hcl2template: extract attr filter code from ds

Datasources use their attribute's expressions to determine whether or
not they depend on another datasource, in order to get the list of
dependencies and execute them before executing a datasource.

This code may be useful later on for figuring out the dependencies for
any block, so we move this code to the utils.go file, and use this for
datasources.

hcl2template/types.packer_config.go

@@ -5,7 +5,6 @@ package hcl2template
 
 import (
 	"fmt"
-	"log"
 	"sort"
 	"strings"
 
@@ -313,27 +312,17 @@ func (cfg *PackerConfig) evaluateDatasources(skipExecution bool) hcl.Diagnostics
 		// with the datasources in its context.
 		dependencies[ref] = []DatasourceRef{}
 
-		block := ds.block
-		body := block.Body
-		attrs, _ := body.JustAttributes()
-
-		skipFirstEval := false
-		for _, attr := range attrs {
-			vars := attr.Expr.Variables()
-			for _, v := range vars {
-				// check whether the variable is a data source
-				if v.RootName() == "data" {
-					// construct, backwards, the data source type and name we
-					// need to evaluate before this one can be evaluated.
-					dependsOn := DatasourceRef{
-						Type: v[1].(hcl.TraverseAttr).Name,
-						Name: v[2].(hcl.TraverseAttr).Name,
-					}
-					log.Printf("The data source %#v depends on datasource %#v", ref, dependsOn)
-					dependencies[ref] = append(dependencies[ref], dependsOn)
-					skipFirstEval = true
-				}
+		// Note: when looking at the expressions, we only need to care about
+		// attributes, as HCL2 expressions are not allowed in a block's labels.
+		vars := GetVarsByType(ds.block, "data")
+		for _, v := range vars {
+			// construct, backwards, the data source type and name we
+			// need to evaluate before this one can be evaluated.
+			dependsOn := DatasourceRef{
+				Type: v[1].(hcl.TraverseAttr).Name,
+				Name: v[2].(hcl.TraverseAttr).Name,
 			}
+			dependencies[ref] = append(dependencies[ref], dependsOn)
 		}
 	}
 

hcl2template/utils.go

@@ -186,3 +186,23 @@ func ConvertPluginConfigValueToHCLValue(v interface{}) (cty.Value, error) {
 	}
 	return buildValue, nil
 }
+
+func GetVarsByType(block *hcl.Block, topLevelLabels ...string) []hcl.Traversal {
+	attributes, _ := block.Body.JustAttributes()
+
+	var vars []hcl.Traversal
+
+	for _, attr := range attributes {
+		for _, variable := range attr.Expr.Variables() {
+			rootLabel := variable.RootName()
+			for _, label := range topLevelLabels {
+				if label == rootLabel {
+					vars = append(vars, variable)
+					break
+				}
+			}
+		}
+	}
+
+	return vars
+}