Add blocking: blocklists and block/allow rules with BIND response policy zones

jarld downloads blocklists (Adblock, hosts or plain domains), checks them
daily and reloads named only when a list changed; your own block and
allow rules take precedence, and the most specific rule wins. Lists and
rules apply per view (default: every view that offers recursion). Web UI
Blocking page with name checks, jarl blocklist and jarl block, the
/blocking API, blocked counts in the statistics. Database schema 6.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/blocking/blocking_test.go

@@ -0,0 +1,180 @@
+package blocking
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestParse(t *testing.T) {
+	src := "\uFEFF[Adblock Plus]\r\n" + `! Title: test list
+# a comment
+
+||ads.example^
+||Tracker.Example.^$important
+||cosmetic.example^$third-party
+@@||allowed.example^
+example.org##.banner
+|https://url.example/path
+0.0.0.0 hosts.example other.example  # two names
+127.0.0.1 localhost
+127.0.0.1 localhost.localdomain
+::1 ip6-localhost
+0.0.0.0 0.0.0.0
+plain.example
+*.wild.example
+single
+1.2.3.4
+bad_label!.example
+münchen.example
+www.ads.example
+` + strings.Repeat("a", 64) + ".example\n"
+	l, err := Parse(strings.NewReader(src), Limits{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := map[string]uint8{
+		"ads.example": Exact | Below, "tracker.example": Exact | Below,
+		"hosts.example": Exact, "other.example": Exact, "plain.example": Exact,
+		"wild.example": Exact | Below, "xn--mnchen-3ya.example": Exact, "www.ads.example": Exact,
+	}
+	if len(l.Domains) != len(want) {
+		t.Errorf("domains = %v", l.Domains)
+	}
+	for n, f := range want {
+		if l.Domains[n] != f {
+			t.Errorf("%s = %d, want %d", n, l.Domains[n], f)
+		}
+	}
+	// Skipped: $third-party, @@, ##, |url, localhost (x3), 0.0.0.0, single, 1.2.3.4, bad_label!, 64-char label.
+	if l.Skipped != 12 || l.Lines != 19 {
+		t.Errorf("lines %d, skipped %d", l.Lines, l.Skipped)
+	}
+
+	Compact(l)
+	if _, ok := l.Domains["www.ads.example"]; ok {
+		t.Error("www.ads.example is covered by ||ads.example^ and should be dropped")
+	}
+	var sb strings.Builder
+	n, hash, err := FeedZone(&sb, "test", 7, l)
+	if err != nil || n != 10 {
+		t.Fatalf("FeedZone: %d records, %v", n, err)
+	}
+	for _, line := range []string{"ads.example\tCNAME\t.\n", "*.ads.example\tCNAME\t.\n", "plain.example\tCNAME\t.\n", "@\tSOA\tlocalhost. hostmaster.localhost. 7 "} {
+		if !strings.Contains(sb.String(), line) {
+			t.Errorf("zone lacks %q:\n%s", line, sb.String())
+		}
+	}
+	if strings.Contains(sb.String(), "*.plain.example") || strings.Contains(sb.String(), "$ORIGIN") {
+		t.Errorf("zone:\n%s", sb.String())
+	}
+	// The hash ignores the serial.
+	var other strings.Builder
+	if _, h2, _ := FeedZone(&other, "test", 8, l); h2 != hash {
+		t.Error("hash depends on the serial")
+	}
+}
+
+func TestParseLimits(t *testing.T) {
+	_, err := Parse(strings.NewReader("a.example\nb.example\na.example\nc.example\n"), Limits{MaxEntries: 2})
+	if !errors.Is(err, ErrTooManyEntries) {
+		t.Errorf("err = %v", err)
+	}
+	if _, err := Parse(strings.NewReader("a.example\nb.example\na.example\n"), Limits{MaxEntries: 2}); err != nil {
+		t.Errorf("a repeated name counted against the limit: %v", err)
+	}
+	l, _ := Parse(strings.NewReader("long-name.example\nshort.example\n"), Limits{MaxNameLen: 15})
+	if len(l.Domains) != 1 || l.Skipped != 1 {
+		t.Errorf("max name length: %v", l.Domains)
+	}
+}
+
+func TestRuleEntries(t *testing.T) {
+	rules := []Rule{
+		{Name: "ads.example", Action: ActionBlock},                        // and everything below
+		{Name: "cdn.ads.example", Action: ActionAllow, Exact: true},       // just this name
+		{Name: "wide.example", Action: ActionBlock},                       // and below
+		{Name: "ok.wide.example", Action: ActionAllow},                    // and below
+		{Name: "deep.x.y.wide.example", Action: ActionAllow, Exact: true}, // two empty names in between
+		{Name: "solo.example", Action: ActionBlock, Exact: true},
+	}
+	e := RuleEntries(rules)
+	for name, want := range map[string]string{
+		"ads.example":                 ActionBlock,
+		"x.ads.example":               ActionBlock,
+		"cdn.ads.example":             ActionAllow,
+		"y.cdn.ads.example":           ActionBlock, // the wildcard above still applies
+		"ok.wide.example":             ActionAllow,
+		"a.ok.wide.example":           ActionAllow,
+		"y.wide.example":              ActionBlock, // exists only because of a deeper rule
+		"x.y.wide.example":            ActionBlock,
+		"z.x.y.wide.example":          ActionBlock,
+		"deep.x.y.wide.example":       ActionAllow,
+		"below.deep.x.y.wide.example": ActionBlock,
+		"solo.example":                ActionBlock,
+		"other.example":               "",
+		"sub.solo.example":            "",
+		"example":                     "",
+		"unrelated.org":               "",
+	} {
+		_, act, ok := MatchRules(e, name)
+		if !ok {
+			act = ""
+		}
+		if act != want {
+			t.Errorf("%s: %q, want %q", name, act, want)
+		}
+	}
+	z := RulesZoneFile("rules", rules)
+	if !strings.Contains(z, "cdn.ads.example\tCNAME\trpz-passthru.\n*.cdn.ads.example\tCNAME\t.\n") {
+		t.Errorf("rules zone:\n%s", z)
+	}
+	if strings.Index(z, "ads.example\t") > strings.Index(z, "*.ads.example\t") {
+		t.Errorf("a name should come before its wildcard:\n%s", z)
+	}
+}
+
+func TestMatchFeed(t *testing.T) {
+	l, _ := Parse(strings.NewReader("||tracker.example^\nexact.example\n||deep.a.example^\n"), Limits{})
+	Compact(l)
+	path := filepath.Join(t.TempDir(), "list.rpz")
+	f, _ := os.Create(path)
+	FeedZone(f, "t", 1, l)
+	f.Close()
+	for name, want := range map[string]string{
+		"tracker.example":     "tracker.example",
+		"a.b.tracker.example": "*.tracker.example",
+		"exact.example":       "exact.example",
+		"sub.exact.example":   "",
+		"x.deep.a.example":    "*.deep.a.example",
+		"a.example":           "",
+		"example":             "",
+	} {
+		owner, ok, err := MatchFeed(path, name)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !ok {
+			owner = ""
+		}
+		if owner != want {
+			t.Errorf("%s: %q, want %q", name, owner, want)
+		}
+	}
+}
+
+func TestNames(t *testing.T) {
+	if ListZone("hagezi") != "hagezi.blocklist.jarl.invalid" || ListName("hagezi.blocklist.jarl.invalid.") != "hagezi" || ListName(RulesZone) != "" || ListName("x.y.blocklist.jarl.invalid") != "" {
+		t.Error("zone names")
+	}
+	if n, err := NormalizeName("Ads.Example.COM."); err != nil || n != "ads.example.com" {
+		t.Errorf("NormalizeName = %q, %v", n, err)
+	}
+	for _, bad := range []string{"com", "1.2.3.4", "a b.example", "*.example.com", ""} {
+		if _, err := NormalizeName(bad); err == nil {
+			t.Errorf("%q accepted", bad)
+		}
+	}
+}

internal/blocking/parse.go

@@ -0,0 +1,198 @@
+// Package blocking turns blocklists and block/allow rules into BIND
+// response policy zones (RPZ), and answers which rule or list matches a
+// name.
+//
+// A policy zone holds one rule per owner name: "ads.example CNAME ."
+// answers NXDOMAIN for ads.example, "*.ads.example CNAME ." for every name
+// below it, and "CNAME rpz-passthru." answers normally (an exception).
+// named looks names up in a policy zone like in any zone, so wildcards
+// follow DNS rules: a wildcard doesn't apply below a name that exists in
+// the zone. The writers here add the rules that keep "the most specific
+// rule wins" true despite that (see RulesZone).
+package blocking
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"net/netip"
+	"strings"
+	"unicode/utf8"
+
+	"jarl/internal/dnsname"
+)
+
+// Flags say what a list entry covers.
+const (
+	Exact uint8 = 1 << iota // the name itself
+	Below                   // every name below it
+)
+
+// List is a parsed blocklist.
+type List struct {
+	// Domains maps names (lowercase, without the final dot) to flags.
+	Domains map[string]uint8
+	// Lines is the number of lines with an entry (not comments or blank
+	// lines); Skipped how many of them were not usable.
+	Lines, Skipped int
+}
+
+// Limits bound what Parse accepts.
+type Limits struct {
+	MaxEntries int // distinct domains (0: no limit)
+	MaxNameLen int // longest name in characters (0: 253)
+}
+
+// ErrTooManyEntries is returned when a list has more than MaxEntries
+// domains.
+var ErrTooManyEntries = errors.New("too many entries")
+
+// Parse reads a blocklist. Each line is one of:
+//
+//	||ads.example^          Adblock style: the name and everything below it
+//	*.ads.example           the name and everything below it
+//	0.0.0.0 ads.example     hosts file (any address; several names allowed)
+//	ads.example             just this name
+//
+// Comments (#, !), Adblock headers ([Adblock Plus]) and blank lines are
+// ignored. Lines that can't be used for DNS blocking (exceptions,
+// cosmetic and URL rules, addresses, single-label names, invalid names)
+// are counted as skipped.
+func Parse(r io.Reader, lim Limits) (*List, error) {
+	maxLen := lim.MaxNameLen
+	if maxLen <= 0 || maxLen > 253 {
+		maxLen = 253
+	}
+	l := &List{Domains: map[string]uint8{}}
+	full := false // a new name was refused because of MaxEntries
+	add := func(name string, flags uint8) bool {
+		name, ok := normalize(name, maxLen)
+		if !ok {
+			return false
+		}
+		if _, seen := l.Domains[name]; !seen && lim.MaxEntries > 0 && len(l.Domains) >= lim.MaxEntries {
+			full = true
+			return false
+		}
+		l.Domains[name] |= flags
+		return true
+	}
+	sc := bufio.NewScanner(r)
+	sc.Buffer(make([]byte, 64*1024), 1024*1024)
+	first := true
+	for sc.Scan() {
+		line := sc.Text()
+		if first {
+			line = strings.TrimPrefix(line, "\uFEFF")
+			first = false
+		}
+		line = strings.TrimSpace(line)
+		if line == "" || line[0] == '#' || line[0] == '!' || line[0] == '[' {
+			continue
+		}
+		l.Lines++
+		ok := false
+		switch {
+		case strings.HasPrefix(line, "||"):
+			ok = add(adblockDomain(line[2:]), Exact|Below)
+		case strings.ContainsAny(line, "|/$^@") || strings.Contains(line, "##"):
+			// Exceptions (@@), URL and cosmetic rules: not for DNS.
+		default:
+			if i := strings.Index(line, "#"); i >= 0 {
+				line = strings.TrimSpace(line[:i])
+			}
+			fields := strings.Fields(line)
+			switch {
+			case len(fields) == 1 && strings.HasPrefix(fields[0], "*."):
+				ok = add(fields[0][2:], Exact|Below)
+			case len(fields) == 1:
+				ok = add(fields[0], Exact)
+			case len(fields) > 1 && isAddr(fields[0]):
+				for _, f := range fields[1:] {
+					if add(f, Exact) {
+						ok = true
+					}
+				}
+			}
+		}
+		if full {
+			return l, fmt.Errorf("%w: the list has more than %d domains", ErrTooManyEntries, lim.MaxEntries)
+		}
+		if !ok {
+			l.Skipped++
+		}
+	}
+	if err := sc.Err(); err != nil {
+		return l, err
+	}
+	return l, nil
+}
+
+// adblockDomain returns the domain of an Adblock rule's body
+// ("ads.example^" or "ads.example^$important"), or "" when the rule
+// does more than block a domain (other modifiers, paths, wildcards).
+func adblockDomain(s string) string {
+	d, rest, _ := strings.Cut(s, "^")
+	if rest != "" && rest != "$important" {
+		return ""
+	}
+	if strings.ContainsAny(d, "*/:|$") {
+		return ""
+	}
+	return d
+}
+
+func isAddr(s string) bool {
+	_, err := netip.ParseAddr(s)
+	return err == nil
+}
+
+// localNames are hosts-file names that aren't blocklist entries.
+var localNames = map[string]bool{"localhost.localdomain": true, "ip6-localhost": true, "ip6-loopback": true}
+
+// normalize lowercases a name, removes the final dot, converts
+// international names to their ASCII form, and checks it is usable: at
+// least two labels of letters, digits, hyphens and underscores, and not an
+// address.
+func normalize(name string, maxLen int) (string, bool) {
+	name = strings.TrimSuffix(strings.ToLower(strings.TrimSpace(name)), ".")
+	if name == "" || localNames[name] || isAddr(name) {
+		return "", false
+	}
+	for i := 0; i < len(name); i++ {
+		if name[i] >= utf8.RuneSelf {
+			a, err := dnsname.ToASCII(name)
+			if err != nil {
+				return "", false
+			}
+			name = strings.TrimSuffix(strings.ToLower(a), ".")
+			break
+		}
+	}
+	if len(name) > maxLen || !strings.Contains(name, ".") {
+		return "", false
+	}
+	for _, label := range strings.Split(name, ".") {
+		if label == "" || len(label) > 63 {
+			return "", false
+		}
+		for i := 0; i < len(label); i++ {
+			c := label[i]
+			if !(c >= 'a' && c <= 'z' || c >= '0' && c <= '9' || c == '-' || c == '_') {
+				return "", false
+			}
+		}
+	}
+	return name, true
+}
+
+// NormalizeName checks and normalizes a domain the way list entries are
+// (for rules entered by hand). The name must have at least two labels.
+func NormalizeName(name string) (string, error) {
+	n, ok := normalize(name, 253-len(RulesZone)-2)
+	if !ok {
+		return "", fmt.Errorf("%q is not a domain name that can be blocked (it needs at least two labels of letters, digits, hyphens or underscores, like ads.example.com)", name)
+	}
+	return n, nil
+}

internal/blocking/zone.go

@@ -0,0 +1,306 @@
+package blocking
+
+import (
+	"bufio"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"os"
+	"sort"
+	"strings"
+)
+
+// Policy zone names. They are under .invalid (RFC 6761), so they can
+// never clash with a real zone, and named only uses them internally.
+const (
+	// RulesZone holds the rules entered in Jarl (one copy per view).
+	RulesZone = "rules.jarl.invalid"
+	// listSuffix follows a list's name: hagezi.blocklist.jarl.invalid.
+	listSuffix = ".blocklist.jarl.invalid"
+)
+
+// ListZone returns the policy zone name of a blocklist.
+func ListZone(name string) string { return name + listSuffix }
+
+// ListName returns the blocklist name of a policy zone ("" for the rules
+// zone and zones that aren't Jarl's).
+func ListName(zone string) string {
+	zone = strings.TrimSuffix(strings.ToLower(zone), ".")
+	if n, ok := strings.CutSuffix(zone, listSuffix); ok && !strings.Contains(n, ".") {
+		return n
+	}
+	return ""
+}
+
+// Match says what blocking does with a name in one view.
+type Match struct {
+	View   string `json:"view,omitempty"`
+	Result string `json:"result"`          // blocked, allowed or none
+	Entry  string `json:"entry,omitempty"` // the matching entry: the name, or *.parent
+	RuleID int64  `json:"rule_id,omitempty"`
+	List   string `json:"list,omitempty"` // the blocklist, when a list matched
+}
+
+// Actions of rules.
+const (
+	ActionBlock = "block"
+	ActionAllow = "allow"
+)
+
+// cname is the CNAME target of each action: "." answers NXDOMAIN,
+// rpz-passthru. answers as if there were no rule.
+var cname = map[string]string{ActionBlock: ".", ActionAllow: "rpz-passthru."}
+
+// header starts a policy zone file. There is no $ORIGIN: names are
+// relative to the zone, so a list can be renamed without rewriting its
+// file. The short TTLs make a change reach clients' caches quickly.
+func header(title string, serial uint32) string {
+	return fmt.Sprintf("; %s\n; Generated by Jarl. DO NOT EDIT.\n$TTL 60\n@\tSOA\tlocalhost. hostmaster.localhost. %d 3600 600 86400 60\n\tNS\tlocalhost.\n", title, serial)
+}
+
+// Placeholder is the content of a list's zone file before the list has
+// been downloaded: a valid, empty policy zone.
+func Placeholder() string {
+	return header("Blocklist not downloaded yet.", 1)
+}
+
+// Rule is a block or allow rule entered by hand.
+type Rule struct {
+	Name   string // lowercase, without the final dot
+	Action string // ActionBlock or ActionAllow
+	Exact  bool   // only the name itself, not the names below it
+}
+
+// RuleEntries returns the policy zone owners for rules (relative names,
+// "*." for wildcards) and their actions, with the entries that make the
+// most specific rule win added: named won't apply a wildcard below a name
+// that exists in the zone, so every name below a wildcard that exists
+// (because a more specific rule mentions it) gets the wildcard's action
+// for itself and for the names below it, unless a rule says otherwise.
+// Callers make sure no two rules have the same name and scope.
+func RuleEntries(rules []Rule) map[string]string {
+	e := map[string]string{}
+	for _, r := range rules {
+		e[r.Name] = r.Action
+		if !r.Exact {
+			e["*."+r.Name] = r.Action
+		}
+	}
+	// Every name that exists in the zone: owners and their ancestors.
+	nodes := map[string]bool{}
+	for owner := range e {
+		n := strings.TrimPrefix(owner, "*.")
+		for {
+			nodes[n] = true
+			i := strings.IndexByte(n, '.')
+			if i < 0 {
+				break
+			}
+			n = n[i+1:]
+		}
+	}
+	ordered := make([]string, 0, len(nodes))
+	for n := range nodes {
+		ordered = append(ordered, n)
+	}
+	// Shallow names first, so a filled-in wildcard is inherited further
+	// down.
+	sort.Slice(ordered, func(i, j int) bool {
+		di, dj := strings.Count(ordered[i], "."), strings.Count(ordered[j], ".")
+		if di != dj {
+			return di < dj
+		}
+		return ordered[i] < ordered[j]
+	})
+	for _, n := range ordered {
+		inherited := ""
+		for a := n; ; {
+			i := strings.IndexByte(a, '.')
+			if i < 0 {
+				break
+			}
+			a = a[i+1:]
+			if act, ok := e["*."+a]; ok {
+				inherited = act
+				break
+			}
+		}
+		if inherited == "" {
+			continue
+		}
+		if _, ok := e[n]; !ok {
+			e[n] = inherited
+		}
+		if _, ok := e["*."+n]; !ok {
+			e["*."+n] = inherited
+		}
+	}
+	return e
+}
+
+// RulesZoneFile renders the rules zone of a view.
+func RulesZoneFile(title string, rules []Rule) string {
+	e := RuleEntries(rules)
+	owners := make([]string, 0, len(e))
+	for o := range e {
+		owners = append(owners, o)
+	}
+	sort.Slice(owners, func(i, j int) bool { return ownerLess(owners[i], owners[j]) })
+	var sb strings.Builder
+	sb.WriteString(header(title, 1))
+	for _, o := range owners {
+		fmt.Fprintf(&sb, "%s\tCNAME\t%s\n", o, cname[e[o]])
+	}
+	return sb.String()
+}
+
+// ownerLess sorts names by their labels from the right (example,
+// ads.example, *.ads.example, cdn.ads.example), so related rules are next
+// to each other.
+func ownerLess(a, b string) bool {
+	ka, kb := reverseLabels(a), reverseLabels(b)
+	return ka < kb
+}
+
+func reverseLabels(n string) string {
+	labels := strings.Split(n, ".")
+	for i, j := 0, len(labels)-1; i < j; i, j = i+1, j-1 {
+		labels[i], labels[j] = labels[j], labels[i]
+	}
+	// A wildcard sorts right after the name it belongs to.
+	return strings.ReplaceAll(strings.Join(labels, "\x00"), "*", "\x01")
+}
+
+// MatchRules finds the rule entry named would apply to name (lowercase,
+// without the final dot) in a rules zone with entries e: the name itself,
+// or else the wildcard of its closest existing ancestor. ok is false when
+// no entry applies.
+func MatchRules(e map[string]string, name string) (owner, action string, ok bool) {
+	if act, found := e[name]; found {
+		return name, act, true
+	}
+	nodes := map[string]bool{}
+	for o := range e {
+		n := strings.TrimPrefix(o, "*.")
+		for {
+			nodes[n] = true
+			i := strings.IndexByte(n, '.')
+			if i < 0 {
+				break
+			}
+			n = n[i+1:]
+		}
+	}
+	if nodes[name] {
+		return "", "", false // the name exists without a rule of its own
+	}
+	for a := name; ; {
+		i := strings.IndexByte(a, '.')
+		if i < 0 {
+			return "", "", false
+		}
+		a = a[i+1:]
+		if nodes[a] {
+			if act, found := e["*."+a]; found {
+				return "*." + a, act, true
+			}
+			return "", "", false
+		}
+	}
+}
+
+// Compact drops entries another entry already covers (a name below a
+// name whose subdomains are blocked). That keeps the zone small, and it
+// matters for correctness: no name exists below a wildcard, so every
+// wildcard applies to all names below it.
+func Compact(l *List) {
+	for n := range l.Domains {
+		for a := n; ; {
+			i := strings.IndexByte(a, '.')
+			if i < 0 {
+				break
+			}
+			a = a[i+1:]
+			if l.Domains[a]&Below != 0 {
+				delete(l.Domains, n)
+				break
+			}
+		}
+	}
+}
+
+// FeedZone writes a list's policy zone to w and returns the number of
+// records and a hash of them (the serial isn't part of the hash, so it
+// tells whether the content changed). l must be compacted.
+func FeedZone(w io.Writer, title string, serial uint32, l *List) (records int, hash string, err error) {
+	names := make([]string, 0, len(l.Domains))
+	for n := range l.Domains {
+		names = append(names, n)
+	}
+	sort.Strings(names)
+	bw := bufio.NewWriterSize(w, 256*1024)
+	h := sha256.New()
+	if _, err := bw.WriteString(header(title, serial)); err != nil {
+		return 0, "", err
+	}
+	write := func(owner string) error {
+		line := owner + "\tCNAME\t.\n"
+		h.Write([]byte(line))
+		records++
+		_, err := bw.WriteString(line)
+		return err
+	}
+	for _, n := range names {
+		f := l.Domains[n]
+		if f&Exact != 0 {
+			if err := write(n); err != nil {
+				return 0, "", err
+			}
+		}
+		if f&Below != 0 {
+			if err := write("*." + n); err != nil {
+				return 0, "", err
+			}
+		}
+	}
+	if err := bw.Flush(); err != nil {
+		return 0, "", err
+	}
+	return records, hex.EncodeToString(h.Sum(nil)), nil
+}
+
+// MatchFeed reports which entry of a list's zone file blocks name: the
+// name itself, or a wildcard of one of its ancestors (the closest).
+// Lists are compacted, so no name exists below a wildcard and the
+// closest wildcard ancestor is the one named uses.
+func MatchFeed(path, name string) (owner string, ok bool, err error) {
+	want := map[string]int{name: 0}
+	depth := 1
+	for a := name; ; depth++ {
+		i := strings.IndexByte(a, '.')
+		if i < 0 {
+			break
+		}
+		a = a[i+1:]
+		want["*."+a] = depth
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		return "", false, err
+	}
+	defer f.Close()
+	best := -1
+	sc := bufio.NewScanner(f)
+	for sc.Scan() {
+		line := sc.Text()
+		o, _, found := strings.Cut(line, "\t")
+		if !found {
+			continue
+		}
+		if d, hit := want[o]; hit && (best < 0 || d < best) {
+			best, owner = d, o
+		}
+	}
+	return owner, best >= 0, sc.Err()
+}