Handle signals in main

The kit handles no signals (decision 21): forbidigo refuses signal.Notify and its relatives in every package. The program’s main owns them. Four programs carry the same twenty lines; this is the shape.

func main() {
	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
	defer stop()

	err := run(ctx)
	stop() // a second Ctrl-C now ends the process the usual way
	os.Exit(exitCode(err))
}

Rules#

• One context. run gets ctx and hands it down. Every pool, every child process and every request is made under it, so one interrupt ends them all, and fanout reports the items it did not start as skipped.
• Interrupt once. The first signal cancels ctx. A program that starts child commands sends them SIGINT once, starts none during the shutdown, and uses cmd.WaitDelay so that a child that ignores it is killed rather than waited for.
• The second signal is the default. stop() after run returns, or a second NotifyContext that is released at the first signal, gives back the default action so that a person can end a stuck shutdown.
• Exit code in the program. exitCode maps the error to the program’s numbers. cobratree.IsUsage marks a usage error, and the program’s Classify hook gives progress the skips and failures (decision 21, decision 24). An interrupt exits with the number the program documents, 130 by convention.
• Deferred work runs. os.Exit skips deferred calls, so run returns first and closes the display and the event log, as cliprogress.Session does, and only then does main exit.
• A server shuts down gracefully. A program with a local web server calls Shutdown with a short deadline when ctx is done, then closes the SSE streams, then removes its runtime files.