`ChildProcess.spawn` crashes the program when the program to spawn cannot be started

[gilramir]

Found against: gren 0.6.6, gren-lang/core 7.4.2, gren-lang/node 6.1.3, node 22
Reproduction: https://github.com/gilramir/gren-bug-reports/tree/main/2026-09-16-childprocess-spawn-error-crashes

ChildProcess.spawn with NoShell whose program cannot be started (it does
not exist, or is not executable) ends the whole Gren program with node's
Unhandled 'error' event. Neither onExit nor the connection message arrives,
so the application cannot handle the failure. With DefaultShell the shell
starts, fails to find the program and exits 127, which onExit receives.

Reproduction

gren.json dependencies: gren-lang/core 7.4.2, gren-lang/node 6.1.3. Save as src/Main.gren. Build and run with gren make Main --output=app && node app:

module Main exposing (main)                                                   
                                                                              
import Bytes exposing (Bytes)                                 
import ChildProcess exposing (Shell(..))                 
import Init             
import Node                                                         
import Stream
import Task             
                                                             
                                                                     
rows : Array { label : String, shell : Shell, program : String, arguments : Array String }
rows =            
    [ { label = "`DefaultShell`, `no-such-program`", shell = DefaultShell, program = "no-such-program", arguments = [] }
    , { label = "`NoShell`, `no-such-program`", shell = NoShell, program = "no-such-program", arguments = [] }
    ]                                     
                             
                                                                         
type Msg                                        
    = Spawn Int                                                                      
    | Exited { row : Int, code : Int }                                                       
    | Started

type alias Model =
    { cp : ChildProcess.Permission, stdout : Stream.Writable Bytes }


main : Node.Program Model Msg
main =
    Node.defineProgram { init = init, update = update, subscriptions = \_ -> Sub.none }


init : Node.Environment -> Init.Task { model : Model, command : Cmd Msg }
init env =
    Init.await ChildProcess.initialize <| \cp ->
        Node.startProgram
            { model = { cp = cp, stdout = env.stdout }
            , command = print env.stdout "| spawn | exit code | expected |\n|---|---|---|" (Spawn 0)
            }


print : Stream.Writable Bytes -> String -> Msg -> Cmd Msg
print stdout line next =
    Stream.writeLineAsBytes line stdout |> Task.attempt (\_ -> next)


update : Msg -> Model -> { model : Model, command : Cmd Msg }
update msg model =
    when msg is
        Spawn i ->
            { model = model
            , command =
                when Array.get i rows is
                    Just r ->
                        ChildProcess.spawn model.cp r.program r.arguments
                            { shell = r.shell
                            , workingDirectory = ChildProcess.InheritWorkingDirectory
                            , environmentVariables = ChildProcess.InheritEnvironmentVariables
                            , runDuration = ChildProcess.NoLimit
                            , connection = ChildProcess.Ignored (\_ -> Started)
                            , onExit = \code -> Exited { row = i, code = code }
                            }
                    Nothing ->
                        Cmd.none
            }

        Exited { row = i, code } ->
            { model = model
            , command =
                when Array.get i rows is
                    Just r ->
                        print model.stdout ("| " ++ r.label ++ " | " ++ String.fromInt code ++ " | non-zero |") (Spawn (i + 1))

                    Nothing ->
                        Cmd.none
            }

        Started ->
            { model = model, command = Cmd.none }

Output:

$ node app
| spawn | exit code | expected |
|---|---|---|
| `DefaultShell`, `no-such-program` | 127 | non-zero |
node:events:497
      throw er; // Unhandled 'error' event
      ^

Error: spawn no-such-program ENOENT
    at ChildProcess._handle.onexit (node:internal/child_process:285:19)
    at onErrorNT (node:internal/child_process:483:16)
    at process.processTicksAndRejections (node:internal/process/task_queues:89:21)
Emitted 'error' event on ChildProcess instance at:
    at ChildProcess._handle.onexit (node:internal/child_process:291:12)
    at onErrorNT (node:internal/child_process:483:16)
    at process.processTicksAndRejections (node:internal/process/task_queues:89:21) {
  errno: -2,
  code: 'ENOENT',
  syscall: 'spawn no-such-program',
  path: 'no-such-program',
  spawnargs: []
}

Node.js v22.23.2

Cause

Node reports a failed spawn asynchronously, as an error event on the
ChildProcess, and Gren/Kernel/ChildProcess.js installs no listener for it;
an error event with no listener throws. The kernel handles only a spawn that
throws synchronously, by sending onExit the error's errno:

} catch (e) {
  callback(
    __Scheduler_succeed(
      __Scheduler_rawSpawn(
        sendExitToApp(typeof e.errno === "undefined" ? -1 : e.errno),
      ),
    ),
  );