terraform: error if config graph is missing dependencies

terraform/graph_config.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 
+	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/config"
 	"github.com/hashicorp/terraform/config/module"
 	"github.com/hashicorp/terraform/dag"
@@ -54,19 +55,34 @@ func Graph2(mod *module.Tree) (*dag.Graph, error) {
 		g.Add(n)
 	}
 
+	// Err is where the final error value will go if there is one
+	var err error
+
 	// Go through all the nodes and build up the actual graph edges. We
 	// do this by getting the variables that each node depends on and then
 	// building the dep map based on the fullMap which contains the mapping
 	// of var names to the actual node with that name.
 	for _, n := range nodes {
 		for _, id := range n.Variables() {
-			if target, ok := fullMap[id]; ok {
-				g.Connect(dag.BasicEdge(n, target))
+			if id == "" {
+				// Empty name means its a variable we don't care about
+				continue
 			}
+
+			target, ok := fullMap[id]
+			if !ok {
+				// We can't find the target meaning the dependency
+				// is missing. Accumulate the error.
+				err = multierror.Append(err, fmt.Errorf(
+					"%s: missing dependency: %s", n, id))
+				continue
+			}
+
+			g.Connect(dag.BasicEdge(n, target))
 		}
 	}
 
-	return &g, nil
+	return &g, err
 }
 
 // varNameForVar returns the VarName value for an interpolated variable.

terraform/graph_config_node.go

@@ -11,7 +11,7 @@ import (
 // configuration graph need to implement in order to build the variable
 // dependencies properly.
 type graphNodeConfig interface {
-	dag.Vertex
+	dag.NamedVertex
 
 	// Variables returns the full list of variables that this node
 	// depends on. The values within the slice should map to the VarName()

terraform/graph_config_test.go

@@ -66,6 +66,13 @@ func TestGraph2_modules(t *testing.T) {
 	}
 }
 
+func TestGraph2_errMissingDeps(t *testing.T) {
+	_, err := Graph2(testModule(t, "graph-missing-deps"))
+	if err == nil {
+		t.Fatal("should error")
+	}
+}
+
 const testGraphBasicStr = `
 aws_instance.web
   aws_security_group.firewall

terraform/test-fixtures/graph-missing-deps/main.tf

@@ -0,0 +1,5 @@
+resource "aws_instance" "db" {}
+
+resource "aws_instance" "web" {
+    foo = "${aws_instance.lb.id}"
+}