`mix release` regression under read-only Elixir installations

[chinponya]

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.3.1] [source] [64-bit] [smp:32:32] [ds:32:32:10] [async-threads:1] [jit:ns]
Elixir 1.20.0-rc.4 (compiled with Erlang/OTP 28)

Operating system

NixOS 25.11

Current behavior

Something changed between 1.20.0-rc.2 and 1.20.0-rc.3 that's causing mix release to fail when the Elixir installation has no write permission. Possibly due to this commit: 52c58c7.

This issue directly affects nix, so I'll use nix for the repro steps

# that's the commit hash which introduced elixir 1.20.0-rc.3
# current master which points to rc.4 is broken too
nix shell github:NixOS/nixpkgs/6587ff4208bfff9745bbdd6b4c4e05fada36e800#beam.packages.erlang_28.elixir_1_20
mix new example
cd example
mix release

This gets us:

Compiling 1 file (.ex)
Generated example app
* assembling example-0.1.0 on MIX_ENV=dev
* skipping runtime configuration (config/runtime.exs not found)
* skipping elixir.bat for windows (bin/elixir.bat not found in the Elixir installation)
* skipping iex.bat for windows (bin/iex.bat not found in the Elixir installation)

14:00:26.810 [error] Task #PID<0.263.0> started from #PID<0.95.0> terminating
** (File.CopyError) could not copy recursively from "/nix/store/cg3wmh2h30cn586x3kb87did5qqddrwb-erlang-28.4/lib/erlang/erts-16.3/bin" to "/home/user/dev/example/_build/dev/rel/example/erts-16.3/bin". /nix/store/cg3wmh2h30cn586x3kb87did5qqddrwb-erlang-28.4/lib/erlang/erts-16.3/bin/escript: permission denied
    (elixir 1.20.0-rc.3) lib/file.ex:1236: File.cp_r!/3
    (mix 1.20.0-rc.3) lib/mix/release.ex:797: Mix.Release.copy_erts/1
    (mix 1.20.0-rc.3) lib/mix/tasks/release.ex:1391: Mix.Tasks.Release.copy/2
    (elixir 1.20.0-rc.3) lib/task/supervised.ex:105: Task.Supervised.invoke_mfa/2
    (elixir 1.20.0-rc.3) lib/task/supervised.ex:40: Task.Supervised.reply/4
Function: &:erlang.apply/2
    Args: [#Function<0.18801473/1 in Mix.Tasks.Release.assemble/1>, [:erts]]
** (EXIT from #PID<0.95.0>) an exception was raised:
    ** (File.CopyError) could not copy recursively from "/nix/store/cg3wmh2h30cn586x3kb87did5qqddrwb-erlang-28.4/lib/erlang/erts-16.3/bin" to "/home/user/dev/example/_build/dev/rel/example/erts-16.3/bin". /nix/store/cg3wmh2h30cn586x3kb87did5qqddrwb-erlang-28.4/lib/erlang/erts-16.3/bin/escript: permission denied
        (elixir 1.20.0-rc.3) lib/file.ex:1236: File.cp_r!/3
        (mix 1.20.0-rc.3) lib/mix/release.ex:797: Mix.Release.copy_erts/1
        (mix 1.20.0-rc.3) lib/mix/tasks/release.ex:1391: Mix.Tasks.Release.copy/2
        (elixir 1.20.0-rc.3) lib/task/supervised.ex:105: Task.Supervised.invoke_mfa/2
        (elixir 1.20.0-rc.3) lib/task/supervised.ex:40: Task.Supervised.reply/4

Expected behavior

It should build like it did in 1.20.0-rc.2 and earlier. I don't think this can be corrected on the nixpkgs side.

For example this:

# commit hash which introduced elixir 1.20.0-rc.2
nix shell github:NixOS/nixpkgs/218a9739a409258928ebb0378ccc2a2cbb6bb1fb#beam.packages.erlang_28.elixir_1_20
mix new example
cd example
mix release

Builds just fine:

Compiling 1 file (.ex)
Generated example app
* assembling example-0.1.0 on MIX_ENV=dev
* skipping runtime configuration (config/runtime.exs not found)
* skipping elixir.bat for windows (bin/elixir.bat not found in the Elixir installation)
* skipping iex.bat for windows (bin/iex.bat not found in the Elixir installation)

Release created at _build/dev/rel/example

    # To start your system
    _build/dev/rel/example/bin/example start

Once the release is running:

    # To connect to it remotely
    _build/dev/rel/example/bin/example remote

    # To stop it gracefully (you may also send SIGINT/SIGTERM)
    _build/dev/rel/example/bin/example stop

To list all commands:

    _build/dev/rel/example/bin/example