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/item.go

@@ -0,0 +1,116 @@
+package cache
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/plugin/cache/freq"
+	"github.com/coredns/coredns/plugin/pkg/response"
+	"github.com/miekg/dns"
+)
+
+type item struct {
+	Rcode              int
+	Authoritative      bool
+	AuthenticatedData  bool
+	RecursionAvailable bool
+	Answer             []dns.RR
+	Ns                 []dns.RR
+	Extra              []dns.RR
+
+	origTTL uint32
+	stored  time.Time
+
+	*freq.Freq
+}
+
+func newItem(m *dns.Msg, d time.Duration) *item {
+	i := new(item)
+	i.Rcode = m.Rcode
+	i.Authoritative = m.Authoritative
+	i.AuthenticatedData = m.AuthenticatedData
+	i.RecursionAvailable = m.RecursionAvailable
+	i.Answer = m.Answer
+	i.Ns = m.Ns
+	i.Extra = make([]dns.RR, len(m.Extra))
+	// Don't copy OPT record as these are hop-by-hop.
+	j := 0
+	for _, e := range m.Extra {
+		if e.Header().Rrtype == dns.TypeOPT {
+			continue
+		}
+		i.Extra[j] = e
+		j++
+	}
+	i.Extra = i.Extra[:j]
+
+	i.origTTL = uint32(d.Seconds())
+	i.stored = time.Now().UTC()
+
+	i.Freq = new(freq.Freq)
+
+	return i
+}
+
+// toMsg turns i into a message, it tailors the reply to m.
+// The Authoritative bit is always set to 0, because the answer is from the cache.
+func (i *item) toMsg(m *dns.Msg) *dns.Msg {
+	m1 := new(dns.Msg)
+	m1.SetReply(m)
+
+	m1.Authoritative = false
+	m1.AuthenticatedData = i.AuthenticatedData
+	m1.RecursionAvailable = i.RecursionAvailable
+	m1.Rcode = i.Rcode
+	m1.Compress = true
+
+	m1.Answer = make([]dns.RR, len(i.Answer))
+	m1.Ns = make([]dns.RR, len(i.Ns))
+	m1.Extra = make([]dns.RR, len(i.Extra))
+
+	ttl := uint32(i.ttl(time.Now()))
+	if ttl < minTTL {
+		ttl = minTTL
+	}
+
+	for j, r := range i.Answer {
+		m1.Answer[j] = dns.Copy(r)
+		m1.Answer[j].Header().Ttl = ttl
+	}
+	for j, r := range i.Ns {
+		m1.Ns[j] = dns.Copy(r)
+		m1.Ns[j].Header().Ttl = ttl
+	}
+	for j, r := range i.Extra {
+		m1.Extra[j] = dns.Copy(r)
+		if m1.Extra[j].Header().Rrtype != dns.TypeOPT {
+			m1.Extra[j].Header().Ttl = ttl
+		}
+	}
+	return m1
+}
+
+func (i *item) ttl(now time.Time) int {
+	ttl := int(i.origTTL) - int(now.UTC().Sub(i.stored).Seconds())
+	return ttl
+}
+
+func minMsgTTL(m *dns.Msg, mt response.Type) time.Duration {
+	if mt != response.NoError && mt != response.NameError && mt != response.NoData {
+		return 0
+	}
+
+	minTTL := maxTTL
+	for _, r := range append(m.Answer, m.Ns...) {
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				return time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+	return minTTL
+}