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/setup.go

@@ -0,0 +1,71 @@
+package metadata
+
+import (
+	"fmt"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("metadata", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	m, err := metadataParse(c)
+	if err != nil {
+		return err
+	}
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		m.Next = next
+		return m
+	})
+
+	c.OnStartup(func() error {
+		plugins := dnsserver.GetConfig(c).Handlers()
+		// Collect all plugins which implement Provider interface
+		metadataVariables := map[string]bool{}
+		for _, p := range plugins {
+			if met, ok := p.(Provider); ok {
+				for _, varName := range met.MetadataVarNames() {
+					if _, ok := metadataVariables[varName]; ok {
+						return fmt.Errorf("Metadata variable '%v' has duplicates", varName)
+					}
+					metadataVariables[varName] = true
+				}
+				m.Providers = append(m.Providers, met)
+			}
+		}
+		return nil
+	})
+
+	return nil
+}
+
+func metadataParse(c *caddy.Controller) (*Metadata, error) {
+	m := &Metadata{}
+	c.Next()
+	zones := c.RemainingArgs()
+
+	if len(zones) != 0 {
+		m.Zones = zones
+		for i := 0; i < len(m.Zones); i++ {
+			m.Zones[i] = plugin.Host(m.Zones[i]).Normalize()
+		}
+	} else {
+		m.Zones = make([]string, len(c.ServerBlockKeys))
+		for i := 0; i < len(c.ServerBlockKeys); i++ {
+			m.Zones[i] = plugin.Host(c.ServerBlockKeys[i]).Normalize()
+		}
+	}
+
+	if c.NextBlock() || c.Next() {
+		return nil, plugin.Error("metadata", c.ArgErr())
+	}
+	return m, nil
+}