From f766bb8380e370502b1c2c0d3ef75396811d159c Mon Sep 17 00:00:00 2001
From: James Bardin <j.bardin@gmail.com>
Date: Sat, 28 Sep 2019 20:34:55 -0400
Subject: [PATCH] prune unused resources from apply

If a resource is only destroying instances, there is no reason to
prepare the state and we can remove the Resource (prepare state) nodes.
They normally have pose no issue, but if the instances are being
destroyed along with their dependencies, the resource node may fail to
evaluate due to the missing dependencies (since destroy happens in the
reverse order).

These failures were previously blocked by there being a cycle when the
destroy nodes were directly attached to the resource nodes.
---
 terraform/transform_destroy_edge.go | 42 +++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/terraform/transform_destroy_edge.go b/terraform/transform_destroy_edge.go
index 7fb415bdfc..be1540ffef 100644
--- a/terraform/transform_destroy_edge.go
+++ b/terraform/transform_destroy_edge.go
@@ -277,5 +277,47 @@ func (t *DestroyEdgeTransformer) Transform(g *Graph) error {
 		}
 	}
 
+	return t.pruneResources(g)
+}
+
+// If there are only destroy instances for a particular resource, there's no
+// reason for the resource node to prepare the state. Remove Resource nodes so
+// that they don't fail by trying to evaluate a resource that is only being
+// destroyed along with its dependencies.
+func (t *DestroyEdgeTransformer) pruneResources(g *Graph) error {
+	for _, v := range g.Vertices() {
+		n, ok := v.(*NodeApplyableResource)
+		if !ok {
+			continue
+		}
+
+		// if there are only destroy dependencies, we don't need this node
+		des, err := g.Descendents(n)
+		if err != nil {
+			return err
+		}
+
+		descendents := des.List()
+		nonDestroyInstanceFound := false
+		for _, v := range descendents {
+			if _, ok := v.(*NodeApplyableResourceInstance); ok {
+				nonDestroyInstanceFound = true
+				break
+			}
+		}
+
+		if nonDestroyInstanceFound {
+			continue
+		}
+
+		// connect all the through-edges, then delete the node
+		for _, d := range g.DownEdges(n).List() {
+			for _, u := range g.UpEdges(n).List() {
+				g.Connect(dag.BasicEdge(u, d))
+			}
+		}
+		log.Printf("DestroyEdgeTransformer: pruning unused resource node %s", dag.VertexName(n))
+		g.Remove(n)
+	}
 	return nil
 }