With Elixir 1.18, only a single user may compile a project

[jiegillet]

Elixir and Erlang/OTP versions

Erlang/OTP 27 [erts-15.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]

Elixir 1.18.1 (compiled with Erlang/OTP 27)

Operating system

MacOS

Current behavior

In Elixir 1.18, on compilation a temporary folder is being created in /tmp/mix_pubsub (or potentially in some other temporary directory).
However, due to systems permissions, if that folder was created by a user a, a user b (with the same temporary directory) will not be allowed be re-create the folder and attempting to compile a project will crash.
Here is an example I ran on my machine:

➜  ~ whoami
jie_old
➜  ~ elixir -v                             
Erlang/OTP 27 [erts-15.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]

Elixir 1.18.1 (compiled with Erlang/OTP 27)
➜  ~ elixir -e "IO.puts(System.tmp_dir!())"
/tmp
➜  ~ ls -l /tmp/mix_*
zsh: no matches found: /tmp/mix_*
➜  ~ mix new project1 && cd project1      
* creating README.md
* creating .formatter.exs
* creating .gitignore
* creating mix.exs
* creating lib
* creating lib/project1.ex
* creating test
* creating test/test_helper.exs
* creating test/project1_test.exs

Your Mix project was created successfully.
You can use "mix" to compile it, test it, and more:

    cd project1
    mix test

Run "mix help" for more commands.
➜  project1 mix compile
Compiling 1 file (.ex)
Generated project1 app
➜  project1 ls -l /tmp/mix_*                      
/tmp/mix_lock:
total 0
drwxr-xr-x  4 jie_old  wheel  128 Jan 10 21:03 tARo6JLGiDoBf2MhoMm72g

/tmp/mix_pubsub:
total 0
drwxr-xr-x  3 jie_old  wheel  96 Jan 10 21:03 tARo6JLGiDoBf2MhoMm72g
➜  project1 
➜  project1 su - jie
Password:
➜  ~ whoami
jie
➜  ~ elixir -v                             
Erlang/OTP 27 [erts-15.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

Elixir 1.18.1 (compiled with Erlang/OTP 27)
➜  ~ elixir -e "IO.puts(System.tmp_dir!())"
/tmp
➜  ~ mix new project2 && cd project2
* creating README.md
* creating .formatter.exs
* creating .gitignore
* creating mix.exs
* creating lib
* creating lib/project2.ex
* creating test
* creating test/test_helper.exs
* creating test/project2_test.exs

Your Mix project was created successfully.
You can use "mix" to compile it, test it, and more:

    cd project2
    mix test

Run "mix help" for more commands.
➜  project2 mix compile

21:04:17.346 [error] GenServer Mix.Sync.PubSub terminating
** (File.Error) could not make directory (with -p) "/tmp/mix_pubsub/VNbrPlEkR_STXjO2E53Uug": permission denied
    (elixir 1.18.1) lib/file.ex:346: File.mkdir_p!/1
    (mix 1.18.1) lib/mix/sync/pubsub.ex:222: Mix.Sync.PubSub.create_subscription_file/2
    (mix 1.18.1) lib/mix/sync/pubsub.ex:142: Mix.Sync.PubSub.handle_call/3
    (stdlib 6.0) gen_server.erl:2209: :gen_server.try_handle_call/4
    (stdlib 6.0) gen_server.erl:2238: :gen_server.handle_msg/6
    (stdlib 6.0) proc_lib.erl:329: :proc_lib.init_p_do_apply/3
Last message (from Mix.PubSub.Subscriber): {:subscribe, #PID<0.110.0>, "/Users/jie/project2/_build/dev"}
State: %{port: nil, hash_to_pids: %{}}
Client Mix.PubSub.Subscriber is alive

    (stdlib 6.0) gen.erl:241: :gen.do_call/4
    (elixir 1.18.1) lib/gen_server.ex:1125: GenServer.call/3
    (mix 1.18.1) lib/mix/sync/pubsub.ex:44: Mix.Sync.PubSub.subscribe/1
    (mix 1.18.1) lib/mix/pubsub/subscriber.ex:21: Mix.PubSub.Subscriber.init/1
    (stdlib 6.0) gen_server.erl:2057: :gen_server.init_it/2
    (stdlib 6.0) gen_server.erl:2012: :gen_server.init_it/6
    (stdlib 6.0) proc_lib.erl:329: :proc_lib.init_p_do_apply/3

21:04:17.355 [notice] Application mix exited: shutdown
** (ArgumentError) errors were found at the given arguments:

  * 1st argument: the table identifier does not refer to an existing ETS table

    (stdlib 6.0) :ets.lookup(Mix.State, :debug)
    (mix 1.18.1) lib/mix/state.ex:26: Mix.State.get/2
    (mix 1.18.1) lib/mix/cli.ex:112: Mix.CLI.run_task/2
    /Users/jie/.asdf/installs/elixir/1.18.1-otp-27/bin/mix:2: (file)

We realized this in the context of updating the Exercism test runner. The test runner runs user code and formats the result in a particular way. The app was built in a Docker container by the root user, and then used by some other user, and with 1.18 it started crashing. Once we realized the cause, it was easy enough to fix, but I thought I would signal this.

Expected behavior

I would expect several users on a machine to be able to compile projects.

[josevalim]

I will release v1.18.2 in the following days, so you can update exercism. Thank you for your contributions! ❤️