core: Normalize results from NodeAbstractResource.StateReferences

The underlying References function includes duplicates and returns refs
in the order they appeared in source (approximately), but after we reduce
to just the raw addresses it's better to dedupe and return in a
predictable order.

terraform/node_resource_abstract.go

@@ -3,6 +3,7 @@ package terraform
 import (
 	"fmt"
 	"log"
+	"sort"
 
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs"
@@ -272,9 +273,20 @@ func dottedInstanceAddr(tr addrs.ResourceInstance) string {
 func (n *NodeAbstractResource) StateReferences() []addrs.Referenceable {
 	selfAddrs := n.ReferenceableAddrs()
 
+	// Since we don't include the source location references in our
+	// results from this method, we'll also filter out duplicates:
+	// there's no point in listing the same object twice without
+	// that additional context.
+	seen := map[string]struct{}{}
+
 	depsRaw := n.References()
 	deps := make([]addrs.Referenceable, 0, len(depsRaw))
 	for _, d := range depsRaw {
+		k := d.Subject.String()
+		if _, exists := seen[k]; exists {
+			continue
+		}
+		seen[k] = struct{}{}
 		switch tr := d.Subject.(type) {
 		case addrs.ResourceInstance:
 			deps = append(deps, tr)
@@ -303,6 +315,14 @@ func (n *NodeAbstractResource) StateReferences() []addrs.Referenceable {
 		}
 	}
 
+	// We'll also sort them, since that'll avoid creating changes in the
+	// serialized state that make no semantic difference.
+	sort.Slice(deps, func(i, j int) bool {
+		// Simple string-based sort because we just care about consistency,
+		// not user-friendliness.
+		return deps[i].String() < deps[j].String()
+	})
+
 	return deps
 }