Starting & shutting down iex with a Port - gracefully

PragTob

Hello my fellow elixir enthusiasts!

I want to start iex in a port then shut it down and I want to do so gracefully. Here is my attempt with some things outcommented that I tried:

port = Port.open({:spawn, "iex"}, [:binary])

# wait for startup
receive do
  {^port, {:data, "iex(1)> "}} -> :ok
end

# attemtps at closing iex in the port
# send(port, {:command, "^C"})
# send(port, {:command, "^C"})
# send(port, {:command, "^\\"})
# port_info = port |> Port.info() |> IO.inspect()
# os_pid = Access.get(port_info, :os_pid)
# System.cmd("kill", [to_string(os_pid)])

# attempts at closing just the port
Port.close(port)
# send(port, {self(), :close})
# receive do
#   {^port, :closed} -> :ok
# end
IO.puts("script finished")

Running this leads to the following output:

tobi@qiqi:~/github/elixir_playground(main)$ mix run scripts/close_iex.exs; echo $?
script finished
Failed to write log message to stdout, trying stderr

11:22:42.395 [error] Process #PID<0.101.0> raised an exception
** (FunctionClauseError) no function clause matching in :string.trim_l/2
    (stdlib 5.1.1) string.erl:884: :string.trim_l(:eof, [~c"\r\n", 9, 10, 11, 12, 13, 32, 133, 8206, 8207, 8232, 8233])
    (stdlib 5.1.1) string.erl:303: :string.trim/3
    (kernel 9.1) group.erl:534: :group.get_chars_apply/10
    (kernel 9.1) group.erl:199: :group.io_request/6
    (kernel 9.1) group.erl:125: :group.server_loop/3
Failed to write log message to stdout, trying stderr

11:22:42.406 [info] Failed to write to standard out (:epipe)
0

(you can also check it out at elixir_playground/scripts/close_iex.exs at main · PragTob/elixir_playground · GitHub)

I can not get it to finish without any errors like these. To be clear, the command exits with 0 aka job well done.

I’m running 1.16.0-rc.1 x 26.1.2 on Linux but I’m pretty sure this happens on everything.

As for the inevitable question “why would anyone want to do this?” the script is based off a test in benchee: benchee/test/benchee_test.exs at main · bencheeorg/benchee · GitHub

It’s a test that makes sure Benchee warns when run inside iex. The test works. It passes. However it spouts these error messages and seems to (partially) break the shell as I went down to diagnose that if the tests are initiated via Mix.Shell.IO.cmd that then infinitely hangs which breaks ex_guard which makes me sad (cc: @slashmili / Stopped working for benchee / only runs one command · Issue #68 · slashmili/ex_guard · GitHub ).

I’m confident though that if we solve this script, we solve that test, i.e. I’m confident it’s connected to the error Failed to write log message to stdout, trying stderr

Thanks everyone for taking a look and helping!

7 comments

#iex #port

1 1406 7

2023-12-18 07:45:43 UTC

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garazdawi

Erlang Core Team

It should be possible to do Port.close/1 without seeing that crash. The crash is caused by a disagreement by group and IEx.Server.__parse__/2. group expects __parse__/2 to return {stop, eof, eof}, while it returns {stop, eof, []}. I think that in this case it is group that is wrong and it should be able to handle what __parse__/2 is doing. I’ll write a fix for this.

As a workaround you can close the shell by sending "\aq\n". i.e.

port = Port.open({:spawn, "iex"}, [:binary, :exit_status])
receive do
  {^port, {:data, "iex(1)> "}} -> :ok
end
send(port, {self, {:command, "\a"}})
send(port, {self, {:command, "q\n"}})
receive do
  {^port, {:exit_status, num}} -> num
end

Most likely this error does not exist when using Erlang/OTP < 26.

Post #1

garazdawi

Erlang Core Team

https://github.com/erlang/otp/pull/7966

Post #2

PragTob

Wow, thanks a ton! Sorry for the assumption old erlang versions also had the problem - you’re right, they don’t!

@garazdawi it’s just amazing that I can ask a question here, someone from the Erlang Core team takes the time to look at it and answer it and figures out it’s actually a bug and implements a fix. All within ~3h. Absolutely amazing, tack så mycket!

The script needs some small adustments (self() and the last receive does not work/happen for me):

port = Port.open({:spawn, "iex"}, [:binary])

# wait for startup
receive do
  {^port, {:data, "iex(1)> "}} -> :ok
end

send(port, {self(), {:command, "\a"}})
send(port, {self(), {:command, "q\n"}})

@garazdawi any reference where I can learn about the commands/inputs we’re sending there? quick google wasn’t successful but probably that’s me.

Post #3

garazdawi

Erlang Core Team

The script needs some small adustments (self() and the last receive does not work/happen for me):

I added :exit_status to the Port.open arguments to make the receive work.

@garazdawi any reference where I can learn about the commands/inputs we’re sending there? quick google wasn’t successful but probably that’s me.

“\a” is the same as typing Ctrl+G, which is ascii code 7 i.e. bell/alert. So by sending \a you enter Job Control Mode in the shell, and from there you just type “q\n” to exit the shell.

I suppose sending ":init.stop\n" would achieve almost the same thing, though it would require the shell to be in a clean state.

Post #4

PragTob

woops sorry I should read better

Thanks a ton!

Post #5

PragTob

While the error is gone, something is still odd at least with exit_status it doesn’t work for me, I get notes about syntax errors instead:

port = Port.open({:spawn, "iex"}, [:binary, :exit_status])
receive do
  {^port, {:data, "iex(1)> "}} -> :ok
end
send(port, {self(), {:command, "\a"}})
send(port, {self(), {:command, "q\n"}})
receive do
  {^port, {:exit_status, num}} -> num
after
  5_000 ->
    messages = :erlang.process_info(self(), :messages)
    IO.inspect(messages, label: "messages")
end

tobi@qiqi:~/github/elixir_playground(main)$ mix run wut.exs 
messages: {:messages,
 [
   {#Port<0.6>,
    {:data,
     "Interactive Elixir (1.16.0-rc.1) - press Ctrl+C to exit (type h() ENTER for help)\n"}},
   {#Port<0.6>,
    {:data,
     "** (SyntaxError) invalid syntax found on iex:1:1:\n    error: unexpected token: alert (column 1, code point U+0007)\n    │\n  1 │ \aq\n    │ ^\n    │\n    └─ iex:1:1\n    (iex 1.16.0-rc.1) lib/iex/evaluator.ex:294: IEx.Evaluator.parse_eval_inspect/4\n    (iex 1.16.0-rc.1) lib/iex/evaluator.ex:187: IEx.Evaluator.loop/1\n    (iex 1.16.0-rc.1) lib/iex/evaluator.ex:32: IEx.Evaluator.init/5\n    (stdlib 4.3.1.3) proc_lib.erl:240: :proc_lib.init_p_do_apply/3\n"}},
   {#Port<0.6>, {:data, "iex(1)> "}}
 ]}

(both on erlang 25 and 26)

edit: Maybe I need to wait between the 2 sends ?

Post #6