from_stages - noproc errors when GenStage producer finishes early

[Suraj N. Kurapati]

Hello,

I'm getting :noproc errors when I connect a short-lived GenStage producer to a Flow using Flow.from_stages(). Below is a minimal example that reproduces the problem I'm seeing:

1. Passing through Flow using multiple stages works correctly. ✔️
2. Mapping through Flow using a single stage works correctly. ✔️
3. Mapping through Flow using multiple stages triggers the error. 💥

Erlang/OTP 22 [erts-10.4] [source] [64-bit] [smp:32:32] [ds:32:32:10] [async-threads:1] [hipe]

Interactive Elixir (1.9.0) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> {:ok, producer} = GenStage.from_enumerable(1..3, link: false); Flow.from_stages([producer], stages: 2) |> Enum.to_list()
[1, 2, 3]
iex(2)> {:ok, producer} = GenStage.from_enumerable(1..3, link: false); Flow.from_stages([producer], stages: 1) |> Flow.map(fn i -> ms = :timer.seconds(i); IO.inspect("#{inspect(self)}: sleep #{i}"); Process.sleep(ms);
ms end) |> Enum.to_list()
"#PID<0.194.0>: sleep 1"
"#PID<0.194.0>: sleep 2"
"#PID<0.194.0>: sleep 3"
[1000, 2000, 3000]
iex(3)> {:ok, producer} = GenStage.from_enumerable(1..3, link: false); Flow.from_stages([producer], stages: 2) |> Flow.map(fn i -> ms = :timer.seconds(i); IO.inspect("#{inspect(self)}: sleep #{i}"); Process.sleep(ms);
ms end) |> Enum.to_list()
"#PID<0.200.0>: sleep 1"

12:14:48.166 [info]  GenStage consumer #PID<0.201.0> is stopping after receiving cancel from producer #PID<0.197.0> with reason: :noproc


12:14:48.180 [error] GenServer #PID<0.201.0> terminating
** (stop) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
Last message: {:DOWN, #Reference<0.3205571650.1190133762.109920>, :process, #PID<0.197.0>, :noproc}
State: {%{}, %{done?: true, producers: %{}, trigger: #Function<2.127884580/3 in Flow.Window.Global.materialize/5>}, {1, 2}, [], #Function<33.87744541/4 in Flow.Materialize.mapper_ops/1>}
"#PID<0.200.0>: sleep 2"
"#PID<0.200.0>: sleep 3"
** (exit) exited in: GenStage.close_stream(%{#Reference<0.3205571650.1190133762.109930> => {:subscribed, #PID<0.200.0>, :transient, 500, 1000, 1000}})
    ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
    (gen_stage) lib/gen_stage/stream.ex:160: GenStage.Stream.close_stream/1
    (elixir) lib/stream.ex:1400: Stream.do_resource/5
    (elixir) lib/enum.ex:3023: Enum.reverse/1
    (elixir) lib/enum.ex:2668: Enum.to_list/1
iex(3)>

How can I safely connect a short-lived GenStage producer to a Flow using multiple stages? 😕 See also GenStage: How to cancel a Flow from the producer? whose answer relies on an outdated GenStage API.

Thanks for your consideration.

[José Valim]

What is your flow version? Have you tried flow master?

[José Valim]

Also please report your gen_stage version and make sure you are running on latest gen_stage too.

[Suraj N. Kurapati]

Sorry for omitting such basic information 😅 Here are the versions I'm using, according to mix.lock: 💁‍♂️

  "flow": {:hex, :flow, "0.14.3", "0d92991fe53035894d24aa8dec10dcfccf0ae00c4ed436ace3efa9813a646902", [:mix], [{:gen_stage, "~> 0.14.0", [hex: :gen_stage, repo: "hexpm", optional: false]}], "hexpm"},
  "gen_stage": {:hex, :gen_stage, "0.14.2", "6a2a578a510c5bfca8a45e6b27552f613b41cf584b58210f017088d3d17d0b14", [:mix], [], "hexpm"},

I will try to reproduce the issue using the git master versions of these packages and report back shortly.

[Suraj N. Kurapati]

Thanks! 🎉 Upgrading to the git master version of Flow (although GenStage hasn't changed) has solved it:

-  "flow": {:hex, :flow, "0.14.3", "0d92991fe53035894d24aa8dec10dcfccf0ae00c4ed436ace3efa9813a646902", [:mix], [{:gen_stage, "~> 0.14.0", [hex: :gen_stage, repo: "hexpm", optional: false]}], "hexpm"},
+  "flow": {:git, "https://github.com/plataformatec/flow.git", "1ffac6a801602bf8b02192488e58ce5728b581aa", []},
   "gen_stage": {:hex, :gen_stage, "0.14.2", "6a2a578a510c5bfca8a45e6b27552f613b41cf584b58210f017088d3d17d0b14", [:mix], [], "hexpm"},