Fail fail when the ps format template is invalid.

Fixes error continuing execution when the parsing fails.

Signed-off-by: David Calavera <david.calavera@gmail.com>

api/client/ps/custom.go

@@ -170,9 +170,11 @@ func customFormat(ctx Context, containers []types.Container) {
 		format += "\t{{.Size}}"
 	}
 
-	tmpl, err := template.New("ps template").Parse(format)
+	tmpl, err := template.New("").Parse(format)
 	if err != nil {
-		buffer.WriteString(fmt.Sprintf("Invalid `docker ps` format: %v\n", err))
+		buffer.WriteString(fmt.Sprintf("Template parsing error: %v\n", err))
+		buffer.WriteTo(ctx.Output)
+		return
 	}
 
 	for _, container := range containers {
@@ -181,8 +183,9 @@ func customFormat(ctx Context, containers []types.Container) {
 			c:     container,
 		}
 		if err := tmpl.Execute(buffer, containerCtx); err != nil {
-			buffer = bytes.NewBufferString(fmt.Sprintf("Invalid `docker ps` format: %v\n", err))
-			break
+			buffer = bytes.NewBufferString(fmt.Sprintf("Template parsing error: %v\n", err))
+			buffer.WriteTo(ctx.Output)
+			return
 		}
 		if table && len(header) == 0 {
 			header = containerCtx.fullHeader()

api/client/ps/custom_test.go

@@ -1,6 +1,7 @@
 package ps
 
 import (
+	"bytes"
 	"reflect"
 	"strings"
 	"testing"
@@ -10,7 +11,7 @@ import (
 	"github.com/docker/docker/pkg/stringid"
 )
 
-func TestContainerContextID(t *testing.T) {
+func TestContainerPsContext(t *testing.T) {
 	containerID := stringid.GenerateRandomID()
 	unix := time.Now().Unix()
 
@@ -86,3 +87,16 @@ func TestContainerContextID(t *testing.T) {
 
 	}
 }
+
+func TestContainerPsFormatError(t *testing.T) {
+	out := bytes.NewBufferString("")
+	ctx := Context{
+		Format: "{{InvalidFunction}}",
+		Output: out,
+	}
+
+	customFormat(ctx, make([]types.Container, 0))
+	if out.String() != "Template parsing error: template: :1: function \"InvalidFunction\" not defined\n" {
+		t.Fatalf("Expected format error, got `%v`\n", out.String())
+	}
+}