packer_test: add base tests for DAG eval

Since we introduce the DAG with this series of commits, only on locals
and data sources, we need to make sure that the behaviour is what we
expect.

Therefore, this commit adds a basic test with Packer build, and packer
validate, to evaluate a template with both locals and data sources
depending on one another.

This is rejected with the sequential evaluation methods, as we process
the different types one-by-one, whereas the DAG allows us to mix the
order between the two, while still rejecting circular dependencies (and
doing that before they even get evaluated), and self-references.

packer_test/dag_tests/mix_data_locals_test.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/hashicorp/packer/packer_test/common/check"
+)
+
+func (ts *PackerDAGTestSuite) TestWithBothDataLocalMixedOrder() {
+	pluginDir := ts.MakePluginDir()
+	defer pluginDir.Cleanup()
+
+	for _, cmd := range []string{"build", "validate"} {
+		ts.Run(fmt.Sprintf("%s: evaluating with DAG - success expected", cmd), func() {
+			ts.PackerCommand().UsePluginDir(pluginDir).
+				SetArgs(cmd, "./templates/mixed_data_local.pkr.hcl").
+				Assert(check.MustSucceed())
+		})
+
+		ts.Run(fmt.Sprintf("%s: evaluating sequentially - failure expected", cmd), func() {
+			ts.PackerCommand().UsePluginDir(pluginDir).
+				SetArgs(cmd, "--use-sequential-evaluation", "./templates/mixed_data_local.pkr.hcl").
+				Assert(check.MustFail())
+		})
+	}
+}

packer_test/dag_tests/suite_test.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/hashicorp/packer/packer_test/common"
+	"github.com/stretchr/testify/suite"
+)
+
+type PackerDAGTestSuite struct {
+	*common.PackerTestSuite
+}
+
+func Test_PackerDAGSuite(t *testing.T) {
+	baseSuite, cleanup := common.InitBaseSuite(t)
+	defer cleanup()
+
+	ts := &PackerDAGTestSuite{
+		baseSuite,
+	}
+
+	suite.Run(t, ts)
+}

packer_test/dag_tests/templates/mixed_data_local.pkr.hcl

@@ -0,0 +1,23 @@
+data "null" "head" {
+	input = "foo"
+}
+
+locals {
+	loc = "${data.null.head.output}"
+}
+
+data "null" "tail" {
+	input = "${local.loc}"
+}
+
+locals {
+	last = "final - ${data.null.tail.output}"
+}
+
+source "null" "test" {
+	communicator = "none"
+}
+
+build {
+	sources = ["null.test"]
+}