// fail2ban's own code, in the module (novox/hq ADR 0039). The jails are composed by the mesh from the
// modules a machine runs (to-be 31) and written as declared resources; the daemon is kept running by
// one. This code exists only to read and steer the *live* state the daemon owns: who is banned now
// and until when, and the ban or release an operator asks for — the node-intrusion-prevention seat's
// four verbs (ADR 0179). The daemon's state is fail2ban's, not the mesh's: the mesh composes the
// jails and never writes the ban list.
//
// Spoken through fail2ban-client over the daemon's socket. Client and daemon come from the one
// package this module declares on the machine, and the socket is root's: root is the module's
// concern (ADR 0175 §4), and the runtime launching this binary runs as the operator's account (to-be
// 38 WP4), so the client is run through sudo without a prompt where the account is not root.
package main

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"net"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"time"
)

// Runner runs one command and answers what it printed, so the verbs can be tested without a daemon.
type Runner func(ctx context.Context, name string, args ...string) (string, error)

// escalated is the command as it is run: as given when this process is root, else through sudo
// without a prompt. The daemon's socket answers only to root.
func escalated(uid int, name string, args []string) (string, []string) {
	if uid == 0 {
		return name, args
	}
	return "sudo", append([]string{"-n", name}, args...)
}

// installed is whether a tool is on this machine: an executable of that name on the path, or where
// the system keeps its administration.
func installed(tool, path string) bool {
	dirs := append(filepath.SplitList(path), "/usr/sbin", "/sbin", "/usr/bin")
	for _, dir := range dirs {
		if dir == "" {
			continue
		}
		if info, err := os.Stat(filepath.Join(dir, tool)); err == nil && !info.IsDir() && info.Mode()&0o111 != 0 {
			return true
		}
	}
	return false
}

var socketTrouble = regexp.MustCompile(`(?i)Failed to access socket path|Is fail2ban running|Permission denied to socket`)

func execRunner(ctx context.Context, name string, args ...string) (string, error) {
	if !installed(name, os.Getenv("PATH")) {
		return "", fmt.Errorf("%s is not installed on this machine", name)
	}
	ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
	defer cancel()
	program, argv := escalated(os.Getuid(), name, args)
	var stdout, stderr bytes.Buffer
	cmd := exec.CommandContext(ctx, program, argv...)
	cmd.Stdout, cmd.Stderr = &stdout, &stderr
	err := cmd.Run()
	if err == nil {
		return stdout.String(), nil
	}
	said := strings.TrimSpace(stdout.String() + stderr.String())
	// What failed is named by how it failed: sudo missing is a spawn error, sudo refusing speaks on
	// its own stderr line, and the rest is the client's own answer.
	if program == "sudo" {
		if errors.Is(err, exec.ErrNotFound) {
			return "", fmt.Errorf("%s needs root, and sudo is not installed here for the runtime's account to escalate with", name)
		}
		if regexp.MustCompile(`(?m)^sudo:`).MatchString(said) {
			return "", fmt.Errorf("%s needs root and the runtime's account may not run it without a prompt: %s", name, said)
		}
	}
	if socketTrouble.MatchString(said) {
		return "", errors.New("fail2ban is not running on this machine, or its socket does not answer the runtime's account")
	}
	// fail2ban-client's own last line is the one a person reads ("Sorry but the jail 'x' does not exist").
	var lines []string
	for _, l := range strings.Split(said, "\n") {
		if l = strings.TrimSpace(l); l != "" {
			lines = append(lines, l)
		}
	}
	if len(lines) > 0 {
		return "", errors.New(lines[len(lines)-1])
	}
	return "", fmt.Errorf("%s failed: %v", name, err)
}

// Counted is a jail's count now and since it started.
type Counted struct {
	Now   int `json:"now"`
	Total int `json:"total"`
}

// Held is what a jail holds: the count now and since it started, and the addresses.
type Held struct {
	Now       int      `json:"now"`
	Total     int      `json:"total"`
	Addresses []string `json:"addresses"`
}

// JailStatus is one jail as the daemon reports it.
type JailStatus struct {
	Jail string `json:"jail"`
	// Watching is what the jail is reading: files or journal matches, as fail2ban names them.
	Watching []string `json:"watching"`
	// Failing is the addresses with failures counted against them now, and all failures since the
	// jail started.
	Failing Counted `json:"failing"`
	// Banned is the addresses held right now, and all bans since the jail started.
	Banned Held `json:"banned"`
}

// Ban is one ban as the daemon holds it.
type Ban struct {
	IP   string `json:"ip"`
	Jail string `json:"jail"`
	// Since is when the ban was placed, in the machine's local time as fail2ban prints it.
	Since string `json:"since"`
	// Until is when the ban ends; "never" for a permanent ban.
	Until string `json:"until"`
}

// JailSettings is one jail's effective settings.
type JailSettings struct {
	Jail     string   `json:"jail"`
	Bantime  string   `json:"bantime"`
	Findtime string   `json:"findtime"`
	Maxretry int      `json:"maxretry"`
	Ignoreip []string `json:"ignoreip"`
	Actions  []string `json:"actions"`
	Logpath  []string `json:"logpath"`
	Journal  string   `json:"journalmatch"`
}

// Fail2ban is the daemon as this machine has it, through its own client.
type Fail2ban struct {
	Run Runner
}

func (f Fail2ban) client(ctx context.Context, args ...string) (string, error) {
	return f.Run(ctx, "fail2ban-client", args...)
}

var jailList = regexp.MustCompile(`Jail list:[ \t]*(.*)`)

// Jails is the jails the daemon runs, by name.
func (f Fail2ban) Jails(ctx context.Context) ([]string, error) {
	out, err := f.client(ctx, "status")
	if err != nil {
		return nil, err
	}
	m := jailList.FindStringSubmatch(out)
	if m == nil {
		return []string{}, nil
	}
	var jails []string
	for _, j := range strings.Split(m[1], ",") {
		if j = strings.TrimSpace(j); j != "" {
			jails = append(jails, j)
		}
	}
	return jails, nil
}

func (f Fail2ban) named(ctx context.Context, jail string) ([]string, error) {
	if jail != "" {
		return []string{jail}, nil
	}
	return f.Jails(ctx)
}

// Status is every jail with what it watches and holds, or one jail's detail.
func (f Fail2ban) Status(ctx context.Context, jail string) (map[string][]JailStatus, error) {
	names, err := f.named(ctx, jail)
	if err != nil {
		return nil, err
	}
	jails := []JailStatus{}
	for _, name := range names {
		out, err := f.client(ctx, "status", name)
		if err != nil {
			return nil, err
		}
		jails = append(jails, parseJailStatus(name, out))
	}
	return map[string][]JailStatus{"jails": jails}, nil
}

// Banned is every address banned now, with the jail holding it and when the ban ends, soonest to
// end first.
func (f Fail2ban) Banned(ctx context.Context, jail string) (map[string][]Ban, error) {
	names, err := f.named(ctx, jail)
	if err != nil {
		return nil, err
	}
	banned := []Ban{}
	for _, name := range names {
		out, err := f.client(ctx, "get", name, "banip", "--with-time")
		if err != nil {
			return nil, err
		}
		banned = append(banned, parseBans(name, out)...)
	}
	sort.SliceStable(banned, func(a, b int) bool {
		if banned[a].Until != banned[b].Until {
			return banned[a].Until < banned[b].Until
		}
		return banned[a].IP < banned[b].IP
	})
	return map[string][]Ban{"banned": banned}, nil
}

// BanOutcome is a ban as held, and how many addresses the daemon said it added.
type BanOutcome struct {
	Banned *Ban `json:"banned"`
	Added  int  `json:"added"`
}

// Ban bans one address in one jail now. The daemon's own answer is how many addresses it added.
func (f Fail2ban) Ban(ctx context.Context, ip, jail string) (*BanOutcome, error) {
	if err := address(ip); err != nil {
		return nil, err
	}
	if err := jailName(jail); err != nil {
		return nil, err
	}
	out, err := f.client(ctx, "set", jail, "banip", ip)
	if err != nil {
		return nil, err
	}
	added, _ := strconv.Atoi(strings.TrimSpace(out))
	held, err := f.Banned(ctx, jail)
	if err != nil {
		return nil, err
	}
	outcome := &BanOutcome{Added: added}
	for _, b := range held["banned"] {
		if b.IP == ip {
			b := b
			outcome.Banned = &b
		}
	}
	return outcome, nil
}

// Released is how many bans the daemon let go, of which address, from where.
type Released struct {
	Released int    `json:"released"`
	IP       string `json:"ip"`
	Jail     string `json:"jail"`
}

// Unban lets one address go, from one jail or from every jail. The daemon's answer is how many it
// released.
func (f Fail2ban) Unban(ctx context.Context, ip, jail string) (*Released, error) {
	if err := address(ip); err != nil {
		return nil, err
	}
	var out string
	var err error
	if jail != "" {
		if err := jailName(jail); err != nil {
			return nil, err
		}
		out, err = f.client(ctx, "set", jail, "unbanip", ip)
	} else {
		out, err = f.client(ctx, "unban", ip)
		jail = "every jail"
	}
	if err != nil {
		return nil, err
	}
	released, _ := strconv.Atoi(strings.TrimSpace(out))
	return &Released{Released: released, IP: ip, Jail: jail}, nil
}

// Settings is one jail's effective settings — the module's own tool, beside the seat's verbs.
func (f Fail2ban) Settings(ctx context.Context, jail string) (*JailSettings, error) {
	if err := jailName(jail); err != nil {
		return nil, err
	}
	got := map[string]string{}
	for _, key := range []string{"bantime", "findtime", "maxretry", "ignoreip", "actions", "logpath", "journalmatch"} {
		out, err := f.client(ctx, "get", jail, key)
		if err != nil {
			return nil, err
		}
		got[key] = out
	}
	maxretry, _ := strconv.Atoi(strings.TrimSpace(got["maxretry"]))
	s := &JailSettings{
		Jail:     jail,
		Bantime:  strings.TrimSpace(got["bantime"]),
		Findtime: strings.TrimSpace(got["findtime"]),
		Maxretry: maxretry,
		Ignoreip: listed(got["ignoreip"]),
		Actions:  afterHeading(got["actions"]),
		Logpath:  []string{},
		Journal:  strings.Join(afterHeading(got["journalmatch"]), " "),
	}
	if !strings.Contains(got["logpath"], "No file is currently monitored") {
		s.Logpath = listed(got["logpath"])
	}
	return s, nil
}

var treeMarks = regexp.MustCompile("^[\\s|`-]+")

// listed reads fail2ban's tree listings: lines like "|- 127.0.0.0/8" and "`- ::1", after a heading.
func listed(out string) []string {
	items := []string{}
	for i, l := range strings.Split(out, "\n") {
		l = strings.TrimSpace(treeMarks.ReplaceAllString(l, ""))
		if i > 0 && l != "" {
			items = append(items, l)
		}
	}
	return items
}

// afterHeading is every non-empty line after the first, trimmed.
func afterHeading(out string) []string {
	items := []string{}
	for i, l := range strings.Split(out, "\n") {
		if l = strings.TrimSpace(l); i > 0 && l != "" {
			items = append(items, l)
		}
	}
	return items
}

func parseJailStatus(jail, out string) JailStatus {
	field := func(label string) string {
		m := regexp.MustCompile(regexp.QuoteMeta(label) + `:\t?[ \t]*(.*)`).FindStringSubmatch(out)
		if m == nil {
			return ""
		}
		return strings.TrimSpace(m[1])
	}
	num := func(label string) int {
		n, _ := strconv.Atoi(field(label))
		return n
	}
	watching := []string{}
	for _, w := range []string{field("File list"), field("Journal matches")} {
		if w != "" {
			watching = append(watching, w)
		}
	}
	addresses := strings.Fields(field("Banned IP list"))
	if addresses == nil {
		addresses = []string{}
	}
	return JailStatus{
		Jail:     jail,
		Watching: watching,
		Failing:  Counted{Now: num("Currently failed"), Total: num("Total failed")},
		Banned:   Held{Now: num("Currently banned"), Total: num("Total banned"), Addresses: addresses},
	}
}

var banLine = regexp.MustCompile(`^(\S+)\s+(\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}) \+ (-?\d+) = (\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}|\S+)`)

// parseBans reads `get <jail> banip --with-time`, one ban per line: "IP \tsince + seconds = until".
func parseBans(jail, out string) []Ban {
	bans := []Ban{}
	for _, line := range strings.Split(out, "\n") {
		m := banLine.FindStringSubmatch(line)
		if m == nil {
			continue
		}
		until := m[4]
		if seconds, _ := strconv.Atoi(m[3]); seconds < 0 {
			until = "never"
		}
		bans = append(bans, Ban{IP: m[1], Jail: jail, Since: m[2], Until: until})
	}
	return bans
}

func address(ip string) error {
	if net.ParseIP(ip) == nil {
		return fmt.Errorf("%q is not an address", ip)
	}
	return nil
}

var jailNamed = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9._-]*$`)

func jailName(jail string) error {
	if !jailNamed.MatchString(jail) {
		return fmt.Errorf("%q is not a jail's name", jail)
	}
	return nil
}