html(markdown_file)
           end,
           max_concurrency: System.schedulers_online * 2)
        |> Enum.to_list

    IO.inspect html_conversion_results
  end

The function called in Task.async_stream i.e. write_article_html looks like this:

  defp write_article_html(markdown_file_path) do
    html_dir = article_html_dir(markdown_file_path)
    full_html_dir = Path.join(@build_directory, html_dir)

    File.mkdir!(full_html_dir)
    # ...
  end
end # defmodule SomeModule

When the File.mkdir! fails, I would like the parent process (that made the Task.async_stream) not to crash.

I understand I can do this by trapping exits, and I’ve added the code to do it (Process.flag(:trap_exit, true)

The main process still crashes though (crash output shown below). What am I missing? (Do I need to add a receive somewhere for the :exit message? Both these function are in the same module, for reference)

How could I make sure the caller process can handle the mkdir failing? (If this is explained in any documentation, please let me know).

ERROR

Here is the output when the process crashes:

[ok: :ok, ok: :ok, ok: :ok, ok: :ok, ok: :ok, ok: :ok,
 exit: {%File.Error{action: "make directory", path: "_build/my-new-article",
   reason: :eexist},
  [{File, :mkdir!, 1, [file: 'lib/file.ex', line: 183]},
   {Task.Supervised, :do_apply, 2, [file: 'lib/task/supervised.ex', line: 85]},
   {Task.Supervised, :reply, 5, [file: 'lib/task/supervised.ex', line: 36]},
   {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 247]}]},
 exit: {%File.Error{action: "make directory", path: "_build/my-new-article",
   reason: :eexist},
  [{File, :mkdir!, 1, [file: 'lib/file.ex', line: 183]},
   {Task.Supervised, :do_apply, 2, [file: 'lib/task/supervised.ex', line: 85]},
   {Task.Supervised, :reply, 5, [file: 'lib/task/supervised.ex', line: 36]},
   {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 247]}]}]

20:58:50.968 [error] Task #PID<0.127.0> started from #PID<0.120.0> terminating
** (File.Error) could not make directory "_build/my-new-article": file already exists
    (elixir) lib/file.ex:183: File.mkdir!/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: &:erlang.apply/2
    Args: [#Function<0.35336826/1 in Lydian.Builder.Impl.write_articles_html/1>, ["source/2017-08-23-my-new-article.md"]]

20:58:50.976 [error] Task #PID<0.128.0> started from #PID<0.120.0> terminating
** (File.Error) could not make directory "_build/my-new-article": file already exists
    (elixir) lib/file.ex:183: File.mkdir!/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: &:erlang.apply/2
    Args: [#Function<0.35336826/1 in Lydian.Builder.Impl.write_articles_html/1>, ["source/2017-11-12-my-new-article.md"]]