cp: reduce branching in progress printer

This just makes it easier to reason about what is happening.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
(cherry picked from commit efd011b793)
Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cli/command/container/cp.go

@@ -49,35 +49,39 @@ type cpConfig struct {
 // copying files to/from a container.
 type copyProgressPrinter struct {
 	io.ReadCloser
-	toContainer bool
 	total  *float64
 	writer io.Writer
 	isTerm bool
+	header string
 }
 
+const (
+	copyToContainerHeader   = "Copying to container - "
+	copyFromContainerHeader = "Copying from container - "
+)
+
 func (pt *copyProgressPrinter) Read(p []byte) (int, error) {
 	n, err := pt.ReadCloser.Read(p)
-	if n > 0 {
 	isFirst := *pt.total == 0
+	if n > 0 {
 		*pt.total += float64(n)
-
-		if pt.isTerm {
-			var header string
-			if pt.toContainer {
-				header = "Copying to container - "
-			} else {
-				header = "Copying from container - "
 	}
+	if err != nil && err != io.EOF {
+		return n, err
+	}
+
+	if !pt.isTerm {
+		return n, err
+	}
+
 	if isFirst {
 		fmt.Fprint(pt.writer, aec.Restore)
 		fmt.Fprint(pt.writer, aec.EraseLine(aec.EraseModes.All))
-				fmt.Fprint(pt.writer, header)
+		fmt.Fprint(pt.writer, pt.header)
 	}
-			fmt.Fprint(pt.writer, aec.Column(uint(len(header)+1)))
+	fmt.Fprint(pt.writer, aec.Column(uint(len(pt.header)+1)))
 	fmt.Fprint(pt.writer, aec.EraseLine(aec.EraseModes.Tail))
 	fmt.Fprint(pt.writer, units.HumanSize(*pt.total))
-		}
-	}
 
 	return n, err
 }
@@ -229,10 +233,10 @@ func copyFromContainer(ctx context.Context, dockerCli command.Cli, copyConfig cp
 	if !copyConfig.quiet {
 		content = &copyProgressPrinter{
 			ReadCloser: content,
-			toContainer: false,
 			writer:     dockerCli.Err(),
 			total:      &copiedSize,
 			isTerm:     stderrIsTerm,
+			header:     copyFromContainerHeader,
 		}
 	}
 
@@ -361,10 +365,10 @@ func copyToContainer(ctx context.Context, dockerCli command.Cli, copyConfig cpCo
 		if !copyConfig.quiet {
 			content = &copyProgressPrinter{
 				ReadCloser: content,
-				toContainer: true,
 				writer:     dockerCli.Err(),
 				total:      &copiedSize,
 				isTerm:     stderrIsTerm,
+				header:     copyToContainerHeader,
 			}
 		}
 	}