`ChildProcess.run` puts `null` or a string into `exitCode : Int` after a timeout, a signal or too much output

[gilramir]

Found against: gren 0.6.6, gren-lang/core 7.4.2, gren-lang/node 6.1.3, node 22
Reproduction: in https://github.com/gilramir/gren-bug-reports, 2026-09-14-childprocess-exitcode; ./run.sh prints the run
below.

Summary

ChildProcess.run fails with ProgramError { exitCode : Int, … } whenever the
program was started and did not succeed. When the program did not exit with a
code, which is what runDuration and a signal do, and when it wrote more than
maximumBytesWrittenToStreams, exitCode is not an Int:

how the program ended	String.fromInt e.exitCode	e.exitCode + 1	e.exitCode > 0
sh -c "exit 3"	3	4	True
stopped by runDuration = Milliseconds 100	null	1	False
wrote past maximumBytesWrittenToStreams = 4	ERR_CHILD_PROCESS_STDIO_MAXBUFFER	ERR_CHILD_PROCESS_STDIO_MAXBUFFER1	False
killed by a signal (kill -9 $$)	null	1	False

Every value in the last three rows is one no Int can be, so a program that
checks the exit code gets an answer that means nothing, and arithmetic on it
yields a string. A program cannot tell a timeout from a signal, or either from
too much output, except by inspecting a value its type says is a number.

The cause is in Gren/Kernel/ChildProcess.js, where execFile's error becomes
the ProgramError:

__$exitCode: err.code,

Node's err.code is the exit code only when the process exited. After a
timeout or a signal it is null, and err.killed and err.signal say what
happened. When the output passed maxBuffer it is the error's own code, the
string ERR_CHILD_PROCESS_STDIO_MAXBUFFER.

Reproduction

$ node app
exit 3: ProgramError, exitCode = 3, exitCode + 1 = 4, exitCode > 0 = True
killed by runDuration: ProgramError, exitCode = null, exitCode + 1 = 1, exitCode > 0 = False
past maximumBytesWrittenToStreams: ProgramError, exitCode = ERR_CHILD_PROCESS_STDIO_MAXBUFFER, exitCode + 1 = ERR_CHILD_PROCESS_STDIO_MAXBUFFER1, exitCode > 0 = False
killed by a signal: ProgramError, exitCode = null, exitCode + 1 = 1, exitCode > 0 = False