Propagate json error object to the caller

When plugin errors out, it prints out a JSON object to stdout
describing the failure. This object needs to be propagated out
through the plugins and to the container runtime. This change
also adds Print method to both the result and error structs
for easy serialization to stdout.

pkg/plugin/ipam.go

@@ -41,6 +41,22 @@ func Find(plugin string) string {
 	return ""
 }
 
+func pluginErr(err error, output []byte) error {
+	if _, ok := err.(*exec.ExitError); ok {
+		emsg := Error{}
+		if perr := json.Unmarshal(output, &emsg); perr != nil {
+			return fmt.Errorf("netplugin failed but error parsing its diagnostic message %q: %v", string(output), perr)
+		}
+		details := ""
+		if emsg.Details != "" {
+			details = fmt.Sprintf("; %v", emsg.Details)
+		}
+		return fmt.Errorf("%v%v", emsg.Msg, details)
+	}
+
+	return err
+}
+
 // ExecAdd executes IPAM plugin, assuming CNI_COMMAND == ADD.
 // Parses and returns resulting IPConfig
 func ExecAdd(plugin string, netconf []byte) (*Result, error) {
@@ -63,7 +79,7 @@ func ExecAdd(plugin string, netconf []byte) (*Result, error) {
 		Stderr: os.Stderr,
 	}
 	if err := c.Run(); err != nil {
-		return nil, err
+		return nil, pluginErr(err, stdout.Bytes())
 	}
 
 	res := &Result{}
@@ -82,13 +98,19 @@ func ExecDel(plugin string, netconf []byte) error {
 		return fmt.Errorf("could not find %q plugin", plugin)
 	}
 
+	stdout := &bytes.Buffer{}
+
 	c := exec.Cmd{
 		Path:   pluginPath,
 		Args:   []string{pluginPath},
 		Stdin:  bytes.NewBuffer(netconf),
+		Stdout: stdout,
 		Stderr: os.Stderr,
 	}
-	return c.Run()
+	if err := c.Run(); err != nil {
+		return pluginErr(err, stdout.Bytes())
+	}
+	return nil
 }
 
 // ConfigureIface takes the result of IPAM plugin and
@@ -124,22 +146,3 @@ func ConfigureIface(ifName string, res *Result) error {
 
 	return nil
 }
-
-// PrintResult writes out prettified Result JSON to stdout
-func PrintResult(res *Result) error {
-	return prettyPrint(res)
-}
-
-// PrintError writes out prettified Error JSON to stdout
-func PrintError(err *Error) error {
-	return prettyPrint(err)
-}
-
-func prettyPrint(obj interface{}) error {
-	data, err := json.MarshalIndent(obj, "", "    ")
-	if err != nil {
-		return err
-	}
-	_, err = os.Stdout.Write(data)
-	return err
-}