hcl2template: err on malformed local/data dep

When introducing the DAG for locals and datasources, we forgot to handle
one limit case: if a dependency for a local or data is malformed, we
didn't check that a vertex was associated to it, leading to the final
DAG being malformed, and the DAG library would crash in this case.

This commit fixes this problem by checking that the dependency does
exist before attempting to add it to the graph as an edge for a vertex,
so that it is reported accurately, and do that Packer doesn't crash.

hcl2template/parser.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"reflect"
 
+	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/go-version"
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/hcl/v2/ext/dynblock"
@@ -398,6 +399,8 @@ func (cfg *PackerConfig) buildPrereqsDAG() (*dag.AcyclicGraph, error) {
 
 	verticesMap := map[string]dag.Vertex{}
 
+	var err error
+
 	// Do a first pass to create all the vertices
 	for ref := range cfg.Datasources {
 		// We keep a reference to the datasource separately from where it
@@ -429,27 +432,48 @@ func (cfg *PackerConfig) buildPrereqsDAG() (*dag.AcyclicGraph, error) {
 	for _, ds := range cfg.Datasources {
 		dsName := fmt.Sprintf("data.%s", ds.Name())
 
+		source := verticesMap[dsName]
+		if source == nil {
+			err = multierror.Append(err, fmt.Errorf("unable to find source vertex %q for dependency analysis, this is likely a Packer bug", dsName))
+			continue
+		}
+
 		for _, dep := range ds.Dependencies {
-			retGraph.Connect(
-				dag.BasicEdge(verticesMap[dsName],
-					verticesMap[dep.String()]))
+			target := verticesMap[dep.String()]
+			if target == nil {
+				err = multierror.Append(err, fmt.Errorf("unknown dependency %q for %q", dep.String(), dsName))
+				continue
+			}
+
+			retGraph.Connect(dag.BasicEdge(source, target))
 		}
 	}
 	for _, loc := range cfg.LocalBlocks {
 		locName := fmt.Sprintf("local.%s", loc.LocalName)
 
+		source := verticesMap[locName]
+		if source == nil {
+			err = multierror.Append(err, fmt.Errorf("unable to find source vertex %q for dependency analysis, this is likely a Packer bug", locName))
+			continue
+		}
+
 		for _, dep := range loc.dependencies {
-			retGraph.Connect(
-				dag.BasicEdge(verticesMap[locName],
-					verticesMap[dep.String()]))
+			target := verticesMap[dep.String()]
+
+			if target == nil {
+				err = multierror.Append(err, fmt.Errorf("unknown dependency %q for %q", dep.String(), locName))
+				continue
+			}
+
+			retGraph.Connect(dag.BasicEdge(source, target))
 		}
 	}
 
-	if err := retGraph.Validate(); err != nil {
-		return nil, err
+	if validateErr := retGraph.Validate(); validateErr != nil {
+		err = multierror.Append(err, validateErr)
 	}
 
-	return &retGraph, nil
+	return &retGraph, err
 }
 
 func (cfg *PackerConfig) evaluateBuildPrereqs(skipDatasources bool) hcl.Diagnostics {

packer_test/dag_tests/malformed_dep_test.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/hashicorp/packer/packer_test/common/check"
+)
+
+type malformedDepTestCase struct {
+	name          string
+	command       string
+	templatePath  string
+	useSequential bool
+}
+
+func genMalformedDepTestCases() []malformedDepTestCase {
+	retVals := []malformedDepTestCase{}
+
+	for _, cmd := range []string{"build", "validate"} {
+		for _, template := range []string{"./templates/malformed_data_dep.pkr.hcl", "./templates/malformed_local_dep.pkr.hcl"} {
+			for _, seq := range []bool{true, false} {
+				retVals = append(retVals, malformedDepTestCase{
+					name: fmt.Sprintf("Malformed dep packer %s --use-sequential-evaluation=%t %s",
+						cmd, seq, template),
+					command:       cmd,
+					templatePath:  template,
+					useSequential: seq,
+				})
+			}
+		}
+	}
+
+	return retVals
+}
+
+func (ts *PackerDAGTestSuite) TestMalformedDependency() {
+	pluginDir := ts.MakePluginDir()
+	defer pluginDir.Cleanup()
+
+	for _, tc := range genMalformedDepTestCases() {
+		ts.Run(tc.name, func() {
+			ts.PackerCommand().UsePluginDir(pluginDir).
+				SetArgs(tc.command,
+					fmt.Sprintf("--use-sequential-evaluation=%t", tc.useSequential),
+					tc.templatePath).
+				Assert(check.MustFail())
+		})
+	}
+}

packer_test/dag_tests/templates/malformed_data_dep.pkr.hcl

@@ -0,0 +1,8 @@
+data "http" "trusted_ca_certificates" {
+  method = "GET"
+  url    = "http://example.com/ca-bundle.crt"
+}
+
+locals {
+  test = data.trusted_ca_certificates.url
+}

packer_test/dag_tests/templates/malformed_local_dep.pkr.hcl

@@ -0,0 +1,4 @@
+data "http" "trusted_ca_certificates" {
+  method = "GET"
+  url    = local.no_dep
+}