Add constants for legacy mode values

command/build.go

@@ -113,7 +113,7 @@ func (m *Meta) GetConfig(cla *MetaArgs) (packer.Handler, int) {
 // This is an internal-only method that should be called by commands that don't have
 // a different mode when handling legacy JSON templates; mainly build, validate.
 func DisplayLegacyConfigWarning(ui packersdk.Ui) {
-	if v := os.Getenv("PACKER_LEGACY_MODE"); strings.ToLower(v) == "on" {
+	if v := os.Getenv(PackerLegacyModeEnv); strings.ToUpper(v) == packerLegacyModeOn {
 		return
 	}
 
@@ -130,11 +130,11 @@ templates will continue to work but users are encouraged to move to HCL style te
 See: https://learn.hashicorp.com/tutorials/packer/hcl2-upgrade
 `)
 
-	coloredUi.Say(`In JSON mode automatic loading of vendored plugins is enabled by default.  In future
-this feature will be removed and Packer will rely only on system installed plugins.
-To disable automatic loading of vendored plugins set PACKER_LEGACY_MODE=off
+	coloredUi.Say(fmt.Sprintf(`In JSON mode automatic loading of vendored plugins is enabled by default. In the
+future this feature will be removed and Packer will rely only on system installed plugins.
+To disable automatic loading of vendored plugins set %s=%s
 See: https://packer.io/docs/templates/legacy-mode
-`)
+`, PackerLegacyModeEnv, packerLegacyModeOff))
 
 }
 

command/meta.go

@@ -23,6 +23,19 @@ const (
 	FlagSetVars
 )
 
+const (
+	/*
+		PACKER_LEGACY_MODE is an environment variable used by Packer to control features consider legacy as of 1.8.0.
+		By default when the variable is unset Packer will display warnings to the user.
+
+		Setting PACKER_LEGACY_MODE=OFF will display warnings to the user when running, and may disable other features.
+		Setting PACKER_LEGACY_MODE=ON  will not display warnings, and will Packer with all legacy features enabled.
+	*/
+	PackerLegacyModeEnv string = "PACKER_LEGACY_MODE"
+	packerLegacyModeOn  string = "ON"
+	packerLegacyModeOff string = "OFF"
+)
+
 // Meta contains the meta-options and functionality that nearly every
 // Packer command inherits.
 type Meta struct {

command/vendored_plugins.go

@@ -191,10 +191,10 @@ var VendoredPostProcessors = map[string]packersdk.PostProcessor{
 // set of plugins.
 func init() {
 
-	// By default PACKER_LEGACY_MODE=auto to enable automatic loading of vendored plugins.
+	// By default PACKER_LEGACY_MODE is unset to enable automatic loading of vendored plugins.
 	// In preparation for removing vendored plugins entirely we are providing users a flag to
 	//  to control if vendored plugins should be loaded or not.
-	if v := os.Getenv("PACKER_LEGACY_MODE"); strings.ToLower(v) == "off" {
+	if v := os.Getenv(PackerLegacyModeEnv); strings.ToUpper(v) == packerLegacyModeOff {
 		return
 	}