package daemon

import (
	"fmt"
	"os"
	"os/exec"
)

// Spawn starts `exe daemon run` as an independent background process (own
// session or own process group, detached from the terminal), with
// stdout/stderr going to the log file logPath (created fresh per start;
// empty = discard). env is the child's environment (keys, TALK_*
// overrides).
//
// The return value is a channel that delivers the Wait result exactly once
// when the child exits; the caller uses it to detect a failed start (e.g.
// port in use) without waiting for a timeout. If the caller exits first,
// the daemon keeps running (that is the point).
func Spawn(exe, logPath string, env []string) (<-chan error, error) {
	var logFile *os.File
	if logPath == "" {
		logPath = os.DevNull
	}
	logFile, err := os.OpenFile(logPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o644)
	if err != nil {
		return nil, fmt.Errorf("daemon log %s: %w", logPath, err)
	}
	defer logFile.Close() // the child holds its own handle

	cmd := exec.Command(exe, "daemon", "run")
	cmd.Env = env
	cmd.Stdin = nil
	cmd.Stdout = logFile
	cmd.Stderr = logFile
	detach(cmd)
	if err := cmd.Start(); err != nil {
		return nil, fmt.Errorf("starting daemon (%s daemon run): %w", exe, err)
	}
	exited := make(chan error, 1)
	go func() { exited <- cmd.Wait() }()
	return exited, nil
}