plugin/metadata: metadata is just label=value (#1914)

This revert 17d807f0 and re-adds the metadata plugin as a plugin that
just sets a label to a value function.

Add package documentation on how to use the metadata package. Make it
clear that any caching is up to the Func implemented.

There are now - no in tree users. We could add the request metadata by
default under names that copy request.Request, i.e

request/ip - remote IP
request/port - remote port

Variables.go has been deleted.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/metadata/provider.go

@@ -1,3 +1,33 @@
+// Package metadata provides an API that allows plugins to add metadata to the context.
+// Each metadata is stored under a label that has the form <plugin>/<name>. Each metadata
+// is returned as a Func. When Func is called the metadata is returned. If Func is expensive to
+// execute it is its responsibility to provide some form of caching. During the handling of a
+// query it is expected the metadata stays constant.
+//
+// Basic example:
+//
+// Implement the Provder interface for a plugin:
+//
+//    func (p P) Metadata(ctx context.Context, state request.Request) context.Context {
+//       cached := ""
+//       f := func() string {
+//		if cached != "" {
+//                 return cached
+//             }
+//             cached = expensiveFunc()
+//             return cached
+//       }
+//       metadata.SetValueFunc(ctx, "test/something", f)
+//	 return ctx
+//    }
+//
+// Check the metadata from another plugin:
+//
+//    // ...
+//    valueFunc := metadata.ValueFunc(ctx, "test/something")
+//    value := valueFunc()
+//    // use 'value'
+//
 package metadata
 
 import (
@@ -8,40 +38,62 @@ import (
 
 // Provider interface needs to be implemented by each plugin willing to provide
 // metadata information for other plugins.
-// Note: this method should work quickly, because it is called for every request
-// from the metadata plugin.
 type Provider interface {
-	// List of variables which are provided by current Provider. Must remain constant.
-	MetadataVarNames() []string
-	// Metadata is expected to return a value with metadata information by the key
-	// from 4th argument. Value can be later retrieved from context by any other plugin.
-	// If value is not available by some reason returned boolean value should be false.
-	Metadata(ctx context.Context, state request.Request, variable string) (interface{}, bool)
+	// Metadata adds metadata to the context and returns a (potentially) new context.
+	// Note: this method should work quickly, because it is called for every request
+	// from the metadata plugin.
+	Metadata(ctx context.Context, state request.Request) context.Context
 }
 
-// M is metadata information storage.
-type M map[string]interface{}
+// Func is the type of function in the metadata, when called they return the value of the label.
+type Func func() string
 
-// FromContext retrieves the metadata from the context.
-func FromContext(ctx context.Context) (M, bool) {
-	if metadata := ctx.Value(metadataKey{}); metadata != nil {
-		if m, ok := metadata.(M); ok {
-			return m, true
+// Labels returns all metadata keys stored in the context. These label names should be named
+// as: plugin/NAME, where NAME is something descriptive.
+func Labels(ctx context.Context) []string {
+	if metadata := ctx.Value(key{}); metadata != nil {
+		if m, ok := metadata.(md); ok {
+			return keys(m)
 		}
 	}
-	return M{}, false
+	return nil
 }
 
-// Value returns metadata value by key.
-func (m M) Value(key string) (value interface{}, ok bool) {
-	value, ok = m[key]
-	return value, ok
+// ValueFunc returns the value function of label. If none can be found nil is returned. Calling the
+// function returns the value of the label.
+func ValueFunc(ctx context.Context, label string) Func {
+	if metadata := ctx.Value(key{}); metadata != nil {
+		if m, ok := metadata.(md); ok {
+			return m[label]
+		}
+	}
+	return nil
 }
 
-// SetValue sets the metadata value under key.
-func (m M) SetValue(key string, val interface{}) {
-	m[key] = val
+// SetValueFunc set the metadata label to the value function. If no metadata can be found this is a noop and
+// false is returned. Any existing value is overwritten.
+func SetValueFunc(ctx context.Context, label string, f Func) bool {
+	if metadata := ctx.Value(key{}); metadata != nil {
+		if m, ok := metadata.(md); ok {
+			m[label] = f
+			return true
+		}
+	}
+	return false
 }
 
-// metadataKey defines the type of key that is used to save metadata into the context.
-type metadataKey struct{}
+// md is metadata information storage.
+type md map[string]Func
+
+// key defines the type of key that is used to save metadata into the context.
+type key struct{}
+
+func keys(m map[string]Func) []string {
+	s := make([]string, len(m))
+	i := 0
+	for k := range m {
+		s[i] = k
+		i++
+	}
+	return s
+}