// Package proxy implements a forwarding proxy with connection caching.
// It manages a pool of upstream connections (UDP and TCP) to reuse them for subsequent requests,
// reducing latency and handshake overhead. It supports in-band health checking.
package proxy

import (
	"context"
	"errors"
	"io"
	"strconv"
	"strings"
	"sync/atomic"
	"time"

	"github.com/coredns/coredns/request"

	"github.com/miekg/dns"
)

const (
	ErrTransportStopped = "proxy: transport stopped"
)

// limitTimeout is a utility function to auto-tune timeout values
// average observed time is moved towards the last observed delay moderated by a weight
// next timeout to use will be the double of the computed average, limited by min and max frame.
func limitTimeout(currentAvg *int64, minValue time.Duration, maxValue time.Duration) time.Duration {
	rt := time.Duration(atomic.LoadInt64(currentAvg))
	if rt < minValue {
		return minValue
	}
	if rt < maxValue/2 {
		return 2 * rt
	}
	return maxValue
}

func averageTimeout(currentAvg *int64, observedDuration time.Duration, weight int64) {
	dt := time.Duration(atomic.LoadInt64(currentAvg))
	atomic.AddInt64(currentAvg, int64(observedDuration-dt)/weight)
}

func (t *Transport) dialTimeout() time.Duration {
	return limitTimeout(&t.avgDialTime, minDialTimeout, maxDialTimeout)
}

func (t *Transport) updateDialTimeout(newDialTime time.Duration) {
	averageTimeout(&t.avgDialTime, newDialTime, cumulativeAvgWeight)
}

// Dial dials the address configured in transport, potentially reusing a connection or creating a new one.
func (t *Transport) Dial(proto string) (*persistConn, bool, error) {
	// If tls has been configured; use it.
	if t.tlsConfig != nil {
		proto = "tcp-tls"
	}

	// Check if transport is stopped before attempting to dial
	select {
	case <-t.stop:
		return nil, false, errors.New(ErrTransportStopped)
	default:
	}

	transtype := stringToTransportType(proto)

	t.mu.Lock()
	// FIFO: take the oldest conn (front of slice) for source port diversity
	for len(t.conns[transtype]) > 0 {
		pc := t.conns[transtype][0]
		t.conns[transtype] = t.conns[transtype][1:]
		if time.Since(pc.used) > t.expire {
			pc.c.Close()
			continue
		}
		t.mu.Unlock()
		connCacheHitsCount.WithLabelValues(t.proxyName, t.addr, proto).Add(1)
		return pc, true, nil
	}
	t.mu.Unlock()

	connCacheMissesCount.WithLabelValues(t.proxyName, t.addr, proto).Add(1)

	reqTime := time.Now()
	timeout := t.dialTimeout()
	if proto == "tcp-tls" {
		conn, err := dns.DialTimeoutWithTLS("tcp", t.addr, t.tlsConfig, timeout)
		t.updateDialTimeout(time.Since(reqTime))
		return &persistConn{c: conn}, false, err
	}
	conn, err := dns.DialTimeout(proto, t.addr, timeout)
	t.updateDialTimeout(time.Since(reqTime))
	return &persistConn{c: conn}, false, err
}

// Connect selects an upstream, sends the request and waits for a response.
func (p *Proxy) Connect(ctx context.Context, state request.Request, opts Options) (*dns.Msg, error) {
	start := time.Now()

	var proto string
	switch {
	case opts.ForceTCP: // TCP flag has precedence over UDP flag
		proto = "tcp"
	case opts.PreferUDP:
		proto = "udp"
	default:
		proto = state.Proto()
	}

	pc, cached, err := p.transport.Dial(proto)
	if err != nil {
		return nil, err
	}

	// Set buffer size correctly for this client.
	pc.c.UDPSize = max(uint16(state.Size()), 512) // #nosec G115 -- UDP size fits in uint16

	pc.c.SetWriteDeadline(time.Now().Add(maxTimeout))
	// records the origin Id before upstream.
	originId := state.Req.Id
	state.Req.Id = dns.Id()
	defer func() {
		state.Req.Id = originId
	}()

	if err := pc.c.WriteMsg(state.Req); err != nil {
		pc.c.Close() // not giving it back
		if err == io.EOF && cached {
			return nil, ErrCachedClosed
		}
		return nil, err
	}

	var ret *dns.Msg
	pc.c.SetReadDeadline(time.Now().Add(p.readTimeout))
	for {
		ret, err = pc.c.ReadMsg()
		if err != nil {
			if ret != nil && (state.Req.Id == ret.Id) && p.transport.transportTypeFromConn(pc) == typeUDP && shouldTruncateResponse(err) {
				// For UDP, if the error is an overflow, we probably have an upstream misbehaving in some way.
				// (e.g. sending >512 byte responses without an eDNS0 OPT RR).
				// Instead of returning an error, return an empty response with TC bit set. This will make the
				// client retry over TCP (if that's supported) or at least receive a clean
				// error. The connection is still good so we break before the close.

				// Truncate the response.
				ret = truncateResponse(ret)
				break
			}

			pc.c.Close() // not giving it back
			if err == io.EOF && cached {
				return nil, ErrCachedClosed
			}
			// recovery the origin Id after upstream.
			if ret != nil {
				ret.Id = originId
			}
			return ret, err
		}
		// drop out-of-order responses
		if state.Req.Id == ret.Id {
			break
		}
	}
	// recovery the origin Id after upstream.
	ret.Id = originId

	p.transport.Yield(pc)

	rc, ok := dns.RcodeToString[ret.Rcode]
	if !ok {
		rc = strconv.Itoa(ret.Rcode)
	}

	requestDuration.WithLabelValues(p.proxyName, p.addr, rc).Observe(time.Since(start).Seconds())

	return ret, nil
}

const cumulativeAvgWeight = 4

// Function to determine if a response should be truncated.
func shouldTruncateResponse(err error) bool {
	// This is to handle a scenario in which upstream sets the TC bit, but doesn't truncate the response
	// and we get ErrBuf instead of overflow.
	if _, isDNSErr := err.(*dns.Error); isDNSErr && errors.Is(err, dns.ErrBuf) {
		return true
	} else if strings.Contains(err.Error(), "overflow") {
		return true
	}
	return false
}

// Function to return an empty response with TC (truncated) bit set.
func truncateResponse(response *dns.Msg) *dns.Msg {
	// Clear out Answer, Extra, and Ns sections
	response.Answer = nil
	response.Extra = nil
	response.Ns = nil

	// Set TC bit to indicate truncation.
	response.Truncated = true
	return response
}
