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

1 files changed, 84 insertions, 0 deletions

diff --git a/plugin/trace/trace.go b/plugin/trace/trace.go
new file mode 100644
index 000000000..fa561945e
--- /dev/null
+++ b/plugin/trace/trace.go
@@ -0,0 +1,84 @@
+// Package trace implements OpenTracing-based tracing
+package trace
+
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+
+	"github.com/coredns/coredns/plugin"
+	// Plugin the trace package.
+	_ "github.com/coredns/coredns/plugin/pkg/trace"
+
+	"github.com/miekg/dns"
+	ot "github.com/opentracing/opentracing-go"
+	zipkin "github.com/openzipkin/zipkin-go-opentracing"
+	"golang.org/x/net/context"
+)
+
+type trace struct {
+	Next            plugin.Handler
+	ServiceEndpoint string
+	Endpoint        string
+	EndpointType    string
+	tracer          ot.Tracer
+	serviceName     string
+	clientServer    bool
+	every           uint64
+	count           uint64
+	Once            sync.Once
+}
+
+func (t *trace) Tracer() ot.Tracer {
+	return t.tracer
+}
+
+// OnStartup sets up the tracer
+func (t *trace) OnStartup() error {
+	var err error
+	t.Once.Do(func() {
+		switch t.EndpointType {
+		case "zipkin":
+			err = t.setupZipkin()
+		default:
+			err = fmt.Errorf("unknown endpoint type: %s", t.EndpointType)
+		}
+	})
+	return err
+}
+
+func (t *trace) setupZipkin() error {
+
+	collector, err := zipkin.NewHTTPCollector(t.Endpoint)
+	if err != nil {
+		return err
+	}
+
+	recorder := zipkin.NewRecorder(collector, false, t.ServiceEndpoint, t.serviceName)
+	t.tracer, err = zipkin.NewTracer(recorder, zipkin.ClientServerSameSpan(t.clientServer))
+
+	return err
+}
+
+// Name implements the Handler interface.
+func (t *trace) Name() string {
+	return "trace"
+}
+
+// ServeDNS implements the plugin.Handle interface.
+func (t *trace) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	trace := false
+	if t.every > 0 {
+		queryNr := atomic.AddUint64(&t.count, 1)
+
+		if queryNr%t.every == 0 {
+			trace = true
+		}
+	}
+	if span := ot.SpanFromContext(ctx); span == nil && trace {
+		span := t.Tracer().StartSpan("servedns")
+		defer span.Finish()
+		ctx = ot.ContextWithSpan(ctx, span)
+	}
+	return plugin.NextOrFailure(t.Name(), t.Next, ctx, w, r)
+}