package resolver_test

// Transport-failure classification tests.
//
// These are live tests, not mocks. Nothing here substitutes the
// resolver's DNSClient: the resolver dials a real UDP socket, writes
// a real DNS query with the real miekg/dns client, and applies its
// real deadline and its real classification logic to what comes
// back. The only thing under test control is which address the query
// is sent to, and what — if anything — is listening there.
//
// That distinction is what the no-DNS-mocks rule in TESTING.md is
// about. A fake DNSClient lets the code under test skip DNS entirely
// and hands it a manufactured verdict; a nameserver bound on
// loopback makes it speak DNS for real and earn one. Pointing a live
// query at a nameserver of the test's choosing is no more a mock
// than pointing it at a.root-servers.net.
//
// The public network cannot produce these outcomes on demand. A
// black-holed address is not black-holed everywhere — build
// environments that intercept UDP/53 answer it locally — so a test
// built on one asserts on the network it happens to run on rather
// than on the resolver. A loopback nameserver is deterministic
// everywhere, and it is fast, because the test picks the deadline.

import (
	"context"
	"net"
	"testing"
	"time"

	"github.com/miekg/dns"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"

	"sneak.berlin/go/dnswatcher/internal/resolver"
)

const (
	// transportBudget is the wall time the timeout test must stay
	// under. The resolver asks a nameserver for eight record types
	// in turn and retries each one once, so a nameserver silent on
	// every type would cost sixteen query timeouts. The test's
	// nameserver is silent on exactly one type, which costs two,
	// and this budget fails loudly if that ever stops being true.
	transportBudget = 8 * time.Second

	// transportDeadline is the caller deadline the tests run
	// under. It is generous on purpose: these tests are about the
	// resolver classifying a nameserver's behaviour, so the
	// caller's deadline must never be the thing that expires.
	transportDeadline = 30 * time.Second

	// silentNS and failingNS are the nameserver names reported
	// back in NameserverResponse.Nameserver. They are .test names
	// (RFC 6761) and are never resolved: the tests address the
	// nameserver by its socket address.
	silentNS  = "silent.ns.test."
	failingNS = "servfail.ns.test."

	// transportHostname is the name queried. Nothing resolves it;
	// the point is entirely how the nameserver behaves.
	transportHostname = "example.com"
)

// startNameserver binds a real UDP nameserver on loopback and serves
// every datagram it receives with handle, which returns the reply to
// send or nil to stay silent. It returns the "host:port" address to
// aim a query at, and stops the server when the test ends.
func startNameserver(
	t *testing.T,
	handle func(query *dns.Msg) *dns.Msg,
) string {
	t.Helper()

	var lc net.ListenConfig

	conn, err := lc.ListenPacket(t.Context(), "udp", "127.0.0.1:0")
	require.NoError(t, err, "binding loopback nameserver")

	stopped := make(chan struct{})

	t.Cleanup(func() {
		_ = conn.Close()

		<-stopped
	})

	go serveNameserver(conn, handle, stopped)

	return conn.LocalAddr().String()
}

// serveNameserver reads queries until conn is closed, replying with
// whatever handle produces.
func serveNameserver(
	conn net.PacketConn,
	handle func(query *dns.Msg) *dns.Msg,
	stopped chan<- struct{},
) {
	defer close(stopped)

	buf := make([]byte, dns.MaxMsgSize)

	for {
		n, from, err := conn.ReadFrom(buf)
		if err != nil {
			return
		}

		query := new(dns.Msg)
		if query.Unpack(buf[:n]) != nil {
			continue
		}

		reply := handle(query)
		if reply == nil {
			continue
		}

		wire, err := reply.Pack()
		if err != nil {
			continue
		}

		_, err = conn.WriteTo(wire, from)
		if err != nil {
			return
		}
	}
}

// unservedAddr returns a loopback address with nothing listening on
// it, by binding a port and releasing it again.
func unservedAddr(t *testing.T) string {
	t.Helper()

	var lc net.ListenConfig

	conn, err := lc.ListenPacket(t.Context(), "udp", "127.0.0.1:0")
	require.NoError(t, err, "binding loopback port")

	addr := conn.LocalAddr().String()
	require.NoError(t, conn.Close(), "releasing loopback port")

	return addr
}

// TestQueryNameserverIP_Timeout covers the StatusTimeout branch: a
// nameserver that takes the query and never answers.
func TestQueryNameserverIP_Timeout(t *testing.T) {
	t.Parallel()

	// A real nameserver that drops A queries and answers every
	// other type. Silence on one type is all the resolver needs to
	// classify the response as a timeout, and it keeps the test
	// two query timeouts long instead of sixteen.
	addr := startNameserver(t, func(query *dns.Msg) *dns.Msg {
		if len(query.Question) > 0 &&
			query.Question[0].Qtype == dns.TypeA {
			return nil
		}

		reply := new(dns.Msg)
		reply.SetReply(query)

		return reply
	})

	r := newTestResolver(t)

	ctx, cancel := context.WithTimeout(
		context.Background(), transportDeadline,
	)
	defer cancel()

	start := time.Now()

	resp, err := r.QueryNameserverIP(
		ctx, silentNS, addr, transportHostname,
	)

	elapsed := time.Since(start)

	require.NoError(t, err)
	require.NotNil(t, resp)

	assert.Equal(t, resolver.StatusTimeout, resp.Status)
	assert.Equal(t, "all queries timed out", resp.Error)
	assert.Empty(t, resp.Records)
	assert.Equal(t, silentNS, resp.Nameserver)

	assert.Less(
		t, elapsed, transportBudget,
		"one silent record type must cost one query's retries, "+
			"not every record type's",
	)
}

// TestQueryNameserverIP_ServFail covers the StatusError branch: a
// nameserver that answers, and answers SERVFAIL.
func TestQueryNameserverIP_ServFail(t *testing.T) {
	t.Parallel()

	addr := startNameserver(t, func(query *dns.Msg) *dns.Msg {
		reply := new(dns.Msg)
		reply.SetRcode(query, dns.RcodeServerFailure)

		return reply
	})

	r := newTestResolver(t)

	ctx, cancel := context.WithTimeout(
		context.Background(), transportDeadline,
	)
	defer cancel()

	resp, err := r.QueryNameserverIP(
		ctx, failingNS, addr, transportHostname,
	)

	require.NoError(t, err)
	require.NotNil(t, resp)

	assert.Equal(t, resolver.StatusError, resp.Status)
	assert.Equal(t, "server returned SERVFAIL", resp.Error)
	assert.Empty(t, resp.Records)
	assert.Equal(t, failingNS, resp.Nameserver)
}

// TestQueryNameserverIP_NoListener pins the third transport outcome:
// a refused datagram is not a timeout. The socket fails immediately
// with ECONNREFUSED rather than going quiet, so isTimeout is false,
// no failure flag is set, and the response classifies as NoData.
// Asserting it here is what stops that path being mistaken for the
// timeout path, in either direction.
func TestQueryNameserverIP_NoListener(t *testing.T) {
	t.Parallel()

	addr := unservedAddr(t)

	r := newTestResolver(t)

	ctx, cancel := context.WithTimeout(
		context.Background(), transportDeadline,
	)
	defer cancel()

	resp, err := r.QueryNameserverIP(
		ctx, silentNS, addr, transportHostname,
	)

	require.NoError(t, err)
	require.NotNil(t, resp)

	assert.Equal(t, resolver.StatusNoData, resp.Status)
	assert.Empty(t, resp.Records)
}