plugin/metadata: add metadata plugin (#1894)

* plugin/metadata: add metadata plugin

* plugin/metadata: Add MD struct, refactor code, fix doc

* plugin/metadata: simplify metadata key

* plugin/metadata: improve setup_test

* Support of metadata by rewrite plugin. Move calculated variables to metadata.

* Move variables from metadata to pkg, add UTs, READMEs change, metadata small fixes

* Add client port validation to variables_test

* plugin/metadata: improve README

* plugin/metadata: rename methods

* plugin/metadata: Update Metadataer interface, update doc, cosmetic code changes

* plugin/metadata: move colllisions check to OnStartup(). Fix default variables metadataer.

* plugin/metadata: Fix comment for method setValue

* plugin/metadata: change variables order to fix linter warning

* plugin/metadata: rename Metadataer to Provider

plugin/metadata/metadata_test.go

@@ -0,0 +1,79 @@
+package metadata
+
+import (
+	"context"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+// testProvider implements fake Providers. Plugins which inmplement Provider interface
+type testProvider map[string]interface{}
+
+func (m testProvider) MetadataVarNames() []string {
+	keys := []string{}
+	for k := range m {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
+func (m testProvider) Metadata(ctx context.Context, w dns.ResponseWriter, r *dns.Msg, key string) (val interface{}, ok bool) {
+	value, ok := m[key]
+	return value, ok
+}
+
+// testHandler implements plugin.Handler
+type testHandler struct{ ctx context.Context }
+
+func (m *testHandler) Name() string { return "testHandler" }
+
+func (m *testHandler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	m.ctx = ctx
+	return 0, nil
+}
+
+func TestMetadataServDns(t *testing.T) {
+	expectedMetadata := []testProvider{
+		testProvider{"testkey1": "testvalue1"},
+		testProvider{"testkey2": 2, "testkey3": "testvalue3"},
+	}
+	// Create fake Providers based on expectedMetadata
+	providers := []Provider{}
+	for _, e := range expectedMetadata {
+		providers = append(providers, e)
+	}
+	// Fake handler which stores the resulting context
+	next := &testHandler{}
+
+	metadata := Metadata{
+		Zones:     []string{"."},
+		Providers: providers,
+		Next:      next,
+	}
+	metadata.ServeDNS(context.TODO(), &test.ResponseWriter{}, new(dns.Msg))
+
+	// Verify that next plugin can find metadata in context from all Providers
+	for _, expected := range expectedMetadata {
+		md, ok := FromContext(next.ctx)
+		if !ok {
+			t.Fatalf("Metadata is expected but not present inside the context")
+		}
+		for expKey, expVal := range expected {
+			metadataVal, valOk := md.Value(expKey)
+			if !valOk {
+				t.Fatalf("Value by key %v can't be retrieved", expKey)
+			}
+			if metadataVal != expVal {
+				t.Errorf("Expected value %v, but got %v", expVal, metadataVal)
+			}
+		}
+		wrongKey := "wrong_key"
+		metadataVal, ok := md.Value(wrongKey)
+		if ok {
+			t.Fatalf("Value by key %v is not expected to be recieved, but got: %v", wrongKey, metadataVal)
+		}
+	}
+}