package engine

import (
	"fmt"
	"net"
	"sync"
	"testing"
	"time"

	"github.com/miekg/dns"

	"git.k8s-git-k6n.net/mats/fancy-dns/internal/acl"
	"git.k8s-git-k6n.net/mats/fancy-dns/internal/blocklist"
	"git.k8s-git-k6n.net/mats/fancy-dns/internal/cache"
	"git.k8s-git-k6n.net/mats/fancy-dns/internal/config"
	"git.k8s-git-k6n.net/mats/fancy-dns/internal/qlog"
)

// testZone is the local zone the harness loads when a test asks for one.
const testZone = `$TTL 60
local. SOA ns.local. hostmaster.local. 1 3600 600 604800 60
local. NS ns.local.
ns.local. A 192.168.1.2
host.local. A 192.168.1.5
*.wild.local. A 192.168.1.50
`

// loopbackGroup builds an ACL with one group that matches this test's client
// (127.0.0.1), so a test can attach a policy without inventing CIDRs.
func loopbackGroup(g config.Group) config.ACL {
	if g.Name == "" {
		g.Name = "test"
	}
	if g.Policy == "" {
		g.Policy = "allow"
	}
	if len(g.Match.CIDRs) == 0 {
		g.Match.CIDRs = []string{"127.0.0.0/8"}
	}
	return config.ACL{DefaultPolicy: "allow", Groups: []config.Group{g}}
}

// TestACLDenyWinsOverLocalZone pins the first ordering rule: a denied client
// is refused before the local zone, the blocklist or the cache is consulted.
func TestACLDenyWinsOverLocalZone(t *testing.T) {
	h := newHarness(t, hopt{
		acl:       func(a *config.ACL) { *a = loopbackGroup(config.Group{Policy: "deny"}) },
		rules:     []blocklist.Rule{{Name: "denied.local"}},
		localZone: testZone,
	})

	// host.local. is in the local zone and denied.local. is on the
	// blocklist: both must come back REFUSED, which is only possible if the
	// ACL ran first.
	for _, name := range []string{"host.local.", "denied.local."} {
		res := h.ask(name, dns.TypeA)
		requireRcode(t, res, dns.RcodeRefused)
		requireAction(t, res, qlog.ActionRefused)
		if res.Group != "test" {
			t.Fatalf("group = %q, want %q", res.Group, "test")
		}
		if res.GroupPolicy != "deny" {
			t.Fatalf("group policy = %q, want deny", res.GroupPolicy)
		}
		if res.Client != "127.0.0.1" {
			t.Fatalf("client = %q, want 127.0.0.1", res.Client)
		}
		requireNoUpstreamHit(t, h, name)
	}
}

// TestLocalZoneWinsOverBlocklist pins the second rule: locally defined
// authoritative data is answered before the blocklist, so an operator can
// un-block a name in a zone file.
func TestLocalZoneWinsOverBlocklist(t *testing.T) {
	h := newHarness(t, hopt{
		rules:     []blocklist.Rule{{Name: "host.local"}, {Name: "*.wild.local"}},
		localZone: testZone,
	})

	res := h.ask("host.local.", dns.TypeA)
	requireAction(t, res, qlog.ActionAllow)
	if got := answerA(res.Msg); got != "192.168.1.5" {
		t.Fatalf("answer = %q, want 192.168.1.5 (blocked instead?)", got)
	}
	if res.Msg == nil || !res.Msg.Authoritative {
		t.Fatal("local zone answer is not authoritative")
	}
	requireNoUpstreamHit(t, h, "host.local.")

	// A wildcard rule covers the subdomains of wild.local but the zone's
	// own wildcard record answers first.
	w := h.ask("anything.wild.local.", dns.TypeA)
	if got := answerA(w.Msg); got != "192.168.1.50" {
		t.Fatalf("wildcard answer = %q, want 192.168.1.50", got)
	}
}

// TestLocalZoneNxDomainForAZoneItOwns pins the negative case: a name inside a
// loaded zone that does not exist is NXDOMAIN with the zone's SOA, and never
// reaches the upstream.
func TestLocalZoneNxDomainForAZoneItOwns(t *testing.T) {
	h := newHarness(t, hopt{localZone: testZone})
	res := h.ask("nope.local.", dns.TypeA)
	requireRcode(t, res, dns.RcodeNameError)
	if len(res.Msg.Ns) == 0 {
		t.Fatal("NXDOMAIN without an SOA in the authority section")
	}
	requireNoUpstreamHit(t, h, "nope.local.")
}

// TestBlocklistWinsOverCache pins the third rule: a cached answer must not
// outrank a blocklist rule. A name cached with a real address is still
// blocked, and the blocklist is answered without touching the network.
func TestBlocklistWinsOverCache(t *testing.T) {
	h := newHarness(t, hopt{rules: []blocklist.Rule{{Name: "blocked.example.test"}}})

	// Seed the cache with the exact key the engine will compute, so a pass
	// would be visible as a cache hit.
	seed := new(dns.Msg)
	seed.SetQuestion("blocked.example.test.", dns.TypeA)
	rr, _ := dns.NewRR("blocked.example.test. 60 IN A 192.0.2.55")
	seed.Answer = append(seed.Answer, rr)
	key := cache.QueryKey("blocked.example.test.", dns.TypeA, dns.ClassINET, false, 0)
	h.cache.PutResponse(key, seed, "seed")
	if got := h.cache.Get(key); got.Kind != cache.Fresh {
		t.Fatalf("seeded cache entry is %v, want Fresh", got.Kind)
	}

	res := h.ask("blocked.example.test.", dns.TypeA)
	requireAction(t, res, qlog.ActionBlock)
	requireRcode(t, res, dns.RcodeNameError)
	if res.CacheHit {
		t.Fatal("a blocked query reported a cache hit: the cache outranked the blocklist")
	}
	if res.BlockSource != "test" {
		t.Fatalf("block source = %q, want %q", res.BlockSource, "test")
	}
	if answerA(res.Msg) == "192.0.2.55" {
		t.Fatal("the cached address was served for a blocked name")
	}
	requireNoUpstreamHit(t, h, "blocked.example.test.")
}

// TestBlocklistDropSendsNothing pins the "drop" action: no response at all,
// not an empty one.
func TestBlocklistDropSendsNothing(t *testing.T) {
	h := newHarness(t, hopt{rules: []blocklist.Rule{
		{Name: "drop.example.test", Action: blocklist.ActionDrop},
	}})
	res := h.ask("drop.example.test.", dns.TypeA)
	if !res.Drop {
		t.Fatal("Drop = false, want true")
	}
	if res.Msg != nil {
		t.Fatal("a dropped query produced a message to send")
	}
	requireAction(t, res, qlog.ActionDrop)
	requireNoUpstreamHit(t, h, "drop.example.test.")
}

// TestAllowRuleBeatsBlockRule pins the whitelist: an explicit allow rule wins
// over a list entry, and the name resolves.
func TestAllowRuleBeatsBlockRule(t *testing.T) {
	h := newHarness(t, hopt{rules: []blocklist.Rule{
		{Name: "*.example.test"},
		{Name: "ok.example.test", Action: blocklist.ActionAllow},
	}})
	res := h.ask("ok.example.test.", dns.TypeA)
	requireAction(t, res, qlog.ActionAllow)
	if got := answerA(res.Msg); got != "203.0.113.7" {
		t.Fatalf("answer = %q, want the upstream address 203.0.113.7", got)
	}
	blocked := h.ask("ads.example.test.", dns.TypeA)
	requireAction(t, blocked, qlog.ActionBlock)
}

// TestGlobalConcurrencyCapRefuses pins the overload refusal: a saturated
// global budget is answered SERVFAIL and counted as dropped, not queued.
func TestGlobalConcurrencyCapRefuses(t *testing.T) {
	h := newHarness(t, hopt{
		server: func(s *config.Server) { s.MaxConcurrent = 1 },
	})
	open := h.up.hold()

	var wg sync.WaitGroup
	first := make(chan Response, 1)
	wg.Add(1)
	go func() {
		defer wg.Done()
		first <- h.ask("slow.example.test.", dns.TypeA)
	}()

	// The first query is now parked inside the upstream exchange, holding the
	// one global slot.
	h.up.waitArrived(t)
	second := h.ask("other.example.test.", dns.TypeA)
	requireRcode(t, second, dns.RcodeServerFailure)
	requireAction(t, second, qlog.ActionError)
	if h.eng.Dropped() != 1 {
		t.Fatalf("dropped = %d, want 1", h.eng.Dropped())
	}
	// The refusal must not have reached the network.
	requireNoUpstreamHit(t, h, "other.example.test.")

	open()
	wg.Wait()
	res := <-first
	requireAction(t, res, qlog.ActionAllow)
	if got := answerA(res.Msg); got != "203.0.113.7" {
		t.Fatalf("first answer = %q, want 203.0.113.7", got)
	}
	if n := h.eng.Inflight(); n != 0 {
		t.Fatalf("inflight = %d after both queries finished, want 0", n)
	}
}

// TestPerClientConcurrencyCapRefuses pins the per-client cap, which is checked
// before the global one so one noisy client cannot take the whole budget.
func TestPerClientConcurrencyCapRefuses(t *testing.T) {
	h := newHarness(t, hopt{
		acl: func(a *config.ACL) {
			*a = loopbackGroup(config.Group{MaxConcurrent: 1})
		},
		// A generous global budget, so a refusal can only come from the
		// per-client limiter.
		server: func(s *config.Server) { s.MaxConcurrent = 64 },
	})
	open := h.up.hold()

	var wg sync.WaitGroup
	first := make(chan Response, 1)
	wg.Add(1)
	go func() {
		defer wg.Done()
		first <- h.ask("slow.example.test.", dns.TypeA)
	}()
	h.up.waitArrived(t)

	second := h.ask("other.example.test.", dns.TypeA)
	requireRcode(t, second, dns.RcodeServerFailure)
	// A per-client refusal is not a global overload.
	if h.eng.Dropped() != 0 {
		t.Fatalf("dropped = %d, want 0: the global cap was not the one that refused", h.eng.Dropped())
	}

	open()
	wg.Wait()
	requireAction(t, <-first, qlog.ActionAllow)
}

// TestRateLimitRefuses pins the fixed-window rate limit: the first N queries
// in a window go through, the next is REFUSED.
func TestRateLimitRefuses(t *testing.T) {
	h := newHarness(t, hopt{
		acl: func(a *config.ACL) { *a = loopbackGroup(config.Group{RateLimit: 2}) },
	})
	for i := 0; i < 2; i++ {
		// Distinct names, so the cache cannot turn the second query into a
		// hit and mask what is being asserted.
		res := h.ask(fmt.Sprintf("rate%d.example.test.", i), dns.TypeA)
		requireAction(t, res, qlog.ActionAllow)
	}
	res := h.ask("rate2.example.test.", dns.TypeA)
	requireRcode(t, res, dns.RcodeRefused)
	requireAction(t, res, qlog.ActionRefused)
}

// TestGroupDomainPolicy pins the per-group allow/deny lists, which run after
// the rate limit and before the local zone.
func TestGroupDomainPolicy(t *testing.T) {
	t.Run("deny list", func(t *testing.T) {
		h := newHarness(t, hopt{
			acl: func(a *config.ACL) {
				// "**." is the spelling that covers a domain and its
				// subdomains; a bare name is exact (see acl.NewDomainSet).
				*a = loopbackGroup(config.Group{DenyDomains: []string{"**.denied.example.test."}})
			},
		})
		res := h.ask("denied.example.test.", dns.TypeA)
		requireAction(t, res, qlog.ActionBlock)
		if res.BlockSource != "acl:test" {
			t.Fatalf("block source = %q, want acl:test", res.BlockSource)
		}
		// A subdomain of a denied name is denied too.
		if sub := h.ask("sub.denied.example.test.", dns.TypeA); sub.Action != qlog.ActionBlock {
			t.Fatalf("subdomain action = %q, want block", sub.Action)
		}
		if sib := h.ask("sibling.example.test.", dns.TypeA); sib.Action != qlog.ActionAllow {
			t.Fatalf("sibling action = %q, want allow: the deny list leaked", sib.Action)
		}
		allowed := h.ask("allowed.example.test.", dns.TypeA)
		requireAction(t, allowed, qlog.ActionAllow)
		if got := answerA(allowed.Msg); got != "203.0.113.7" {
			t.Fatalf("answer = %q, want 203.0.113.7", got)
		}
	})

	t.Run("exact deny is exact", func(t *testing.T) {
		h := newHarness(t, hopt{
			acl: func(a *config.ACL) {
				*a = loopbackGroup(config.Group{DenyDomains: []string{"denied.example.test."}})
			},
		})
		if res := h.ask("denied.example.test.", dns.TypeA); res.Action != qlog.ActionBlock {
			t.Fatalf("exact name: action = %q, want block", res.Action)
		}
		// Documented behaviour of a rule without a wildcard: the apex only.
		if sub := h.ask("sub.denied.example.test.", dns.TypeA); sub.Action != qlog.ActionAllow {
			t.Fatalf("subdomain of an exact rule: action = %q, want allow", sub.Action)
		}
	})

	t.Run("allow list is exclusive", func(t *testing.T) {
		h := newHarness(t, hopt{
			acl: func(a *config.ACL) {
				*a = loopbackGroup(config.Group{AllowDomains: []string{"only.example.test."}})
			},
		})
		if res := h.ask("other.example.test.", dns.TypeA); res.Action != qlog.ActionBlock {
			t.Fatalf("non-allowlisted name: action = %q, want block", res.Action)
		}
		allowed := h.ask("only.example.test.", dns.TypeA)
		requireAction(t, allowed, qlog.ActionAllow)
		if got := answerA(allowed.Msg); got != "203.0.113.7" {
			t.Fatalf("answer = %q, want 203.0.113.7", got)
		}
	})
}

// TestSkipToggles pins the two per-group bypasses. Both are security-relevant
// switches, so a test that pins them must fail if the flag stops working.
func TestSkipToggles(t *testing.T) {
	t.Run("skip blocklist", func(t *testing.T) {
		h := newHarness(t, hopt{
			acl:   func(a *config.ACL) { *a = loopbackGroup(config.Group{SkipBlocklist: true}) },
			rules: []blocklist.Rule{{Name: "blocked.example.test"}},
		})
		res := h.ask("blocked.example.test.", dns.TypeA)
		requireAction(t, res, qlog.ActionAllow)
		if got := answerA(res.Msg); got != "203.0.113.7" {
			t.Fatalf("answer = %q, want 203.0.113.7 (the blocklist still applies)", got)
		}
	})

	t.Run("skip cache", func(t *testing.T) {
		h := newHarness(t, hopt{
			acl: func(a *config.ACL) { *a = loopbackGroup(config.Group{SkipCache: true}) },
		})
		for i := 0; i < 2; i++ {
			res := h.ask("nocache.example.test.", dns.TypeA)
			requireAction(t, res, qlog.ActionAllow)
			if res.CacheKind != "" {
				t.Fatalf("query %d: cache kind = %q, want none", i, res.CacheKind)
			}
		}
		if n := h.up.hitCount("nocache.example.test."); n != 2 {
			t.Fatalf("upstream saw %d queries, want 2: the cache was used", n)
		}
	})
}

// TestCacheHitDoesNotReachUpstream pins the cache fast path.
func TestCacheHitDoesNotReachUpstream(t *testing.T) {
	h := newHarness(t, hopt{})
	first := h.ask("cached.example.test.", dns.TypeA)
	requireAction(t, first, qlog.ActionAllow)
	if first.CacheHit {
		t.Fatal("the first query reported a cache hit")
	}
	if n := h.up.hitCount("cached.example.test."); n != 1 {
		t.Fatalf("upstream saw %d queries, want 1", n)
	}

	second := h.ask("cached.example.test.", dns.TypeA)
	if !second.CacheHit || second.CacheKind != "hit" {
		t.Fatalf("second query: hit=%v kind=%q, want a fresh hit", second.CacheHit, second.CacheKind)
	}
	requireAction(t, second, qlog.ActionCached)
	if got := answerA(second.Msg); got != "203.0.113.7" {
		t.Fatalf("answer = %q, want 203.0.113.7", got)
	}
	if ttl := firstTTL(second.Msg); ttl == 0 || ttl > 60 {
		t.Fatalf("cached TTL = %d, want 1..60", ttl)
	}
	if second.Upstream == "" {
		t.Fatal("the cached answer lost its upstream attribution")
	}
	if n := h.up.hitCount("cached.example.test."); n != 1 {
		t.Fatalf("upstream saw %d queries after a cache hit, want 1", n)
	}
}

// TestServeStaleAgesToOneAndRefreshesOnce pins serve-stale: the stale answer
// is served with TTL 1, and the background refresh is single-flight however
// many queries pile up while it is running.
func TestServeStaleAgesToOneAndRefreshesOnce(t *testing.T) {
	h := newHarness(t, hopt{serveStale: 10 * time.Minute, clock: true})

	first := h.ask("stale.example.test.", dns.TypeA)
	requireAction(t, first, qlog.ActionAllow)

	// Expire the entry: the upstream TTL is 60s.
	h.advance(61 * time.Second)

	open := h.up.hold()
	stale := h.ask("stale.example.test.", dns.TypeA)
	if !stale.CacheHit || stale.CacheKind != "stale" {
		t.Fatalf("hit=%v kind=%q, want a stale hit", stale.CacheHit, stale.CacheKind)
	}
	if got := answerA(stale.Msg); got != "203.0.113.7" {
		t.Fatalf("stale answer = %q, want 203.0.113.7", got)
	}
	if ttl := firstTTL(stale.Msg); ttl != 1 {
		t.Fatalf("stale TTL = %d, want 1: a stale answer must not be cached by the client", ttl)
	}

	// The prefetch is now parked in the upstream. More stale queries must not
	// start a second one.
	h.up.waitArrived(t)
	for i := 0; i < 3; i++ {
		if res := h.ask("stale.example.test.", dns.TypeA); res.CacheKind != "stale" {
			t.Fatalf("query %d during refresh: cache kind = %q, want stale", i, res.CacheKind)
		}
	}
	// Give a second prefetch, if one were wrongly launched, time to arrive.
	time.Sleep(250 * time.Millisecond)
	if n := h.up.hitCount("stale.example.test."); n != 2 {
		t.Fatalf("upstream saw %d queries, want 2 (one resolve, one refresh)", n)
	}

	open()
	// After the refresh lands the entry is fresh again.
	deadline := time.Now().Add(5 * time.Second)
	for {
		if res := h.ask("stale.example.test.", dns.TypeA); res.CacheKind == "hit" {
			break
		}
		if time.Now().After(deadline) {
			t.Fatal("the entry never came back fresh after the background refresh")
		}
		time.Sleep(10 * time.Millisecond)
	}
}

// TestTruncationAgainstClientBuffer pins RFC 2181 truncation on the uncached
// path: a 4 KiB answer over a 512-byte client buffer comes back with TC and no
// records, and the same answer is delivered in full over TCP or with a large
// EDNS0 buffer.
func TestTruncationAgainstClientBuffer(t *testing.T) {
	h := newHarness(t, hopt{})

	small := h.ask("big.test.", dns.TypeTXT)
	if small.Msg == nil || !small.Msg.Truncated {
		t.Fatal("a 4 KiB answer over a 512-byte buffer was not truncated")
	}
	if len(small.Msg.Answer) != 0 {
		t.Fatalf("truncated answer carries %d records, want none", len(small.Msg.Answer))
	}
	if small.Msg.Rcode != dns.RcodeSuccess {
		t.Fatalf("rcode = %s, want NOERROR", dns.RcodeToString[small.Msg.Rcode])
	}

	// A different name, so the 4 KiB answer in the cache is not the one being
	// served: TCP clients are not size-limited.
	overTCP := h.askTCP("big2.test.", dns.TypeTXT)
	if overTCP.Msg == nil || overTCP.Msg.Truncated {
		t.Fatal("a TCP client got a truncated answer")
	}
	if firstTTL(overTCP.Msg) == 0 {
		t.Fatal("the TCP answer has no records")
	}

	// A large advertised EDNS0 buffer gets the whole thing too.
	edns := h.askEDNS("big3.test.", dns.TypeTXT, 4096)
	if edns.Msg == nil || edns.Msg.Truncated {
		t.Fatal("a 4096-byte EDNS0 client got a truncated answer")
	}
	if len(edns.Msg.Answer) == 0 {
		t.Fatal("the EDNS0 answer has no records")
	}
	if b, err := edns.Msg.Pack(); err != nil || len(b) > 4096 {
		t.Fatalf("packed size = %d (err %v), want <= 4096", len(b), err)
	}
}

// TestSmallClientDoesNotPoisonTheCache pins the reason the engine caches the
// untruncated upstream answer: a 512-byte client must not leave a truncated
// copy in the cache for every later client.
func TestSmallClientDoesNotPoisonTheCache(t *testing.T) {
	h := newHarness(t, hopt{})

	small := h.ask("big.test.", dns.TypeTXT)
	if small.Msg == nil || !small.Msg.Truncated {
		t.Fatal("precondition: the 512-byte client was not truncated")
	}
	full := h.askEDNS("big.test.", dns.TypeTXT, 4096)
	if full.Msg == nil || full.Msg.Truncated {
		t.Fatal("a large-buffer client got a truncated answer: the cache entry was poisoned")
	}
	if len(full.Msg.Answer) == 0 {
		t.Fatal("the cached answer has no records")
	}
	if n := h.up.hitCount("big.test."); n != 1 {
		t.Fatalf("upstream saw %d queries, want 1: the second query was not served from cache", n)
	}
}

// TestTruncatedUpstreamAnswerIsNotCached pins the other half of the rule: an
// answer the upstream flagged TC is never stored, because it would be served
// truncated forever.
func TestTruncatedUpstreamAnswerIsNotCached(t *testing.T) {
	h := newHarness(t, hopt{})
	for i := 0; i < 2; i++ {
		res := h.ask("tc.test.", dns.TypeA)
		if res.CacheHit {
			t.Fatalf("query %d was served from cache: a TC answer was cached", i)
		}
	}
	if n := h.up.hitCount("tc.test."); n != 2 {
		t.Fatalf("upstream saw %d queries, want 2", n)
	}
}

// TestMalformedQueriesAreFormErr pins the validation step, which runs before
// the ACL: a query with no question, two questions, or the response bit set is
// FORMERR, and still counts as a query.
func TestMalformedQueriesAreFormErr(t *testing.T) {
	h := newHarness(t, hopt{
		acl: func(a *config.ACL) { *a = loopbackGroup(config.Group{Policy: "deny"}) },
	})

	twoQuestions := new(dns.Msg)
	twoQuestions.SetQuestion("a.example.test.", dns.TypeA)
	twoQuestions.Question = append(twoQuestions.Question, dns.Question{Name: "b.example.test.", Qtype: dns.TypeA, Qclass: dns.ClassINET})

	alreadyResponse := new(dns.Msg)
	alreadyResponse.SetQuestion("a.example.test.", dns.TypeA)
	alreadyResponse.Response = true

	empty := new(dns.Msg)

	for name, q := range map[string]*dns.Msg{"no question": empty, "two questions": twoQuestions, "response bit set": alreadyResponse} {
		before := h.totals()
		res := h.askWith(Request{Msg: q, Remote: h.request("a.example.test.", dns.TypeA).Remote, Transport: "udp"})
		requireRcode(t, res, dns.RcodeFormatError)
		requireAction(t, res, qlog.ActionError)
		if got := h.totals().Queries - before.Queries; got != 1 {
			t.Fatalf("%s: metrics recorded %d queries, want 1", name, got)
		}
	}
	if h.up.totalHits() != 0 {
		t.Fatal("a malformed query reached the upstream")
	}
}

// TestMetricsAndQueryLogFireOncePerQuery pins the hook contract: every query
// that reaches Handle is counted exactly once and logged exactly once,
// including the ones refused before resolution.
func TestMetricsAndQueryLogFireOncePerQuery(t *testing.T) {
	cases := []struct {
		name string
		opt  hopt
		// queries is how many queries the case sends: every one of them must
		// be counted exactly once.
		queries int
		send    func(*harness)
		want    qlog.Action
	}{
		{
			name:    "upstream answer",
			opt:     hopt{},
			queries: 1,
			send:    func(h *harness) { h.ask("once.example.test.", dns.TypeA) },
			want:    qlog.ActionAllow,
		},
		{
			name:    "cache hit",
			opt:     hopt{},
			queries: 2,
			send: func(h *harness) {
				h.ask("once.example.test.", dns.TypeA)
				h.ask("once.example.test.", dns.TypeA)
			},
			want: qlog.ActionCached,
		},
		{
			name:    "blocked",
			opt:     hopt{rules: []blocklist.Rule{{Name: "blocked.example.test"}}},
			queries: 1,
			send:    func(h *harness) { h.ask("blocked.example.test.", dns.TypeA) },
			want:    qlog.ActionBlock,
		},
		{
			name:    "refused",
			opt:     hopt{acl: func(a *config.ACL) { *a = loopbackGroup(config.Group{Policy: "deny"}) }},
			queries: 1,
			send: func(h *harness) {
				h.ask("denied.example.test.", dns.TypeA)
			},
			want: qlog.ActionRefused,
		},
	}

	for _, tc := range cases {
		t.Run(tc.name, func(t *testing.T) {
			h := newHarness(t, tc.opt)
			before := h.totals()
			beforeLog := h.logged()
			tc.send(h)
			after := h.totals()
			if got := after.Queries - before.Queries; got != uint64(tc.queries) {
				t.Fatalf("metrics counted %d queries, want exactly %d", got, tc.queries)
			}
			if got := h.logged() - beforeLog; got != tc.queries {
				t.Fatalf("query log holds %d new entries, want exactly %d", got, tc.queries)
			}
			entries := h.entries()
			if len(entries) == 0 {
				t.Fatal("the newest entry is missing from the live ring")
			}
			e := entries[0]
			if e.Action != tc.want {
				t.Fatalf("logged action = %q, want %q", e.Action, tc.want)
			}
			if e.Client != "127.0.0.1" {
				t.Fatalf("logged client = %q, want 127.0.0.1", e.Client)
			}
			if e.Protocol != "udp" {
				t.Fatalf("logged protocol = %q, want udp", e.Protocol)
			}
			if len(entries) != tc.queries {
				t.Fatalf("the live ring holds %d entries, want %d: the hook fired more than once per query",
					len(entries), tc.queries)
			}
			for i := 0; i+1 < len(entries); i++ {
				if entries[i].ID-entries[i+1].ID != 1 {
					t.Fatalf("entries %d and %d are not adjacent (ids %d, %d): the hook fired more than once per query",
						i+1, i, entries[i+1].ID, entries[i].ID)
				}
			}
		})
	}
}

// TestNoUpstreamIsServfail pins the degenerate wiring: a pool-less engine
// answers SERVFAIL rather than panicking or hanging.
func TestNoUpstreamIsServfail(t *testing.T) {
	h := newHarness(t, hopt{noPool: true})
	res := h.ask("nowhere.example.test.", dns.TypeA)
	requireRcode(t, res, dns.RcodeServerFailure)
	requireAction(t, res, qlog.ActionError)
}

// TestDeadUpstreamIsServfail pins upstream failure handling: the exchange
// error becomes a SERVFAIL to the client and an error in the metrics.
func TestDeadUpstreamIsServfail(t *testing.T) {
	pc, err := net.ListenPacket("udp", "127.0.0.1:0")
	if err != nil {
		t.Fatalf("reserve port: %v", err)
	}
	dead := pc.LocalAddr().String()
	_ = pc.Close()

	h := newHarness(t, hopt{
		timeout:   500 * time.Millisecond,
		upstreams: []config.UpstreamServer{{Addr: dead, Proto: "udp", Weight: 1}},
	})
	before := h.totals()
	res := h.ask("dead.example.test.", dns.TypeA)
	requireRcode(t, res, dns.RcodeServerFailure)
	requireAction(t, res, qlog.ActionError)
	if got := h.totals().Errors - before.Errors; got != 1 {
		t.Fatalf("errors rose by %d, want 1", got)
	}
}

// TestForwardedQueryShape pins what goes upstream: our own OPT (so an
// upstream can answer EDNS0), the client's ECS forwarded verbatim, our own
// NSID rather than the client's, and never the client's AD bit.
func TestForwardedQueryShape(t *testing.T) {
	t.Run("plain query", func(t *testing.T) {
		h := newHarness(t, hopt{})
		h.ask("plain.example.test.", dns.TypeA)
		got := h.up.received()
		if len(got) != 1 {
			t.Fatalf("upstream saw %d queries, want 1", len(got))
		}
		if got[0].IsEdns0() == nil {
			t.Fatal("the forwarded query has no OPT record")
		}
		if opt := findOption(got[0], dns.EDNS0NSID); opt != nil {
			t.Fatal("we leaked our own NSID to a client that did not ask for one")
		}
		if !got[0].RecursionDesired {
			t.Fatal("the forwarded query lost RD")
		}
	})

	t.Run("client subnet is forwarded", func(t *testing.T) {
		h := newHarness(t, hopt{})
		req := h.request("ecs.example.test.", dns.TypeA)
		req.Msg.SetEdns0(1232, false)
		sub := &dns.EDNS0_SUBNET{Code: dns.EDNS0SUBNET, Family: 1, SourceNetmask: 24, Address: net.IP{203, 0, 113, 0}}
		req.Msg.IsEdns0().Option = append(req.Msg.IsEdns0().Option, sub)
		h.askWith(req)
		got := h.up.received()
		if len(got) != 1 {
			t.Fatalf("upstream saw %d queries, want 1", len(got))
		}
		opt := findOption(got[0], dns.EDNS0SUBNET)
		if opt == nil {
			t.Fatal("the client's EDNS0 client subnet was not forwarded")
		}
		fwd, ok := opt.(*dns.EDNS0_SUBNET)
		if !ok || !fwd.Address.Equal(sub.Address) || fwd.SourceNetmask != 24 {
			t.Fatalf("forwarded subnet = %+v, want %+v", fwd, sub)
		}
	})

	// The NSID branch of prepareUpstream is deliberately not pinned here:
	// miekg/dns v1.1.73 reports an EDNS0_NSID option as zero length, so any
	// message carrying one fails to pack ("dns: overflow packing opt") and a
	// client that asks for NSID gets SERVFAIL. That is a library defect, not
	// a decision this package owns; see STATUS.md.
}

func findOption(m *dns.Msg, code uint16) dns.EDNS0 {
	opt := m.IsEdns0()
	if opt == nil {
		return nil
	}
	for _, o := range opt.Option {
		if o.Option() == code {
			return o
		}
	}
	return nil
}

// TestReplyEchoesTheClientQuestionAndID pins the wire contract every resolver
// owes a client: the reply carries the client's question verbatim (including
// its case, for 0x20 style checks) and the client's transaction ID.
func TestReplyEchoesTheClientQuestionAndID(t *testing.T) {
	h := newHarness(t, hopt{})
	req := h.request("HoSt.ExAmPlE.TeSt.", dns.TypeA)
	req.Msg.Id = 0x1234
	res := h.askWith(req)
	requireAction(t, res, qlog.ActionAllow)
	if res.Msg.Id != 0x1234 {
		t.Fatalf("reply id = %#x, want 0x1234", res.Msg.Id)
	}
	if len(res.Msg.Question) != 1 {
		t.Fatalf("reply carries %d questions, want 1", len(res.Msg.Question))
	}
	if got := res.Msg.Question[0].Name; got != "HoSt.ExAmPlE.TeSt." {
		t.Fatalf("reply question = %q, want the client's name verbatim", got)
	}
	if got := answerA(res.Msg); got != "203.0.113.7" {
		t.Fatalf("answer = %q, want 203.0.113.7", got)
	}
}

// TestGroupStatsCountEveryQuery pins the per-group counters the UI reads.
func TestGroupStatsCountEveryQuery(t *testing.T) {
	h := newHarness(t, hopt{
		acl:   func(a *config.ACL) { *a = loopbackGroup(config.Group{}) },
		rules: []blocklist.Rule{{Name: "blocked.example.test"}},
	})
	h.ask("stats.example.test.", dns.TypeA)
	h.ask("blocked.example.test.", dns.TypeA)
	h.ask("blocked.example.test.", dns.TypeA)

	var found *acl.GroupSummary
	for i, g := range h.eng.GroupStats() {
		if g.Name == "test" {
			found = &h.eng.GroupStats()[i]
			break
		}
	}
	if found == nil {
		t.Fatalf("group %q missing from the stats table: %+v", "test", h.eng.GroupStats())
	}
	if found.QueryCount != 3 {
		t.Fatalf("query count = %d, want 3", found.QueryCount)
	}
	if found.BlockedCount != 2 {
		t.Fatalf("blocked count = %d, want 2", found.BlockedCount)
	}
}