Add Node.ID

Fixes #2071

hugolib/node.go

@@ -14,10 +14,12 @@
 package hugolib
 
 import (
-	"github.com/spf13/cast"
 	"html/template"
 	"sync"
+	"sync/atomic"
 	"time"
+
+	"github.com/spf13/cast"
 )
 
 type Node struct {
@@ -37,6 +39,24 @@ type Node struct {
 	paginator     *Pager
 	paginatorInit sync.Once
 	scratch       *Scratch
+	id            int
+	idInit        sync.Once
+}
+
+// This should probably be owned by Site and new ids assigned on creation,
+// but that would lead to massive changes; do it simple for now.
+var nodeIDCounter uint64
+
+func nextNodeID() int {
+	return int(atomic.AddUint64(&nodeIDCounter, 1))
+}
+
+// ID returns an integer that identifies this Node.
+// This is unique for a given Hugo build, but must not be considered stable.
+// See UniqueID on Page for an identify that is stable for repeated builds.
+func (n *Node) ID() int {
+	n.idInit.Do(func() { n.id = nextNodeID() })
+	return n.id
 }
 
 func (n *Node) Now() time.Time {