terraform: Handle module depedency inversion

terraform/graph.go

@@ -274,7 +274,7 @@ func Graph(opts *GraphOpts) (*depgraph.Graph, error) {
 
 	// If we have a diff, then make sure to add that in
 	if modDiff != nil {
-		if err := graphAddDiff(g, modDiff); err != nil {
+		if err := graphAddDiff(g, opts.Diff, modDiff); err != nil {
 			return nil, err
 		}
 	}
@@ -544,10 +544,22 @@ func graphAddConfigResources(
 // destroying the VPC's subnets first, whereas creating a VPC requires
 // doing it before the subnets are created. This function handles inserting
 // these nodes for you.
-func graphAddDiff(g *depgraph.Graph, d *ModuleDiff) error {
+func graphAddDiff(g *depgraph.Graph, gDiff *Diff, d *ModuleDiff) error {
 	var nlist []*depgraph.Noun
+	var modules []*depgraph.Noun
 	injected := make(map[*depgraph.Dependency]struct{})
 	for _, n := range g.Nouns {
+		// A module is being destroyed if all it's resources are being
+		// destroyed (via a destroy plan) or if it is orphaned. Only in
+		// those cases do we need to handle depedency inversion.
+		if mod, ok := n.Meta.(*GraphNodeModule); ok {
+			md := gDiff.ModuleByPath(mod.Path)
+			if mod.Flags&FlagOrphan != 0 || (md != nil && md.Destroy) {
+				modules = append(modules, n)
+			}
+			continue
+		}
+
 		rn, ok := n.Meta.(*GraphNodeResource)
 		if !ok {
 			continue
@@ -693,9 +705,11 @@ func graphAddDiff(g *depgraph.Graph, d *ModuleDiff) error {
 		rn.Resource.Diff = rd
 	}
 
-	// Go through each noun and make sure we calculate all the dependencies
-	// properly.
-	for _, n := range nlist {
+	// Go through each resource and module and make sure we
+	// calculate all the dependencies properly.
+	invertDeps := [][]*depgraph.Noun{nlist, modules}
+	for _, list := range invertDeps {
+		for _, n := range list {
 			deps := n.Deps
 			num := len(deps)
 			for i := 0; i < num; i++ {
@@ -766,6 +780,7 @@ func graphAddDiff(g *depgraph.Graph, d *ModuleDiff) error {
 			}
 			n.Deps = deps[:num]
 		}
+	}
 
 	// Add the nouns to the graph
 	g.Nouns = append(g.Nouns, nlist...)