Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/cache/handler.go

@@ -0,0 +1,119 @@
+package cache
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	"github.com/prometheus/client_golang/prometheus"
+	"golang.org/x/net/context"
+)
+
+// ServeDNS implements the plugin.Handler interface.
+func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+
+	qname := state.Name()
+	qtype := state.QType()
+	zone := plugin.Zones(c.Zones).Matches(qname)
+	if zone == "" {
+		return c.Next.ServeDNS(ctx, w, r)
+	}
+
+	do := state.Do() // TODO(): might need more from OPT record? Like the actual bufsize?
+
+	now := time.Now().UTC()
+
+	i, ttl := c.get(now, qname, qtype, do)
+	if i != nil && ttl > 0 {
+		resp := i.toMsg(r)
+
+		state.SizeAndDo(resp)
+		resp, _ = state.Scrub(resp)
+		w.WriteMsg(resp)
+
+		i.Freq.Update(c.duration, now)
+
+		pct := 100
+		if i.origTTL != 0 { // you'll never know
+			pct = int(float64(ttl) / float64(i.origTTL) * 100)
+		}
+
+		if c.prefetch > 0 && i.Freq.Hits() > c.prefetch && pct < c.percentage {
+			// 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.
+			prr := &ResponseWriter{ResponseWriter: w, Cache: c, prefetch: true}
+			plugin.NextOrFailure(c.Name(), c.Next, ctx, prr, r)
+
+			if i1, _ := c.get(now, qname, qtype, do); i1 != nil {
+				i1.Freq.Reset(now, i.Freq.Hits())
+			}
+		}
+
+		return dns.RcodeSuccess, nil
+	}
+
+	crr := &ResponseWriter{ResponseWriter: w, Cache: c}
+	return plugin.NextOrFailure(c.Name(), c.Next, ctx, crr, r)
+}
+
+// Name implements the Handler interface.
+func (c *Cache) Name() string { return "cache" }
+
+func (c *Cache) get(now time.Time, qname string, qtype uint16, do bool) (*item, int) {
+	k := hash(qname, qtype, do)
+
+	if i, ok := c.ncache.Get(k); ok {
+		cacheHits.WithLabelValues(Denial).Inc()
+		return i.(*item), i.(*item).ttl(now)
+	}
+
+	if i, ok := c.pcache.Get(k); ok {
+		cacheHits.WithLabelValues(Success).Inc()
+		return i.(*item), i.(*item).ttl(now)
+	}
+	cacheMisses.Inc()
+	return nil, 0
+}
+
+var (
+	cacheSize = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "size",
+		Help:      "The number of elements in the cache.",
+	}, []string{"type"})
+
+	cacheCapacity = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "capacity",
+		Help:      "The cache's capacity.",
+	}, []string{"type"})
+
+	cacheHits = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "hits_total",
+		Help:      "The count of cache hits.",
+	}, []string{"type"})
+
+	cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: subsystem,
+		Name:      "misses_total",
+		Help:      "The count of cache misses.",
+	})
+)
+
+const subsystem = "cache"
+
+func init() {
+	prometheus.MustRegister(cacheSize)
+	prometheus.MustRegister(cacheCapacity)
+	prometheus.MustRegister(cacheHits)
+	prometheus.MustRegister(cacheMisses)
+}