Allow to use build variables in a post processor (#11323)

* hcl: add test to verify that build.name can be used in a pp

* allow to use build vars in post processors

command/build_test.go

@@ -415,6 +415,17 @@ func TestBuild(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "hcl - using build variables in post-processor",
+			args: []string{
+				testFixture("hcl", "build-var-in-pp.pkr.hcl"),
+			},
+			fileCheck: fileCheck{
+				expectedContent: map[string]string{
+					"example.2.txt": two,
+				},
+			},
+		},
 	}
 
 	for _, tt := range tc {

command/test-fixtures/hcl/build-var-in-pp.pkr.hcl

@@ -0,0 +1,12 @@
+source "null" "example" {
+  communicator = "none"
+}
+
+build {
+  name    = "example"
+  sources = ["source.null.example"]
+
+  post-processor "shell-local" {
+    inline = ["echo 2 > ${build.name}.2.txt"]
+  }
+}

hcl2template/types.hcl_post-processor.go

@@ -31,6 +31,9 @@ func (p *HCL2PostProcessor) HCL2Prepare(buildVars map[string]interface{}) error
 	if len(buildVars) > 0 {
 		ectx = p.evalContext.NewChild()
 		buildValues := map[string]cty.Value{}
+		if !p.evalContext.Variables[buildAccessor].IsNull() {
+			buildValues = p.evalContext.Variables[buildAccessor].AsValueMap()
+		}
 		for k, v := range buildVars {
 			val, err := ConvertPluginConfigValueToHCLValue(v)
 			if err != nil {