Fix "docker ps" with no containers regression

The header row was not being printed when "docker ps" was invoked without containers thanks to the new format support, and we instead received a single blank line.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

api/client/ps/custom.go

@@ -191,6 +191,13 @@ func customFormat(ctx Context, containers []types.Container) {
 	}
 
 	if table {
+		if len(header) == 0 {
+			// if we still don't have a header, we didn't have any containers so we need to fake it to get the right headers from the template
+			containerCtx := &containerContext{}
+			tmpl.Execute(bytes.NewBufferString(""), containerCtx)
+			header = containerCtx.fullHeader()
+		}
+
 		t := tabwriter.NewWriter(ctx.Output, 20, 1, 3, ' ', 0)
 		t.Write([]byte(header))
 		t.Write([]byte("\n"))

integration-cli/docker_cli_ps_test.go

@@ -539,3 +539,18 @@ func (s *DockerSuite) TestPsFormatMultiNames(c *check.C) {
 	}
 
 }
+
+func (s *DockerSuite) TestPsFormatHeaders(c *check.C) {
+	// make sure no-container "docker ps" still prints the header row
+	out, _ := dockerCmd(c, "ps", "--format", "table {{.ID}}")
+	if out != "CONTAINER ID\n" {
+		c.Fatalf(`Expected 'CONTAINER ID\n', got %v`, out)
+	}
+
+	// verify that "docker ps" with a container still prints the header row also
+	dockerCmd(c, "run", "--name=test", "-d", "busybox", "top")
+	out, _ = dockerCmd(c, "ps", "--format", "table {{.Names}}")
+	if out != "NAMES\ntest\n" {
+		c.Fatalf(`Expected 'NAMES\ntest\n', got %v`, out)
+	}
+}