Incorporated the hack demonstrated by @SwampDragons in order to deal with packer.rpc.Ui not exporting any information about what it's doing to anybody.

common/progress.go

@@ -3,11 +3,17 @@ package common
 import (
 	"fmt"
 	"github.com/cheggaaa/pb"
+	"github.com/hashicorp/packer/helper/multistep"
 	"github.com/hashicorp/packer/packer"
+	"github.com/hashicorp/packer/packer/rpc"
 	"log"
+	"reflect"
 	"time"
 )
 
+// This is the arrow from packer/ui.go -> TargetedUI.prefixLines
+const targetedUIArrowText = "==>"
+
 // The ProgressBar interface is used for abstracting cheggaaa's progress-
 // bar, or any other progress bar. If a UI does not support a progress-
 // bar, then it must return a null progress bar.
@@ -82,16 +88,19 @@ func calculateUiPrefixLength(ui packer.Ui) int {
 			return recursiveCalculateUiPrefixLength(u.Ui, agg)
 
 		case *packer.TargetedUI:
-			// A TargetedUI added the .Target and an arrow by default
-			const targetedArrowText = "==>"
+			// A TargetedUI adds the .Target and an arrow by default
 			u := ui.(*packer.TargetedUI)
-			res := fmt.Sprintf("%s %s: ", targetedArrowText, u.Target)
+			res := fmt.Sprintf("%s %s: ", targetedUIArrowText, u.Target)
 			return recursiveCalculateUiPrefixLength(u.Ui, agg+len(res))
 
 		case *packer.BasicUi:
 			// The standard BasicUi appends only a newline
 			return agg + len("\n")
 
+		// packer.rpc.Ui returns 0 here to trigger the hack described later
+		case *rpc.Ui:
+			return 0
+
 		case *packer.MachineReadableUi:
 			// MachineReadableUi doesn't emit anything...like at all
 			return 0
@@ -103,12 +112,30 @@ func calculateUiPrefixLength(ui packer.Ui) int {
 	return recursiveCalculateUiPrefixLength(ui, 0)
 }
 
-func GetProgressBar(ui packer.Ui) ProgressBar {
+func GetPackerConfigFromStateBag(state multistep.StateBag) *PackerConfig {
+	config := state.Get("config")
+	rConfig := reflect.Indirect(reflect.ValueOf(config))
+	iPackerConfig := rConfig.FieldByName("PackerConfig").Interface()
+	packerConfig := iPackerConfig.(PackerConfig)
+	return &packerConfig
+}
+
+func GetProgressBar(ui packer.Ui, config *PackerConfig) ProgressBar {
+	// Figure out the prefix length by quering the UI
 	uiPrefixLength := calculateUiPrefixLength(ui)
+
+	// hack to deal with packer.rpc.Ui courtesy of @Swampdragons
+	if _, ok := ui.(*rpc.Ui); uiPrefixLength == 0 && config != nil && ok {
+		res := fmt.Sprintf("%s %s: \n", targetedUIArrowText, config.PackerBuildName)
+		uiPrefixLength = len(res)
+	}
+
+	// Now we can use the prefix length to calculate the progress bar width
 	width := calculateProgressBarWidth(uiPrefixLength)
 
 	log.Printf("ProgressBar: Using progress bar width: %d\n", width)
 
+	// Get a default progress bar and set some output defaults
 	bar := GetDefaultProgressBar()
 	bar.SetWidth(width)
 	bar.Callback = func(message string) {

common/step_download.go

@@ -64,7 +64,7 @@ func (s *StepDownload) Run(_ context.Context, state multistep.StateBag) multiste
 	ui.Say(fmt.Sprintf("Retrieving %s", s.Description))
 
 	// Get a progress bar from the ui so we can hand it off to the download client
-	bar := GetProgressBar(ui)
+	bar := GetProgressBar(ui, GetPackerConfigFromStateBag(state))
 
 	// First try to use any already downloaded file
 	// If it fails, proceed to regular download logic
@@ -144,7 +144,7 @@ func (s *StepDownload) download(config *DownloadConfig, state multistep.StateBag
 	ui := state.Get("ui").(packer.Ui)
 
 	// Get a progress bar and hand it off to the download client
-	bar := GetProgressBar(ui)
+	bar := GetProgressBar(ui, GetPackerConfigFromStateBag(state))
 
 	// Create download client with config and progress bar
 	download := NewDownloadClient(config, bar)

provisioner/file/provisioner.go

@@ -127,7 +127,7 @@ func (p *Provisioner) ProvisionDownload(ui packer.Ui, comm packer.Communicator)
 		defer f.Close()
 
 		// Get a default progress bar
-		pb := common.GetProgressBar(ui)
+		pb := common.GetProgressBar(ui, &p.config.PackerConfig)
 		bar := pb.Start()
 		defer bar.Finish()
 
@@ -176,7 +176,7 @@ func (p *Provisioner) ProvisionUpload(ui packer.Ui, comm packer.Communicator) er
 		}
 
 		// Get a default progress bar
-		pb := common.GetProgressBar(ui)
+		pb := common.GetProgressBar(ui, &p.config.PackerConfig)
 		bar := pb.Start()
 		defer bar.Finish()