core: Move CountBoundaryTransformer to the plan graph builder

This fixes interpolation issues on grandchild data sources that have
multiple instances (ie: counts). For example, baz depends on bar, which
depends on foo.

In this instance, after an initial TF run is done and state is saved,
the next refresh/plan is not properly transformed, and instead of the
graph/state coming through as data.x.bar.0, it comes through as
data.x.bar.  This breaks interpolations that rely on splat operators -
ie: data.x.bar.*.out.

builtin/providers/test/data_source_test.go

@@ -99,3 +99,59 @@ resource "test_resource" "foo" {
 		},
 	})
 }
+
+// Test that a grandchild data source that is based off of count works, ie:
+// dependency chain foo -> bar -> baz. This was failing because
+// CountBoundaryTransformer is being run during apply instead of plan, which
+// meant that it wasn't firing after data sources were potentially changing
+// state and causing diff/interpolation issues.
+//
+// This happens after the initial apply, after state is saved.
+func TestDataSource_dataSourceCountGrandChild(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers: testAccProviders,
+		CheckDestroy: func(s *terraform.State) error {
+			return nil
+		},
+		Steps: []resource.TestStep{
+			{
+				Config: dataSourceCountGrandChildConfig,
+			},
+			{
+				Config: dataSourceCountGrandChildConfig,
+				Check: func(s *terraform.State) error {
+					for _, v := range []string{"foo", "bar", "baz"} {
+						count := 0
+						for k := range s.RootModule().Resources {
+							if strings.HasPrefix(k, fmt.Sprintf("data.test_data_source.%s.", v)) {
+								count++
+							}
+						}
+
+						if count != 2 {
+							return fmt.Errorf("bad count for data.test_data_source.%s: %d", v, count)
+						}
+					}
+					return nil
+				},
+			},
+		},
+	})
+}
+
+const dataSourceCountGrandChildConfig = `
+data "test_data_source" "foo" {
+  count = 2
+  input = "one"
+}
+
+data "test_data_source" "bar" {
+  count = "${length(data.test_data_source.foo.*.id)}"
+  input = "${data.test_data_source.foo.*.output[count.index]}"
+}
+
+data "test_data_source" "baz" {
+  count = "${length(data.test_data_source.bar.*.id)}"
+  input = "${data.test_data_source.bar.*.output[count.index]}"
+}
+`

terraform/graph_builder_apply.go

@@ -117,9 +117,6 @@ func (b *ApplyGraphBuilder) Steps() []GraphTransformer {
 		// Connect references so ordering is correct
 		&ReferenceTransformer{},
 
-		// Add the node to fix the state count boundaries
-		&CountBoundaryTransformer{},
-
 		// Target
 		&TargetsTransformer{Targets: b.Targets},
 

terraform/graph_builder_plan.go

@@ -120,6 +120,9 @@ func (b *PlanGraphBuilder) Steps() []GraphTransformer {
 		&CloseProviderTransformer{},
 		&CloseProvisionerTransformer{},
 
+		// Add the node to fix the state count boundaries
+		&CountBoundaryTransformer{},
+
 		// Single root
 		&RootTransformer{},
 	}