Return Valid JSON for empty data

For commands that ask for JSON results, if the input to the Go JSON
marshaller is empty, it will return a byte array with a literal
"null" in it.  If that is the case, we should output [] instead
as at least that is valid JSON and will not break consumers of the
data.

Signed-off-by: baude <bbaude@redhat.com>

cmd/kpod/formats/formats.go

@@ -8,6 +8,7 @@ import (
 	"text/tabwriter"
 	"text/template"
 
+	"bytes"
 	"github.com/ghodss/yaml"
 	"github.com/pkg/errors"
 )
@@ -59,6 +60,16 @@ func (j JSONStructArray) Out() error {
 	if err != nil {
 		return err
 	}
+
+	// JSON returns a byte array with a literal null [110 117 108 108] in it
+	// if it is passed empty data.  We used bytes.Compare to see if that is
+	// the case.
+	if diff := bytes.Compare(data, []byte("null")); diff == 0 {
+		data = []byte("[]")
+	}
+
+	// If the we did get NULL back, we should spit out {} which is
+	// at least valid JSON for the consumer.
 	fmt.Printf("%s\n", data)
 	return nil
 }

cmd/kpod/ps.go

@@ -398,7 +398,9 @@ func getJSONOutput(containers []*libkpod.ContainerData, nSpace bool) (psOutput [
 
 func generatePsOutput(containers []*libkpod.ContainerData, server *libkpod.ContainerServer, opts psOptions) error {
 	containersOutput := getContainers(containers, opts)
-	if len(containersOutput) == 0 {
+	// In the case of JSON, we want to continue so we at least pass
+	// {} --valid JSON-- to the consumer
+	if len(containersOutput) == 0 && opts.format != formats.JSONString {
 		return nil
 	}