// The node-resolver seat's verbs, done by systemd-resolved (novox/hq ADR 0247): what is routed where,
// route a set of domains to a set of servers over one link, and take a link's route away.
//
// **resolved holds the routes, not this code.** A link's servers and routing domains are resolved's own
// per-link state, set through resolvectl and forgotten by resolved when the link goes. So nothing here
// keeps a table that could disagree with what resolved does: `routes` reads resolved, and the two
// transports that serve these verbs — the mesh, through the node's runtime as the operator account, and
// the machine, through the guard's socket as root — run the same code against the same daemon.
//
// **It knows nothing of any VPN.** A link, domains and servers. What a VPN client pushed is its own
// module's to read and hand over.
package main

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

// Runner runs one command — as root when it changes something — and answers what it printed.
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 (the guard), else through
// sudo without a prompt (the runtime's account), as the hosts file's and the packet filter's verbs do.
func escalated(uid int, name string, args []string) (string, []string) {
	if uid == 0 {
		return name, args
	}
	return "sudo", append([]string{"-n", name}, args...)
}

// execRunner runs a command that changes resolved's state, escalated.
func execRunner(ctx context.Context, name string, args ...string) (string, error) {
	return run(ctx, true, name, args...)
}

// readRunner runs a command that only reads, as whoever this process is.
func readRunner(ctx context.Context, name string, args ...string) (string, error) {
	return run(ctx, false, name, args...)
}

func run(ctx context.Context, escalate bool, name string, args ...string) (string, error) {
	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
	defer cancel()
	program, argv := name, args
	if escalate {
		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())
	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 said != "" {
		return "", fmt.Errorf("%s: %s", name, said)
	}
	return "", fmt.Errorf("%s failed: %v", name, err)
}

// Resolver is systemd-resolved on this machine, as the seat's verbs see it.
type Resolver struct {
	// Change runs what changes resolved (escalated); Read what only reads it.
	Change, Read Runner
	// NetDir is where the machine's links are listed (/sys/class/net).
	NetDir string
	// SuffixFile holds the mesh's own domain, which is never routed elsewhere.
	SuffixFile string
}

// ThisResolver is the machine's.
func ThisResolver() *Resolver {
	return &Resolver{Change: execRunner, Read: readRunner, NetDir: "/sys/class/net", SuffixFile: SuffixPath}
}

// SuffixPath is the file the mesh renders the mesh's own domain into (the manifest's fact `suffix`).
const SuffixPath = "/etc/node-resolver/suffix"

// Scope is one place resolved sends names: the machine's global servers (the mesh's resolvers), or a link.
type Scope struct {
	Link    string   `json:"link,omitempty"`
	Servers []string `json:"servers"`
	// Domains are the routing domains, without resolved's `~`: every name under one goes to these servers.
	Domains []string `json:"domains"`
	// DefaultRoute is whether names no domain routes may also go here. Only the mesh's resolvers are.
	DefaultRoute *bool `json:"default_route,omitempty"`
}

// Routes is what resolved sends where.
type Routes struct {
	// Mesh is the global scope: the mesh's resolvers, which answer every name nothing routes elsewhere.
	Mesh Scope `json:"mesh"`
	// Links is every link given servers of its own.
	Links []Scope `json:"links"`
}

var linkLine = regexp.MustCompile(`^Link\s+\d+\s+\(([^)]+)\):\s*(.*)$`)

// perScope reads one resolvectl listing (`dns`, `domain`, `default-route`) into the global line and one
// line per link.
func perScope(out string) (global []string, links map[string][]string) {
	links = map[string][]string{}
	for _, line := range strings.Split(out, "\n") {
		line = strings.TrimSpace(line)
		if rest, ok := strings.CutPrefix(line, "Global:"); ok {
			global = strings.Fields(rest)
			continue
		}
		if m := linkLine.FindStringSubmatch(line); m != nil {
			links[m[1]] = strings.Fields(m[2])
		}
	}
	return global, links
}

func unrouted(domains []string) []string {
	out := make([]string, 0, len(domains))
	for _, d := range domains {
		out = append(out, strings.TrimPrefix(d, "~"))
	}
	return out
}

// Routes reads what resolved sends where. It changes nothing and needs no root.
func (r *Resolver) Routes(ctx context.Context) (*Routes, error) {
	dns, err := r.Read(ctx, "resolvectl", "dns")
	if err != nil {
		return nil, fmt.Errorf("systemd-resolved does not answer: %w", err)
	}
	domain, err := r.Read(ctx, "resolvectl", "domain")
	if err != nil {
		return nil, fmt.Errorf("systemd-resolved does not answer: %w", err)
	}
	defaults, _ := r.Read(ctx, "resolvectl", "default-route")
	gServers, lServers := perScope(dns)
	gDomains, lDomains := perScope(domain)
	_, lDefault := perScope(defaults)
	out := &Routes{Mesh: Scope{Servers: orEmpty(gServers), Domains: orEmpty(unrouted(gDomains))}, Links: []Scope{}}
	names := make([]string, 0, len(lServers))
	for name, servers := range lServers {
		if len(servers) > 0 {
			names = append(names, name)
		}
	}
	sort.Strings(names)
	for _, name := range names {
		s := Scope{Link: name, Servers: lServers[name], Domains: orEmpty(unrouted(lDomains[name]))}
		if d, ok := lDefault[name]; ok && len(d) > 0 {
			yes := d[0] == "yes"
			s.DefaultRoute = &yes
		}
		out.Links = append(out.Links, s)
	}
	return out, nil
}

func orEmpty(s []string) []string {
	if s == nil {
		return []string{}
	}
	return s
}

// Routed is what a route did.
type Routed struct {
	Link    string   `json:"link"`
	Servers []string `json:"servers"`
	Domains []string `json:"domains"`
	Said    string   `json:"said"`
}

var (
	linkName   = regexp.MustCompile(`^[A-Za-z0-9_.:@-]{1,15}$`)
	domainName = regexp.MustCompile(`^([a-z0-9_]([a-z0-9_-]{0,61}[a-z0-9_])?\.)*[a-z0-9_]([a-z0-9_-]{0,61}[a-z0-9_])?$`)
	separators = regexp.MustCompile(`[\s,]+`)
)

// Split reads a list given as one string, separated by spaces or commas, or as a list.
func Split(v any) []string {
	var raw []string
	switch v := v.(type) {
	case string:
		raw = separators.Split(v, -1)
	case []any:
		for _, x := range v {
			if s, ok := x.(string); ok {
				raw = append(raw, separators.Split(s, -1)...)
			}
		}
	case []string:
		for _, s := range v {
			raw = append(raw, separators.Split(s, -1)...)
		}
	}
	out := []string{}
	for _, s := range raw {
		if s = strings.TrimSpace(s); s != "" {
			out = append(out, s)
		}
	}
	return out
}

// suffix is the mesh's own domain, as the mesh rendered it; "internal" when it has not been yet.
func (r *Resolver) suffix() string {
	raw, err := os.ReadFile(r.SuffixFile)
	if s := strings.Trim(strings.TrimSpace(string(raw)), "."); err == nil && s != "" {
		return strings.ToLower(s)
	}
	return "internal"
}

// checkLink refuses a link that is not one, loopback, and one that is not on this machine now.
func (r *Resolver) checkLink(link string) error {
	if !linkName.MatchString(link) {
		return fmt.Errorf("%q is not a link's name", link)
	}
	if link == "lo" {
		return errors.New("loopback carries no servers of its own")
	}
	if _, err := os.Stat(filepath.Join(r.NetDir, link)); err != nil {
		return fmt.Errorf("there is no link %q on this machine now", link)
	}
	return nil
}

// Domains reads the domains to route: lower-cased, without resolved's `~` or a final dot, each once. The
// root and the mesh's own domain are refused: routing either away would send the mesh's names, or every
// name, to servers that are not the mesh's (ADR 0223, ADR 0247).
func (r *Resolver) Domains(given []string) ([]string, error) {
	suffix := r.suffix()
	seen := map[string]bool{}
	out := []string{}
	for _, d := range given {
		d = strings.ToLower(strings.TrimSuffix(strings.TrimPrefix(d, "~"), "."))
		if d == "" {
			return nil, errors.New("the root domain is every name: only the mesh's resolvers answer every name")
		}
		if !domainName.MatchString(d) || len(d) > 253 {
			return nil, fmt.Errorf("%q is not a domain", d)
		}
		if d == suffix || strings.HasSuffix(d, "."+suffix) {
			return nil, fmt.Errorf("%q is the mesh's own domain: the mesh's names are answered by the mesh's resolvers alone", d)
		}
		if !seen[d] {
			seen[d] = true
			out = append(out, d)
		}
	}
	if len(out) == 0 {
		return nil, errors.New("no domain given: a link's servers answer only the domains routed to them")
	}
	if len(out) > 64 {
		return nil, fmt.Errorf("%d domains; at most 64", len(out))
	}
	return out, nil
}

// Servers reads the servers: addresses, each once, at most eight.
func Servers(given []string) ([]string, error) {
	seen := map[string]bool{}
	out := []string{}
	for _, s := range given {
		a, err := netip.ParseAddr(s)
		if err != nil {
			return nil, fmt.Errorf("%q is not an address", s)
		}
		if a.IsUnspecified() || a.IsMulticast() {
			return nil, fmt.Errorf("%s cannot answer names", s)
		}
		if !seen[a.String()] {
			seen[a.String()] = true
			out = append(out, a.String())
		}
	}
	if len(out) == 0 {
		return nil, errors.New("no server given")
	}
	if len(out) > 8 {
		return nil, fmt.Errorf("%d servers; at most 8", len(out))
	}
	return out, nil
}

// Route sends these domains, and every name under them, to these servers over this link — and only them.
// Whatever the link was given before is replaced. resolved forgets it when the link goes.
func (r *Resolver) Route(ctx context.Context, link string, domains, servers []string) (*Routed, error) {
	if err := r.checkLink(link); err != nil {
		return nil, err
	}
	ds, err := r.Domains(domains)
	if err != nil {
		return nil, err
	}
	ss, err := Servers(servers)
	if err != nil {
		return nil, err
	}
	routing := make([]string, len(ds))
	for i, d := range ds {
		routing[i] = "~" + d
	}
	// The link is never a default route: names no domain routes go to the mesh's resolvers, so set
	// first, before the servers, that no question but these domains' ever reaches it.
	steps := [][]string{
		{"default-route", link, "false"},
		append([]string{"domain", link}, routing...),
		append([]string{"dns", link}, ss...),
	}
	for _, s := range steps {
		if _, err := r.Change(ctx, "resolvectl", s...); err != nil {
			return nil, err
		}
	}
	return &Routed{Link: link, Servers: ss, Domains: ds,
		Said: fmt.Sprintf("%d domains go to %d servers over %s; every other name to the mesh's resolvers", len(ds), len(ss), link)}, nil
}

// Unrouted is what taking a route away did.
type Unrouted struct {
	Link string `json:"link"`
	Said string `json:"said"`
}

// Unroute takes one link's route away. A link that has gone has nothing to take away.
func (r *Resolver) Unroute(ctx context.Context, link string) (*Unrouted, error) {
	if !linkName.MatchString(link) || link == "lo" {
		return nil, fmt.Errorf("%q is not a link's name", link)
	}
	if _, err := os.Stat(filepath.Join(r.NetDir, link)); err != nil {
		return &Unrouted{Link: link, Said: link + " is not on this machine; resolved forgot its route with it"}, nil
	}
	if _, err := r.Change(ctx, "resolvectl", "revert", link); err != nil {
		return nil, err
	}
	return &Unrouted{Link: link, Said: link + "'s domains go to the mesh's resolvers again"}, nil
}

// OnlyTheMeshIsADefaultRoute keeps every link that has servers of its own from answering names nothing
// routes to it: a network manager telling resolved a network's servers makes them a default route, and
// then resolved asks them every name beside the mesh's resolvers. Their routing domains are kept — a
// link's own domains still go to it. Answers the links it changed.
func (r *Resolver) OnlyTheMeshIsADefaultRoute(ctx context.Context) ([]string, error) {
	routes, err := r.Routes(ctx)
	if err != nil {
		return nil, err
	}
	var changed []string
	for _, l := range routes.Links {
		if l.DefaultRoute == nil || !*l.DefaultRoute {
			continue
		}
		if _, err := r.Change(ctx, "resolvectl", "default-route", l.Link, "false"); err != nil {
			return changed, err
		}
		changed = append(changed, l.Link)
	}
	return changed, nil
}