Improve mackerel-agent configtest

[wafuwafu13]

Now, we run resolveConfig to do configtest and detect Invalid syntax (config_test.go shows that).
But, we can't detect unexpected key, so I want to improve by using Undecoded().
It would be better if it could suggest intended key.

This is sample of mackerel-agent.conf which includes unexpected key.

1  apikey = "abcdefg"
2  podfile = "/path/to/pidfile"
3 
4  [foo]
5  command = "bar"
6 
7  [plugins.checks.foo]
8  command = "bar"
9 
10 [plugin.check.foo]
11 command = "bar"

Actual behavior

$ mackerel-agent configtest
/path/to/mackerel-agent.conf Syntax OK

Expected behavior

$ mackerel-agent configtest
warn: podfile is invalid key. Do you mean pidfile ?(line 2)
warn: foo is invalid key (line 4)
warn: plugins.checks.foo is invalid key. Do you mean plugin.checks.foo ? (line 7)
warn: plugin.check.foo is invalid key. Do you mean plugin.checks.foo ? (line 10)

Implementation image

Create config/validate.go#ValidateConfigFile and use it in commands.go#doConfigtest

func doConfigtest(fs *flag.FlagSet, argv []string) error {
	conf, err := resolveConfig(fs, argv)
	if err != nil {
		return fmt.Errorf("failed to test config: %s", err)
	}
	warnings := config.ValidateConfigFile(conf.Conffile)
	if warnings != nil {
                 return <display warnings>
        }
        fmt.Fprintf(os.Stderr, "%s Syntax OK\n", conf.Conffile)
	return nil
}

Implementation stage

• detect unexpected key Improve mackerel-agent configtest: detect unexpected key #813
• show suggestion Improve mackerel-agent configtest: Add suggestion to unexpected keys #818