golang trouble traversing, populating xml from file

I'm relatively new to working with golang and I could use some advice on populating a struct tree from an XML file. edit: I fixed an inconsistency with the XML structure vs the struct definitions. Updated the playground link; the full code with a sample XML is at http://play.golang.org/p/1ymyESO2jp .

The XML file has a combination of attributes and values (chardata)

<?xml version="1.0" encoding="UTF-8"?>
<system name="SystemA123" enabled="true" open="9" close="15" timeZone="America/Denver" freq="4h" dailyMax="1" override="false">
	<hosts>
		<host address="10.1.2.3">
			<command>"free -mo</command>
			<command>"cat /proc/cpuinfo | grep processor"</command>
			<command>"ifconfig eth0 down"</command>
			<command>"shutdown -r now"</command>
			<command>"cat /proc/loadavg"</command>
		</host>
        <host address="10.1.2.4">
                  ... more commands>command elements
        </host>

I've build the corresponding structs like so:

type SystemConfig struct {
	XMLName   xml.Name `xml:"system"`
	SysName   string   `xml:"name,attr"`
	Enabled   bool     `xml:"enabled,attr"`
	OpenHour  int      `xml:"open,attr"`
	CloseHour int      `xml:"close,attr"`
	TimeZone  string   `xml:"timeZone,attr"`
	Frequency string   `xml:"freq,attr"` //will use time.ParseDuration to read the interval specified here
	DailyMax  int      `xml:"dailyMax,attr"`
	Override  bool     `xml:"override,attr"`
	Hosts     []*Hosts `xml:"hosts"`
}

type Hosts struct {
	XMLName xml.Name `xml:"hosts"`
	Host    []*Host  `xml:host"`
}

type Host struct {
	XMLName  xml.Name        `xml:"host"`
	IPaddr   string          `xml:"address,attr"`
	Commands []*HostCommands `xml:"command"`
}

type HostCommands struct {
	XMLName xml.Name `xml:"command"`
	Command string   `xml:",chardata"`
}

and I'm reading the XML file into the structs with this code:

var sc *SystemConfig
xmlConf, err := os.Open(appConfFile)
defer xmlConf.Close()
sc, err = ReadSystemConfig(xmlConf)

with this method definition for ReadSystemConfig

func ReadSystemConfig(reader io.Reader) (*SystemConfig, error) {
	sysConf := &SystemConfig{}
	decoder := xml.NewDecoder(reader)
	if err := decoder.Decode(sysConf); err != nil {
		//fmt.Println("error decoding sysConf in ReadSystemConfig: %v", err)
		return nil, err
	}
	return sysConf, nil
}

When I run this as-is, including some fmt.Printf statements to verify the data got loaded properly, the SystemConfig attributes are loaded correctly, but I can't get any of the Hosts data loaded. Where I've asked for fmt.Printf("first IP address: %v\n", sc.Hosts[0].Host[0].IPaddr, the index out of range panic is thrown.

Hi:app pd$ ./app -f ../config/conf.xml
SystemConfig:SysName SystemA123
SystemConfig:Enabled true
SystemConfig:OpenHour 9
SystemConfig:CloseHour 15
SystemConfig:TimeZone America/Denver
SystemConfig:Frequency 4h
SystemConfig:DailyMax 1
SystemConfig:Override false
panic: runtime error: index out of range

goroutine 1 [running]:
runtime.panic(0xd9cc0, 0x1fecf7)
	/usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
main.main()
	/Users/pd/golang/src/local/boxofsand/app/boxofsand.go:95 +0x8ef

I feel like I have the structs setup properly (happy to take advice if I don't); I have tried removing the <system> tags and modifying the code to start from the <hosts> tags, but I wasn't able to pull any data that way, either (same index out of range error). So, I'm confident that I have borked up 2-3 lines of code here, but I can't find a realistic example online that reads relatively simple, multi-level XML from a file.

Lastly, I'd be open to advice on doing this configuration in JSON instead. I didn't choose that initially because I thought the deeper nesting would be harder to read; also, a lot of what I work with on other projects is XML-based, so I'd like to learn something here that I can apply to those other (for now, Java-based) projects.

As always, thanks in advance for any advice or help you're willing to lend.

edit: I saw a consistency issue in the XML skeleton vs the struct definitions. I modified the XML to suit the structs by removing the <commands> tagset

Patrick, it looks like your structs had a few inconsistencies (I ran into the same issues when I first started trying to use Go to parse nested xml/json). Mostly just that you had arrays of arrays of Host objects. I tweaked your structs and this seems to be parsing well for me:

type SystemConfig struct {
	XMLName   xml.Name `xml:"system"`
	SysName   string   `xml:"name,attr"`
	Enabled   bool     `xml:"enabled,attr"`
	OpenHour  int      `xml:"open,attr"`
	CloseHour int      `xml:"close,attr"`
	TimeZone  string   `xml:"timeZone,attr"`
	Frequency string   `xml:"freq,attr"` //will use time.ParseDuration to read the interval specified here
	DailyMax  int      `xml:"dailyMax,attr"`
	Override  bool     `xml:"override,attr"`
	Hosts     []Host   `xml:"hosts>host"`
}

type Host struct {
	XMLName  xml.Name      `xml:"host"`
	IPaddr   string        `xml:"address,attr"`
	Commands []HostCommand `xml:"commands>command"`
}

type HostCommand struct {
	XMLName xml.Name `xml:"command"`
	Command string   `xml:",chardata"`
}

As an additional note, one of the most useful things I have found while working through encoding issues in Go, build your structures, throw some mock data into them, then spit out the data and see how Go formats it. From there, it's usually pretty easy to see where you went wrong.

For example, here is the code I used to iterate and figure out what was wrong with your structs:

package main

import (
	"encoding/xml"
	"fmt"
)

var (
	xmlData = `... your xml chunk ...`
)

// ... structs ...

func main() {
	conf, _ := ReadSystemConfig(xmlData)
	fmt.Printf("%#v\n", conf)
	data, _ := WriteSystemConfig(conf)
	fmt.Printf("%#v\n", data)
}

func ReadSystemConfig(data string) (*SystemConfig, error) {
	sysConf := &SystemConfig{}
	if err := xml.Unmarshal([]byte(data), sysConf); err != nil {
		return nil, err
	}
	return sysConf, nil
}

func WriteSystemConfig(sysConf *SystemConfig) (string, error) {
	dataBytes, err := xml.Marshal(sysConf)
	if err != nil {
		return "", err
	}
	return string(dataBytes), nil
}

With that, I was able to read in your xml and spit it back out to see what Go was able to parse, then I guessed a few times (having done xml parsing in Go before), and iterated until all the data came back out.

Hope this helps!

I decided that the <Hosts> tagset was extraneous and removed them from the XML structure, also removing the Hosts struct. When I made this change, the XML data got loaded properly.

I realized that I was using a variable named Host of type []*Host, which may have been confusing the runtime.

type SystemConfig struct {
	XMLName   xml.Name `xml:"system"`
	SysName   string   `xml:"name,attr"`
	Enabled   bool     `xml:"enabled,attr"`
	OpenHour  int      `xml:"open,attr"`
	CloseHour int      `xml:"close,attr"`
	TimeZone  string   `xml:"timeZone,attr"`
	Frequency string   `xml:"freq,attr"` //will use time.ParseDuration to read the interval specified here
	DailyMax  int      `xml:"dailyMax,attr"`
	Override  bool     `xml:"override,attr"`
	Hosts     []*Host  `xml:"host"`
}
type Host struct {
	XMLName  xml.Name        `xml:"host"`
	IPaddr   string          `xml:"address,attr"`
	Commands []*HostCommands `xml:"command"`
}

type HostCommands struct {
	XMLName xml.Name `xml:"command"`
	Command string   `xml:",chardata"`
}