terraform: detect compute counts and show a nicer error

This will detect computed counts (which we don't currently support) and
change the error to be more informative that we don't allow computed
counts. Prior to this, the error would instead be something like
`strconv.ParseInt: "${var.foo}" cannot be parsed as int`.

terraform/context_plan_test.go

@@ -1309,6 +1309,26 @@ func TestContext2Plan_countComputed(t *testing.T) {
 	}
 }
 
+func TestContext2Plan_countComputedModule(t *testing.T) {
+	m := testModule(t, "plan-count-computed-module")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	})
+
+	_, err := ctx.Plan()
+
+	expectedErr := "aws_instance.bar: value of 'count'"
+	if !strings.Contains(fmt.Sprintf("%s", err), expectedErr) {
+		t.Fatalf("expected err would contain %q\nerr: %s\n",
+			expectedErr, err)
+	}
+}
+
 func TestContext2Plan_countIndex(t *testing.T) {
 	m := testModule(t, "plan-count-index")
 	p := testProvider("aws")

terraform/eval_count_computed.go

@@ -0,0 +1,25 @@
+package terraform
+
+import (
+	"fmt"
+
+	"github.com/hashicorp/terraform/config"
+)
+
+// EvalCountCheckComputed is an EvalNode that checks if a resource count
+// is computed and errors if so. This can possibly happen across a
+// module boundary and we don't yet support this.
+type EvalCountCheckComputed struct {
+	Resource *config.Resource
+}
+
+// TODO: test
+func (n *EvalCountCheckComputed) Eval(ctx EvalContext) (interface{}, error) {
+	if n.Resource.RawCount.Value() == unknownValue() {
+		return nil, fmt.Errorf(
+			"%s: value of 'count' cannot be computed",
+			n.Resource.Id())
+	}
+
+	return nil, nil
+}

terraform/graph_config_node_resource.go

@@ -215,6 +215,7 @@ func (n *GraphNodeConfigResource) EvalTree() EvalNode {
 	return &EvalSequence{
 		Nodes: []EvalNode{
 			&EvalInterpolate{Config: n.Resource.RawCount},
+			&EvalCountCheckComputed{Resource: n.Resource},
 			&EvalOpFilter{
 				Ops:  []walkOperation{walkValidate},
 				Node: &EvalValidateCount{Resource: n.Resource},

terraform/node_resource_plan.go

@@ -31,6 +31,7 @@ func (n *NodePlannableResource) EvalTree() EvalNode {
 			// into the proper number of instances.
 			&EvalInterpolate{Config: n.Config.RawCount},
 
+			&EvalCountCheckComputed{Resource: n.Config},
 			&EvalCountFixZeroOneBoundary{Resource: n.Config},
 		},
 	}

terraform/test-fixtures/plan-count-computed-module/child/main.tf

@@ -0,0 +1,5 @@
+variable "value" {}
+
+resource "aws_instance" "bar" {
+    count = "${var.value}"
+}

terraform/test-fixtures/plan-count-computed-module/main.tf

@@ -0,0 +1,8 @@
+resource "aws_instance" "foo" {
+    compute = "foo"
+}
+
+module "child" {
+    source = "./child"
+    value = "${aws_instance.foo.foo}"
+}