package rdns

import (
	"fmt"
	"io"
	"net"
	"sync"
	"time"

	"github.com/miekg/dns"
)

// Defines how long to wait for a response from the resolver if no other timeout is given.
const defaultQueryTimeout = 2 * time.Second

// Tear down an upstream connection if nothing has been received for this long.
const idleTimeout = 10 * time.Second

// Pipeline is a DNS client that is able to use pipelining for multiple requests over
// one connection, handle out-of-order responses and deals with disconnects
// gracefully. It opens a single connection on demand and uses it for all queries.
// It can manage UDP, TCP, DNS-over-TLS, and DNS-over-DTLS connections.
type Pipeline struct {
	addr     string
	client   DNSDialer
	requests chan *request
	metrics  *ListenerMetrics
	timeout  time.Duration
}

// DNSDialer is an abstraction for a dns.Client that returns a *dns.Conn.
type DNSDialer interface {
	Dial(address string) (*dns.Conn, error)
}

// NewPipeline returns an initialized (and running) DNS connection manager.
func NewPipeline(id string, addr string, client DNSDialer, timeout time.Duration) *Pipeline {
	if timeout == 0 {
		timeout = defaultQueryTimeout
	}
	c := &Pipeline{
		addr:     addr,
		client:   client,
		requests: make(chan *request),
		metrics:  NewListenerMetrics("client", id),
		timeout:  timeout,
	}
	go c.start()
	return c
}

// Resolve a single query using this connection.
func (c *Pipeline) Resolve(q *dns.Msg) (*dns.Msg, error) {
	r := newRequest(q)

	timeout := time.NewTimer(c.timeout)
	defer timeout.Stop()

	// Queue up the request or time out
	select {
	case c.requests <- r:
	case <-timeout.C:
		c.metrics.err.Add("querytimeout", 1)
		return nil, QueryTimeoutError{q}
	}

	// Wait for the request to complete or time out
	select {
	case <-r.done:
	case <-timeout.C:
		c.metrics.err.Add("querytimeout", 1)
		return nil, QueryTimeoutError{q}
	}

	return r.waitFor()
}

// Starts a loop that will wait for queries and open an upstream connection on-demand, writing queries
// and reading answers concurrently using the same connection. It also handles errors like idle
// close from upstream.
func (c *Pipeline) start() {
	var (
		wg       sync.WaitGroup
		inFlight inFlightQueue
	)
	log := Log.With("addr", c.addr)
	for req := range c.requests { // Lazy connection. Only open a real connection if there's a request
		done := make(chan struct{})
		log.Debug("opening connection")
		conn, err := c.client.Dial(c.addr)
		if err != nil {
			c.metrics.err.Add("open", 1)
			log.Warn("failed to open connection", "error", err)
			req.markDone(nil, err)
			continue
		}
		wg.Add(2)

		go func(r *request) { c.requests <- r }(req) // re-queue the request that triggered the upstream connection

		go func() { // writer
			for {
				select {
				case req := <-c.requests:
					query := inFlight.add(req)
					log.With("qname", qName(query)).Debug("sending query")
					c.metrics.query.Add(1)
					if err := conn.WriteMsg(query); err != nil {
						req.markDone(nil, err) // fail the request
						inFlight.get(query)    // clean up the in-flight queue so it doesn't keep growing
						conn.Close()           // throw away this connection, should wake up the reader as well
						wg.Done()
						c.metrics.err.Add("send_query", 1)
						log.With("qname", qName(query)).Debug("failed sending query",
							"error", err)
						return
					}
				case <-done: // the reader ran into an error and we want to stop using this connection
					wg.Done()
					return
				}
			}
		}()
		go func() { // reader
			for {
				// Set the idle deadline on the reader, not the writer since when using UDP "connections",
				// a network topology change wouldn't be noticed. Putting the idle timeout here ensures
				// a reconnect in that case as well. This does create a very slight race however if the
				// sender is using the connection right at the time of the timeout in the receiver.
				_ = conn.SetReadDeadline(time.Now().Add(idleTimeout))
				a, err := conn.ReadMsg()
				if err != nil {
					switch e := err.(type) {
					case net.Error:
						if e.Timeout() {
							log.Debug("connection terminated by idle timeout")
						} else {
							c.metrics.err.Add("server_term", 1)
							log.Debug("connection terminated by server")
						}
						close(done) // tell the writer to not use this connection anymore
						wg.Done()
						return
					default:
						if err == io.EOF {
							c.metrics.err.Add("server_eof", 1)
							log.Debug("connection terminated by server")
							close(done) // tell the writer to not use this connection anymore
							wg.Done()
							return
						}
						// It's possible the response can't be correctly parsed, but we do have a response.
						// In this case, return it and carry on, don't terminate the connection because we
						// got a bad packet (like a truncated one for example).
						if a == nil {
							c.metrics.err.Add("read", 1)
							log.Warn("read failed", "error", err)
							close(done) // tell the writer to not use this connection anymore
							wg.Done()
							return
						}
						log.Warn("failed to read response", "error", err, "qname", qName(a))
					}
				}
				req := inFlight.get(a) // match the answer to an in-flight query
				if req == nil {
					c.metrics.err.Add("unexpected_a", 1)
					log.With("qname", qName(a)).Warn("unexpected answer received, ignoring")
					continue
				}
				c.metrics.response.Add(rCode(a), 1)
				req.markDone(a, nil)
				ql := inFlight.maxQueueLen()
				if ql > c.metrics.maxQueueLen.Value() {
					c.metrics.maxQueueLen.Set(ql)
				}
			}
		}()

		// wait for both, sender and receiver to terminate before trying to reconnect
		wg.Wait()
	}
}

// Request received from a client. It also contains the response and a channel that is
// closed when the request is done.
type request struct {
	q, a *dns.Msg
	err  error
	done chan struct{}
}

func newRequest(q *dns.Msg) *request {
	return &request{
		q:    q,
		done: make(chan struct{}),
	}
}

// Wait for the request to be completed and return the answer.
func (r *request) waitFor() (*dns.Msg, error) {
	<-r.done

	if r.err == nil {
		// As per https://tools.ietf.org/html/rfc7858#section-3.3, we need to double check this
		// really is the correct response.
		if len(r.a.Question) > 0 && len(r.q.Question) > 0 {
			q := r.q.Question[0]
			a := r.a.Question[0]
			if a.Name != q.Name || a.Qclass != q.Qclass || a.Qtype != q.Qtype {
				return nil, fmt.Errorf("expected answer for %s, got %s", q.String(), a.String())
			}
		}
	}

	return r.a, r.err
}

// Mark the request as complete.
func (r *request) markDone(a *dns.Msg, err error) {
	if a != nil {
		a.Id = r.q.Id // Fix the query ID in the answer to match the query
	}
	r.a = a
	r.err = err
	close(r.done)
}

// Queue to manage requests that are in flight. Used to asynchronously match received
// responses with their requests.
type inFlightQueue struct {
	requests  map[uint16]*request
	mu        sync.Mutex
	idCounter uint16
	maxLen    int
}

// Add a request to the queue and return an updated DNS query with a new ID. The ID needs
// to be unique per connection, and we could be receiving multiple queries with the same
// ID. So make up a new ID, used that in the query upstream, then map it back to the
// request and replace the ID with the original one.
func (q *inFlightQueue) add(r *request) *dns.Msg {
	q.mu.Lock()
	defer q.mu.Unlock()
	if q.requests == nil {
		q.requests = make(map[uint16]*request)
	}
	q.idCounter++
	q.requests[q.idCounter] = r
	query := r.q.Copy()
	query.Id = q.idCounter
	if len(q.requests) > q.maxLen {
		q.maxLen = len(q.requests)
	}
	return query
}

// Returns the request for a given query ID, or nil if the request isn't in the queue. The
// request is removed from the queue.
func (q *inFlightQueue) get(a *dns.Msg) *request {
	q.mu.Lock()
	defer q.mu.Unlock()
	id := a.Id
	r, ok := q.requests[id]
	if !ok {
		return nil
	}
	delete(q.requests, id)
	return r
}

func (q *inFlightQueue) maxQueueLen() int64 {
	q.mu.Lock()
	defer q.mu.Unlock()
	return int64(q.maxLen)
}