agent/config/config.go
func NewConfig(ec2client ec2.EC2MetadataClient) (*Config, error) {
	var errs []error
	envConfig, err := environmentConfig() //Environment overrides all else
	if err != nil {
		errs = append(errs, err)
	}
	config := &envConfig

	if config.complete() {
		// No need to do file / network IO
		return config, nil
	}

	fcfg, err := fileConfig()
	if err != nil {
		errs = append(errs, err)
	}
	config.Merge(fcfg)

	config.Merge(userDataConfig(ec2client))

	if config.AWSRegion == "" {
		if config.NoIID {
			// get it from AWS SDK if we don't have instance identity document
			awsRegion, err := ec2client.Region()
			if err != nil {
				errs = append(errs, err)
			}
			config.AWSRegion = awsRegion
		} else {
			// Get it from metadata only if we need to (network io)
			config.Merge(ec2MetadataConfig(ec2client))
		}
	}

	return config, config.mergeDefaultConfig(errs)
}
func NewConfig(ec2client ec2.EC2MetadataClient) (*Config, error) {
	var errs []error
	envConfig, err := environmentConfig() //Environment overrides all else
	if err != nil {
		errs = append(errs, err)
	}
	config := &envConfig

	if config.complete() {
		// No need to do file / network IO
		return config, nil
	}

	fcfg, err := fileConfig()
	if err != nil {
		errs = append(errs, err)
	}
	config.Merge(fcfg)

	config.Merge(userDataConfig(ec2client))

	if config.AWSRegion == "" {
		if config.NoIID {
			// get it from AWS SDK if we don't have instance identity document
			awsRegion, err := ec2client.Region()
			if err != nil {
				errs = append(errs, err)
			}
			config.AWSRegion = awsRegion
		} else {
			// Get it from metadata only if we need to (network io)
			config.Merge(ec2MetadataConfig(ec2client))
		}
	}

	return config, config.mergeDefaultConfig(errs)
}
// NewConfig returns a config struct created by merging environment variables,
// a config file, and EC2 Metadata info.
// The 'config' struct it returns can be used, even if an error is returned. An
// error is returned, however, if the config is incomplete in some way that is
// considered fatal.