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/plugin.go

@@ -0,0 +1,102 @@
+// Package plugin provides some types and functions common among plugin.
+package plugin
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/miekg/dns"
+	ot "github.com/opentracing/opentracing-go"
+	"golang.org/x/net/context"
+)
+
+type (
+	// Plugin is a middle layer which represents the traditional
+	// idea of plugin: it chains one Handler to the next by being
+	// passed the next Handler in the chain.
+	Plugin func(Handler) Handler
+
+	// Handler is like dns.Handler except ServeDNS may return an rcode
+	// and/or error.
+	//
+	// If ServeDNS writes to the response body, it should return a status
+	// code. If the status code is not one of the following:
+	//
+	// * SERVFAIL (dns.RcodeServerFailure)
+	//
+	// * REFUSED (dns.RecodeRefused)
+	//
+	// * FORMERR (dns.RcodeFormatError)
+	//
+	// * NOTIMP (dns.RcodeNotImplemented)
+	//
+	// CoreDNS assumes *no* reply has yet been written. All other response
+	// codes signal other handlers above it that the response message is
+	// already written, and that they should not write to it also.
+	//
+	// If ServeDNS encounters an error, it should return the error value
+	// so it can be logged by designated error-handling plugin.
+	//
+	// If writing a response after calling another ServeDNS method, the
+	// returned rcode SHOULD be used when writing the response.
+	//
+	// If handling errors after calling another ServeDNS method, the
+	// returned error value SHOULD be logged or handled accordingly.
+	//
+	// Otherwise, return values should be propagated down the plugin
+	// chain by returning them unchanged.
+	Handler interface {
+		ServeDNS(context.Context, dns.ResponseWriter, *dns.Msg) (int, error)
+		Name() string
+	}
+
+	// HandlerFunc is a convenience type like dns.HandlerFunc, except
+	// ServeDNS returns an rcode and an error. See Handler
+	// documentation for more information.
+	HandlerFunc func(context.Context, dns.ResponseWriter, *dns.Msg) (int, error)
+)
+
+// ServeDNS implements the Handler interface.
+func (f HandlerFunc) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	return f(ctx, w, r)
+}
+
+// Name implements the Handler interface.
+func (f HandlerFunc) Name() string { return "handlerfunc" }
+
+// Error returns err with 'plugin/name: ' prefixed to it.
+func Error(name string, err error) error { return fmt.Errorf("%s/%s: %s", "plugin", name, err) }
+
+// NextOrFailure calls next.ServeDNS when next is not nill, otherwise it will return, a ServerFailure
+// and a nil error.
+func NextOrFailure(name string, next Handler, ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	if next != nil {
+		if span := ot.SpanFromContext(ctx); span != nil {
+			child := span.Tracer().StartSpan(next.Name(), ot.ChildOf(span.Context()))
+			defer child.Finish()
+			ctx = ot.ContextWithSpan(ctx, child)
+		}
+		return next.ServeDNS(ctx, w, r)
+	}
+
+	return dns.RcodeServerFailure, Error(name, errors.New("no next plugin found"))
+}
+
+// ClientWrite returns true if the response has been written to the client.
+// Each plugin to adhire to this protocol.
+func ClientWrite(rcode int) bool {
+	switch rcode {
+	case dns.RcodeServerFailure:
+		fallthrough
+	case dns.RcodeRefused:
+		fallthrough
+	case dns.RcodeFormatError:
+		fallthrough
+	case dns.RcodeNotImplemented:
+		return false
+	}
+	return true
+}
+
+// Namespace is the namespace used for the metrics.
+const Namespace = "coredns"