// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

// Package system implements native-host lifecycle: systemd unit install /
// uninstall, plus an optional self-update timer. Linux-only by nature; on
// other OSes the commands fail with a clear message rather than pretending.
package system

import (
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strings"
)

const (
	unitPath        = "/etc/systemd/system/echolot-server.service"
	updateUnitPath  = "/etc/systemd/system/echolot-server-update.service"
	updateTimerPath = "/etc/systemd/system/echolot-server-update.timer"
	envFilePath     = "/etc/echolot-server.env"
)

const unitTemplate = `[Unit]
Description=Echolot probe server
Documentation=https://echo-lot.app
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
ExecStart=%s --serve
Restart=on-failure
RestartSec=5
StateDirectory=echolot-server
Environment=ECHOLOT_STATE_DIR=/var/lib/echolot-server
# Host-specific config (listen addresses etc.) lives here, not in the unit:
EnvironmentFile=-%s
# Hardening — the server needs sockets and its state dir, nothing else.
NoNewPrivileges=true
ProtectSystem=strict
ProtectHome=true
ReadWritePaths=/var/lib/echolot-server
PrivateTmp=true

[Install]
WantedBy=multi-user.target
`

const updateUnitTemplate = `[Unit]
Description=Echolot server self-update
After=network-online.target

[Service]
Type=oneshot
ExecStart=%s --self-update --self-update-api=%s
# The updater only replaces the binary; the restart activates it.
ExecStartPost=/usr/bin/systemctl try-restart echolot-server.service
`

const updateTimerTemplate = `[Unit]
Description=Daily Echolot server self-update check

[Timer]
OnCalendar=daily
RandomizedDelaySec=1h
Persistent=true

[Install]
WantedBy=timers.target
`

const envFileTemplate = `# Echolot server host configuration (systemd EnvironmentFile).
# Bind explicit addresses on multi-IP hosts — a wildcard would also claim
# management-only addresses. Comma-separated lists are supported.
#ECHOLOT_CONTROL_LISTEN=203.0.113.10:8443,[2001:db8::10]:8443
#ECHOLOT_UDP_LISTEN=203.0.113.10:8442,[2001:db8::10]:8442
#ECHOLOT_TCP_LISTEN=203.0.113.10:8441,[2001:db8::10]:8441
#ECHOLOT_ADMIN_LISTEN=127.0.0.1:8444
#ECHOLOT_NAME=my-server
`

// InstallSystemd writes the unit(s) for THIS binary (absolute path), reloads
// systemd, and enables the service. When selfUpdateAPI is non-empty, a daily
// self-update timer is installed alongside. Idempotent.
func InstallSystemd(selfUpdateAPI string) error {
	if runtime.GOOS != "linux" {
		return fmt.Errorf("--install-systemd is Linux-only (this is %s)", runtime.GOOS)
	}
	self, err := os.Executable()
	if err != nil {
		return err
	}
	self, err = filepath.EvalSymlinks(self)
	if err != nil {
		return err
	}
	if err := os.WriteFile(unitPath, []byte(fmt.Sprintf(unitTemplate, self, envFilePath)), 0o644); err != nil {
		return fmt.Errorf("writing %s (need root?): %w", unitPath, err)
	}
	// Seed the env file once; never overwrite an existing one.
	if _, err := os.Stat(envFilePath); os.IsNotExist(err) {
		_ = os.WriteFile(envFilePath, []byte(envFileTemplate), 0o644)
	}
	cmds := [][]string{
		{"systemctl", "daemon-reload"},
		{"systemctl", "enable", "--now", "echolot-server.service"},
	}
	if selfUpdateAPI != "" {
		if err := os.WriteFile(updateUnitPath,
			[]byte(fmt.Sprintf(updateUnitTemplate, self, selfUpdateAPI)), 0o644); err != nil {
			return err
		}
		if err := os.WriteFile(updateTimerPath, []byte(updateTimerTemplate), 0o644); err != nil {
			return err
		}
		cmds = append(cmds, []string{"systemctl", "enable", "--now", "echolot-server-update.timer"})
	}
	for _, cmd := range cmds {
		if out, err := exec.Command(cmd[0], cmd[1:]...).CombinedOutput(); err != nil {
			return fmt.Errorf("%v: %s: %w", cmd, out, err)
		}
	}
	fmt.Printf("installed echolot-server.service (ExecStart=%s, config: %s)\n", self, envFilePath)
	if selfUpdateAPI != "" {
		fmt.Println("installed echolot-server-update.timer (daily, randomized)")
	}
	return nil
}

func UninstallSystemd() error {
	if runtime.GOOS != "linux" {
		return fmt.Errorf("--uninstall-systemd is Linux-only (this is %s)", runtime.GOOS)
	}
	// Stop/disable first; ignore "not loaded" errors so uninstall is idempotent.
	_ = exec.Command("systemctl", "disable", "--now", "echolot-server-update.timer").Run()
	_ = exec.Command("systemctl", "disable", "--now", "echolot-server.service").Run()
	for _, p := range []string{unitPath, updateUnitPath, updateTimerPath} {
		if err := os.Remove(p); err != nil && !os.IsNotExist(err) {
			return err
		}
	}
	_ = exec.Command("systemctl", "daemon-reload").Run()
	fmt.Println("removed echolot-server units (state dir and env file left in place)")
	return nil
}

// RepairExecStart brings an already-installed unit up to date with the current invocation.
//
// Serving became an explicit verb (--serve), which means every unit written before that change
// would start the binary with no arguments — and the binary now answers that with usage and a
// non-zero exit. A self-update replaces the binary but never the unit, so without this a routine
// update would leave a service that cannot start, discovered whenever the host next reboots.
//
// Only a unit this program wrote is touched, identified by its description line. Editing an
// operator's hand-written unit would be overreach; leaving ours broken would be negligence.
func RepairExecStart() (repaired bool, err error) {
	b, err := os.ReadFile(unitPath)
	if err != nil {
		return false, nil // no unit installed: nothing to repair, and not an error
	}
	text := string(b)
	if !strings.Contains(text, "Echolot probe server") {
		return false, nil // somebody else's unit
	}
	lines := strings.Split(text, "\n")
	changed := false
	for i, ln := range lines {
		t := strings.TrimSpace(ln)
		// Only the serving unit's ExecStart; the timer's own line already carries its verb.
		if strings.HasPrefix(t, "ExecStart=") && !strings.Contains(t, "--") {
			lines[i] = ln + " --serve"
			changed = true
		}
	}
	if !changed {
		return false, nil
	}
	if err := os.WriteFile(unitPath, []byte(strings.Join(lines, "\n")), 0o644); err != nil {
		return false, fmt.Errorf("updating %s: %w", unitPath, err)
	}
	_ = exec.Command("systemctl", "daemon-reload").Run()
	return true, nil
}