handler.go

package cache

import (
	"context"
	"math"
	"time"

	"github.com/coredns/coredns/plugin"
	"github.com/coredns/coredns/plugin/metadata"
	"github.com/coredns/coredns/plugin/metrics"
	"github.com/coredns/coredns/request"

	"github.com/miekg/dns"
)

// ServeDNS implements the plugin.Handler interface.
func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
	rc := r.Copy() // We potentially modify r, to prevent other plugins from seeing this (r is a pointer), copy r into rc.
	state := request.Request{W: w, Req: rc}
	do := state.Do()
	cd := r.CheckingDisabled
	ad := r.AuthenticatedData

	zone := plugin.Zones(c.Zones).Matches(state.Name())
	if zone == "" {
		return plugin.NextOrFailure(c.Name(), c.Next, ctx, w, rc)
	}

	now := c.now().UTC()
	server := metrics.WithServer(ctx)

	// On cache refresh, we will just use the DO bit from the incoming query for the refresh since we key our cache
	// with the query DO bit. That means two separate cache items for the query DO bit true or false. In the situation
	// in which upstream doesn't support DNSSEC, the two cache items will effectively be the same. Regardless, any
	// DNSSEC RRs in the response are written to cache with the response.

	var i *item
	key := hash(state.Name(), state.QType(), state.Do(), state.Req.CheckingDisabled)
	if c.NeedEarlyRefresh(state) {
		i = c.getEarly(now, state, server)
		if i == nil {
			crr := &ResponseWriter{
				ResponseWriter: w, Cache: c, state: state, server: server, do: do, ad: ad, cd: cd,
				nexcept: c.nexcept, pexcept: c.pexcept, wildcardFunc: wildcardFunc(ctx),
			}
			return c.doRefresh(ctx, state, crr)
		} else if c.shouldPrefetch(i, now) {
			cw := newPrefetchResponseWriter(server, state, c)
			go c.doPrefetch(ctx, state, cw, i, now)
		}
	} else {
		i = c.getLate(now, state, server)
		if i == nil {
			i = c.getEarly(now, state, server)
			if i == nil {
				crr := &ResponseWriter{
					ResponseWriter: w, Cache: c, state: state, server: server, do: do, ad: ad, cd: cd,
					nexcept: c.nexcept, pexcept: c.pexcept, wildcardFunc: wildcardFunc(ctx),
				}
				return c.doRefresh(ctx, state, crr)
			} else {
				c.copyToLate(key, i, now)
				if c.shouldPrefetch(i, now) {
					cw := newPrefetchResponseWriter(server, state, c)
					go c.doPrefetch(ctx, state, cw, i, now)
				}
			}
		} else {
			ttl := i.ttl(now)
			if ttl < 0 {
				// serve stale behavior
				if c.verifyStale {
					crr := &ResponseWriter{ResponseWriter: w, Cache: c, state: state, server: server, do: do, cd: cd}
					cw := newVerifyStaleResponseWriter(crr)
					ret, err := c.doRefresh(ctx, state, cw)
					if cw.refreshed {
						return ret, err
					}
				}

				// Adjust the time to get a 0 TTL in the reply built from a stale item.
				now = now.Add(time.Duration(ttl) * time.Second)
				if !c.verifyStale {
					cw := newPrefetchResponseWriter(server, state, c)
					go c.doPrefetch(ctx, state, cw, i, now)
				}
				servedStale.WithLabelValues(server, c.zonesMetricLabel, c.viewMetricLabel).Inc()
			} else if c.shouldPrefetch(i, now.Add(-c.extrattl)) {
				cw := newPrefetchResponseWriter(server, state, c)
				go c.doPrefetch(ctx, state, cw, i, now)
			}
		}
	}

	if i.wildcard != "" {
		// Set wildcard source record name to metadata
		metadata.SetValueFunc(ctx, "zone/wildcard", func() string {
			return i.wildcard
		})
	}

	if c.keepttl {
		// If keepttl is enabled we fake the current time to the stored
		// one so that we always get the original TTL
		now = i.stored
	}
	resp := i.toMsg(r, now, do, ad)
	w.WriteMsg(resp)
	return dns.RcodeSuccess, nil
}

func wildcardFunc(ctx context.Context) func() string {
	return func() string {
		// Get wildcard source record name from metadata
		if f := metadata.ValueFunc(ctx, "zone/wildcard"); f != nil {
			return f()
		}
		return ""
	}
}

func (c *Cache) doPrefetch(ctx context.Context, state request.Request, cw *ResponseWriter, i *item, now time.Time) {
	cachePrefetches.WithLabelValues(cw.server, c.zonesMetricLabel, c.viewMetricLabel).Inc()
	c.doRefresh(ctx, state, cw)

	// When prefetching we loose the item i, and with it the frequency
	// that we've gathered sofar. See we copy the frequencies info back
	// into the new item that was stored in the cache.
	if i1 := c.exists(state); i1 != nil {
		i1.Freq.Reset(now, i.Freq.Hits())
	}
}

func (c *Cache) doRefresh(ctx context.Context, state request.Request, cw dns.ResponseWriter) (int, error) {
	return plugin.NextOrFailure(c.Name(), c.Next, ctx, cw, state.Req)
}

func (c *Cache) shouldPrefetch(i *item, now time.Time) bool {
	if c.prefetch <= 0 {
		return false
	}
	i.Freq.Update(c.duration, now)
	threshold := int(math.Ceil(float64(c.percentage) / 100 * float64(i.origTTL)))
	return i.Freq.Hits() >= c.prefetch && i.ttl(now) <= threshold
}

// Name implements the Handler interface.
func (c *Cache) Name() string { return "cache" }

// getIgnoreTTL unconditionally returns an item if it exists in the cache.
func (c *Cache) getIgnoreTTL(now time.Time, state request.Request, server string) *item {
	k := hash(state.Name(), state.QType(), state.Do(), state.Req.CheckingDisabled)
	cacheRequests.WithLabelValues(server, c.zonesMetricLabel, c.viewMetricLabel).Inc()

	if i, ok := c.ncache.Get(k); ok {
		itm := i.(*item)
		ttl := itm.ttl(now)
		if itm.matches(state) && (ttl > 0 || (c.staleUpTo > 0 && -ttl < int(c.staleUpTo.Seconds()))) {
			cacheHits.WithLabelValues(server, Denial, c.zonesMetricLabel, c.viewMetricLabel).Inc()
			return i.(*item)
		}
	}
	if i, ok := c.pcache.Get(k); ok {
		itm := i.(*item)
		ttl := itm.ttl(now)
		if itm.matches(state) && (ttl > 0 || (c.staleUpTo > 0 && -ttl < int(c.staleUpTo.Seconds()))) {
			cacheHits.WithLabelValues(server, Success, c.zonesMetricLabel, c.viewMetricLabel).Inc()
			return i.(*item)
		}
	}
	cacheMisses.WithLabelValues(server, c.zonesMetricLabel, c.viewMetricLabel).Inc()
	return nil
}

func (c *Cache) exists(state request.Request) *item {
	k := hash(state.Name(), state.QType(), state.Do(), state.Req.CheckingDisabled)
	if i, ok := c.ncache.Get(k); ok {
		return i.(*item)
	}
	if i, ok := c.pcache.Get(k); ok {
		return i.(*item)
	}
	return nil
}