MacOS – Launchd based daemon exit with abnormal code 1, without crash log

Summarized from man launchd.plist:

LowPriorityBackgroundIO: Specifies whether the kernel should consider this daemon to be low priority when doing file system I/O when the process is throttled with Darwin-background classification.

ProcessType: This optional key describes, at a high level, the intended purpose of the job. The system will apply resource limits based on what kind of job it is. If left unspecified, the system will apply light resource limits to the job, throttling its CPU usage and I/O bandwidth.

ProcessType=Background: Background jobs are generally processes that do work that was not directly requested by the user. The resource limits applied to the Background are intended to prevent them from disrupting the user experience.

The traditional approach, e.g. with cron jobs, is to pipe standard error to a program like mail that's smart enough not to send you empty mail. The difference with launchd, as you've discovered, is that the mechanism for redirecting standard error is giving a StandardErrorPath for it to be written to, which isn't as convenient for this purpose as ending your crontab entry with | mail ….

My usual solution is to have a wrapper script check the StandardErrorPath and notify me if there's a problem. This can either be part of the same launchd job, so the checking happens before the next scheduled run, or you can have a separate job that just manages the error logs (maybe using a QueueDirectories key).

I think you could also, for example, use a named pipe as your job's StandardErrorPath, but I've never actually tried that.