command/validate: add option to eval datasources

When packer validate is invoked, it does not try to evaluate the
datasources before attempting to decide if the template is valid.

In many cases, this works, but sometimes it will fail as the value is
unknown by the validation code.

Since the validation code for all the elements of a Packer template is
left to be implemented by plugins, we cannot rely on checking for
unknown values everywhere, especially since the unknown references are
replaced automatically by a value of the right type for the
configuration expected.

So, in order for such configurations to be validable, we add an extra
option to packer validate, that will let users evaluate the datasources
from a template.

command/cli.go

@@ -129,14 +129,16 @@ type FixArgs struct {
 
 func (va *ValidateArgs) AddFlagSets(flags *flag.FlagSet) {
 	flags.BoolVar(&va.SyntaxOnly, "syntax-only", false, "check syntax only")
+	flags.BoolVar(&va.EvaluateDatasources, "evaluate-datasources", false, "evaluate datasources for validation (HCL2 only, may incur costs)")
 
 	va.MetaArgs.AddFlagSets(flags)
 }
 
 // ValidateArgs represents a parsed cli line for a `packer validate`
 type ValidateArgs struct {
 	MetaArgs
-	SyntaxOnly bool
+	SyntaxOnly          bool
+	EvaluateDatasources bool
 }
 
 func (va *InspectArgs) AddFlagSets(flags *flag.FlagSet) {

command/test-fixtures/hcl/local-ds-validate.pkr.hcl

@@ -0,0 +1,17 @@
+data "null" "dep" {
+	input = "upload"
+}
+
+source "null" "test" {
+	communicator = "none"
+}
+
+build {
+	sources = ["sources.null.test"]
+
+	provisioner "file" {
+		source = "test-fixtures/hcl/force.pkr.hcl"
+		destination = "dest"
+		direction = "${data.null.dep.output}"
+	}
+}

command/validate.go

@@ -57,7 +57,7 @@ func (c *ValidateCommand) RunContext(ctx context.Context, cla *ValidateArgs) int
 	}
 
 	diags := packerStarter.Initialize(packer.InitializeOptions{
-		SkipDatasourcesExecution: true,
+		SkipDatasourcesExecution: !cla.EvaluateDatasources,
 	})
 	ret = writeDiags(c.Ui, nil, diags)
 	if ret != 0 {

command/validate_test.go

@@ -11,8 +11,9 @@ import (
 
 func TestValidateCommand(t *testing.T) {
 	tt := []struct {
-		path     string
-		exitCode int
+		path      string
+		exitCode  int
+		extraArgs []string
 	}{
 		{path: filepath.Join(testFixture("validate"), "build.json")},
 		{path: filepath.Join(testFixture("validate"), "build.pkr.hcl")},
@@ -39,6 +40,11 @@ func TestValidateCommand(t *testing.T) {
 
 		// datasource could be unknown at that moment
 		{path: filepath.Join(testFixture("hcl", "data-source-validation.pkr.hcl")), exitCode: 0},
+
+		// datasource unknown at validation-time without datasource evaluation -> fail on provisioner
+		{path: filepath.Join(testFixture("hcl", "local-ds-validate.pkr.hcl")), exitCode: 1},
+		// datasource unknown at validation-time with datasource evaluation -> success
+		{path: filepath.Join(testFixture("hcl", "local-ds-validate.pkr.hcl")), exitCode: 0, extraArgs: []string{"--evaluate-datasources"}},
 	}
 
 	for _, tc := range tt {
@@ -47,7 +53,8 @@ func TestValidateCommand(t *testing.T) {
 				Meta: TestMetaFile(t),
 			}
 			tc := tc
-			args := []string{tc.path}
+			args := tc.extraArgs
+			args = append(args, tc.path)
 			if code := c.Run(args); code != tc.exitCode {
 				fatalCommand(t, c.Meta)
 			}