add module expansion to validation

We cannot evaluate expansion during validation, since the values may not
be known at that time.

Inject a nodeValidateModule, using the "Concrete" pattern used for other
node types during graph building. This node will always evaluate to a
single module instance, so that we have a valid context within which to
evaluate all sub resources.

terraform/graph_builder_plan.go

@@ -52,6 +52,7 @@ type PlanGraphBuilder struct {
 	ConcreteProvider       ConcreteProviderNodeFunc
 	ConcreteResource       ConcreteResourceNodeFunc
 	ConcreteResourceOrphan ConcreteResourceInstanceNodeFunc
+	ConcreteModule         ConcreteModuleNodeFunc
 
 	once sync.Once
 }
@@ -140,7 +141,10 @@ func (b *PlanGraphBuilder) Steps() []GraphTransformer {
 		// Create expansion nodes for all of the module calls. This must
 		// come after all other transformers that create nodes representing
 		// objects that can belong to modules.
-		&ModuleExpansionTransformer{Config: b.Config},
+		&ModuleExpansionTransformer{
+			Concrete: b.ConcreteModule,
+			Config:   b.Config,
+		},
 
 		// Connect so that the references are ready for targeting. We'll
 		// have to connect again later for providers and so on.

terraform/graph_builder_validate.go

@@ -27,6 +27,12 @@ func ValidateGraphBuilder(p *PlanGraphBuilder) GraphBuilder {
 		}
 	}
 
+	p.ConcreteModule = func(n *nodeExpandModule) dag.Vertex {
+		return &nodeValidateModule{
+			nodeExpandModule: *n,
+		}
+	}
+
 	// We purposely don't set any other concrete types since they don't
 	// require validation.
 

terraform/node_module_expand.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs"
+	"github.com/hashicorp/terraform/dag"
 	"github.com/hashicorp/terraform/lang"
 )
 
@@ -14,6 +15,8 @@ type graphNodeModuleCloser interface {
 	CloseModule() addrs.Module
 }
 
+type ConcreteModuleNodeFunc func(n *nodeExpandModule) dag.Vertex
+
 // nodeExpandModule represents a module call in the configuration that
 // might expand into multiple module instances depending on how it is
 // configured.
@@ -214,3 +217,40 @@ func (n *evalPrepareModuleExpansion) Eval(ctx EvalContext) (interface{}, error)
 
 	return nil, nil
 }
+
+// nodeValidateModule wraps a nodeExpand module for validation, ensuring that
+// no expansion is attempted during evaluation, when count and for_each
+// expressions may not be known.
+type nodeValidateModule struct {
+	nodeExpandModule
+}
+
+// GraphNodeEvalable
+func (n *nodeValidateModule) EvalTree() EvalNode {
+	return &evalValidateModule{
+		Addr:       n.Addr,
+		Config:     n.Config,
+		ModuleCall: n.ModuleCall,
+	}
+}
+
+type evalValidateModule struct {
+	Addr       addrs.Module
+	Config     *configs.Module
+	ModuleCall *configs.ModuleCall
+}
+
+func (n *evalValidateModule) Eval(ctx EvalContext) (interface{}, error) {
+	_, call := n.Addr.Call()
+	expander := ctx.InstanceExpander()
+
+	// Modules all evaluate to single instances during validation, only to
+	// create a proper context within which to evaluate. All parent modules
+	// will be a single instance, but still get our address in the expected
+	// manner anyway to ensure they've been registered correctly.
+	for _, module := range expander.ExpandModule(n.Addr.Parent()) {
+		// now set our own mode to single
+		ctx.InstanceExpander().SetModuleSingle(module, call)
+	}
+	return nil, nil
+}

terraform/transform_module_expansion.go

@@ -17,6 +17,7 @@ import (
 // that can be contained within modules have already been added.
 type ModuleExpansionTransformer struct {
 	Config   *configs.Config
+	Concrete ConcreteModuleNodeFunc
 }
 
 func (t *ModuleExpansionTransformer) Transform(g *Graph) error {
@@ -36,11 +37,16 @@ func (t *ModuleExpansionTransformer) transform(g *Graph, c *configs.Config, pare
 	_, call := c.Path.Call()
 	modCall := c.Parent.Module.ModuleCalls[call.Name]
 
-	v := &nodeExpandModule{
+	n := &nodeExpandModule{
 		Addr:       c.Path,
 		Config:     c.Module,
 		ModuleCall: modCall,
 	}
+	var v dag.Vertex = n
+	if t.Concrete != nil {
+		v = t.Concrete(n)
+	}
+
 	g.Add(v)
 	log.Printf("[TRACE] ModuleExpansionTransformer: Added %s as %T", c.Path, v)