mesh-catalog/modules/hostname/cmd/hostname-tools/hostsfile.go

// The hosts file's own code (novox/hq ADR 0199): read /etc/hosts as the machine has it, and change the
// operator's lines — every line outside a `# BEGIN … / # END …` block — leaving every block, the mesh's
// and any other tool's, byte for byte. The mesh writes this module's block; these verbs never touch it.
//
// Root is the module's concern (ADR 0175 §4): the runtime launching this binary runs as the operator's
// account, so the file is written through sudo without a prompt where the account is not root, as the
// packet filter's is.
package main

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

// HostsPath is where the file is. The manifest's resource names the same path; a test holds the two
// together.
const HostsPath = "/etc/hosts"

// Operator is the owner of every line outside a block.
const Operator = "operator"

// Runner runs one command as root and answers what it printed, so the writes can be tested without a
// machine.
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.
func escalated(uid int, name string, args []string) (string, []string) {
	if uid == 0 {
		return name, args
	}
	return "sudo", append([]string{"-n", name}, args...)
}

func execRunner(ctx context.Context, name string, args ...string) (string, error) {
	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())
	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)
}

// Line is one line of the file, as a reader sees it.
type Line struct {
	// Text is the line exactly as it is in the file.
	Text string `json:"text"`
	// Owner is whose it is: the block's id (`mesh hostname.own`, or another tool's) or "operator".
	Owner string `json:"owner"`
	// Address and Names are an entry's; absent for a comment or a blank line.
	Address string   `json:"address,omitempty"`
	Names   []string `json:"names,omitempty"`
}

var (
	begin = regexp.MustCompile(`^#\s*BEGIN\s+(.+?)\s*$`)
	end   = regexp.MustCompile(`^#\s*END\s+(.+?)\s*$`)
)

// isAddress is whether s is an IPv4 or IPv6 address, as a hosts file's first field must be.
func isAddress(s string) bool {
	_, err := netip.ParseAddr(s)
	return err == nil
}

// Parse is every line of a hosts file, each marked whose it is.
func Parse(text string) []Line {
	out := []Line{}
	block := ""
	for _, raw := range strings.Split(text, "\n") {
		if block == "" {
			if m := begin.FindStringSubmatch(raw); m != nil {
				block = m[1]
				out = append(out, Line{Text: raw, Owner: block})
				continue
			}
		}
		owner := block
		if owner == "" {
			owner = Operator
		}
		line := Line{Text: raw, Owner: owner}
		entry, _, _ := strings.Cut(raw, "#")
		if fields := strings.Fields(entry); len(fields) >= 2 && isAddress(fields[0]) {
			line.Address, line.Names = fields[0], fields[1:]
		}
		out = append(out, line)
		if m := end.FindStringSubmatch(raw); block != "" && m != nil && m[1] == block {
			block = ""
		}
	}
	// A trailing newline splits into one empty last element; it is the file's ending, not a line.
	if n := len(out); n > 0 && out[n-1].Text == "" && strings.HasSuffix(text, "\n") {
		out = out[:n-1]
	}
	return out
}

var label = regexp.MustCompile(`^[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?$`)

// Refused input says why, so a caller is one edit from right.
func checkAddress(address string) error {
	if !isAddress(address) {
		return fmt.Errorf("%q is not an IPv4 or IPv6 address", address)
	}
	return nil
}

func checkName(name string) error {
	bad := fmt.Errorf("%q is not a host name", name)
	if len(name) < 1 || len(name) > 253 {
		return bad
	}
	for _, l := range strings.Split(strings.TrimSuffix(name, "."), ".") {
		if !label.MatchString(l) {
			return bad
		}
	}
	return nil
}

// WithAdded is the file with one address and its names added to the operator's lines; unchanged when
// they are already there.
func WithAdded(text, address string, names []string) (string, error) {
	if err := checkAddress(address); err != nil {
		return "", err
	}
	if len(names) == 0 {
		return "", errors.New("add names at least one name for the address")
	}
	for _, n := range names {
		if err := checkName(n); err != nil {
			return "", err
		}
	}
	have := map[string]bool{}
	for _, l := range Parse(text) {
		if l.Owner == Operator && l.Address == address {
			for _, n := range l.Names {
				have[n] = true
			}
		}
	}
	var missing []string
	for _, n := range names {
		if !have[n] && !slices.Contains(missing, n) {
			missing = append(missing, n)
		}
	}
	if len(missing) == 0 {
		return text, nil
	}
	body := text
	if body != "" && !strings.HasSuffix(body, "\n") {
		body += "\n"
	}
	return body + address + "\t" + strings.Join(missing, " ") + "\n", nil
}

// WithRemoved is the file with one name, or every line of one address, taken out of the operator's
// lines, and how many lines it touched. Blocks are never touched: a name only the mesh or another tool
// writes is refused, naming whose it is.
func WithRemoved(text, what string) (string, int, error) {
	byAddress := isAddress(what)
	if !byAddress {
		if err := checkName(what); err != nil {
			return "", 0, err
		}
	}
	matches := func(l Line) bool {
		if byAddress {
			return l.Address == what
		}
		return slices.Contains(l.Names, what)
	}
	lines := Parse(text)
	removed := 0
	kept := []string{}
	for _, l := range lines {
		if l.Owner != Operator || l.Address == "" || !matches(l) {
			kept = append(kept, l.Text)
			continue
		}
		removed++
		if byAddress {
			continue
		}
		var rest []string
		for _, n := range l.Names {
			if n != what {
				rest = append(rest, n)
			}
		}
		if len(rest) > 0 {
			kept = append(kept, l.Address+"\t"+strings.Join(rest, " "))
		}
	}
	if removed == 0 {
		for _, l := range lines {
			if l.Owner != Operator && matches(l) {
				return "", 0, fmt.Errorf("%s is written by %s, not the operator; it is not this verb's to remove", what, l.Owner)
			}
		}
	}
	return strings.Join(kept, "\n") + "\n", removed, nil
}

// HostsFile is the machine's hosts file.
type HostsFile struct {
	Path string
	Run  Runner
}

// Entries is the file's lines, each marked whose.
type Entries struct {
	Path  string `json:"path"`
	Lines []Line `json:"lines"`
}

func (h HostsFile) read() (string, error) {
	b, err := os.ReadFile(h.Path)
	return string(b), err
}

// Entries is every line of the file, each marked whose it is.
func (h HostsFile) Entries() (*Entries, error) {
	text, err := h.read()
	if err != nil {
		return nil, err
	}
	return &Entries{Path: h.Path, Lines: Parse(text)}, nil
}

// Added is whether add changed the file, and the line it wrote.
type Added struct {
	Added bool   `json:"added"`
	Line  string `json:"line,omitempty"`
}

// Add adds one address and its names to the operator's lines.
func (h HostsFile) Add(ctx context.Context, address string, names []string) (*Added, error) {
	before, err := h.read()
	if err != nil {
		return nil, err
	}
	after, err := WithAdded(before, address, names)
	if err != nil {
		return nil, err
	}
	if after == before {
		return &Added{Added: false}, nil
	}
	if err := h.write(ctx, after); err != nil {
		return nil, err
	}
	return &Added{Added: true, Line: strings.TrimSpace(after[len(before):])}, nil
}

// Removed is how many of the operator's lines remove touched.
type Removed struct {
	Removed int `json:"removed"`
}

// Remove takes one name, or every line of one address, out of the operator's lines.
func (h HostsFile) Remove(ctx context.Context, what string) (*Removed, error) {
	before, err := h.read()
	if err != nil {
		return nil, err
	}
	after, removed, err := WithRemoved(before, what)
	if err != nil {
		return nil, err
	}
	if removed > 0 {
		if err := h.write(ctx, after); err != nil {
			return nil, err
		}
	}
	return &Removed{Removed: removed}, nil
}

// write puts the file in place whole, so a reader never sees half of it: the content is staged in a
// private copy, installed as root beside the file — the same directory, so the same filesystem — and
// renamed over it.
func (h HostsFile) write(ctx context.Context, content string) error {
	dir, err := os.MkdirTemp("", "hosts-")
	if err != nil {
		return err
	}
	defer os.RemoveAll(dir)
	staged := filepath.Join(dir, "hosts")
	if err := os.WriteFile(staged, []byte(content), 0o644); err != nil {
		return err
	}
	beside := filepath.Join(filepath.Dir(h.Path), "."+filepath.Base(h.Path)+".hostname-tools")
	if _, err := h.Run(ctx, "install", "-m", "0644", staged, beside); err != nil {
		return err
	}
	if _, err := h.Run(ctx, "mv", "-f", beside, h.Path); err != nil {
		_, _ = h.Run(ctx, "rm", "-f", beside)
		return err
	}
	return nil
}