collect all dependencies for create_before_destroy

An earlier change to eliminate the large amount of duplicate edges being
added by the original CreateBeforeDestroy dependency mapper mistakingly
prevented adding edges when there are multiple CBD dependencies.

This updates the algorithm to use a map to collect all possible edges
and de-deplucating them before processing.

terraform/transform_destroy_cbd.go

@@ -259,7 +259,9 @@ func (t *CBDEdgeTransformer) depMap(g *Graph, destroyMap map[string][]dag.Vertex
 	// Build the list of destroy nodes that each resource address should depend
 	// on. For example, when we find B, we map the address of B to A_d in the
 	// "depMap" variable below.
-	depMap := make(map[string][]dag.Vertex)
+
+	// Use a nested map to remove duplicate edges.
+	depMap := make(map[string]map[dag.Vertex]struct{})
 	for _, v := range g.Vertices() {
 		// We're looking for resources.
 		rn, ok := v.(GraphNodeResource)
@@ -294,12 +296,24 @@ func (t *CBDEdgeTransformer) depMap(g *Graph, destroyMap map[string][]dag.Vertex
 			// needs to depend on.
 			key := rn.ResourceAddr().String()
 
-			// we only need to record the same dns once
-			if _, ok := depMap[key]; !ok {
-				depMap[key] = append(depMap[key], dns...)
+			deps, ok := depMap[key]
+			if !ok {
+				deps = make(map[dag.Vertex]struct{})
 			}
+
+			for _, d := range dns {
+				deps[d] = struct{}{}
+			}
+			depMap[key] = deps
 		}
 	}
 
-	return depMap, nil
+	result := map[string][]dag.Vertex{}
+	for k, m := range depMap {
+		for v := range m {
+			result[k] = append(result[k], v)
+		}
+	}
+
+	return result, nil
 }