distribution/configuration/parser.go

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package configuration
import (
"fmt"
"os"
"reflect"
"sort"
"strconv"
"strings"
"github.com/Sirupsen/logrus"
"gopkg.in/yaml.v2"
)
// Version is a major/minor version pair of the form Major.Minor
// Major version upgrades indicate structure or type changes
// Minor version upgrades should be strictly additive
type Version string
// MajorMinorVersion constructs a Version from its Major and Minor components
func MajorMinorVersion(major, minor uint) Version {
return Version(fmt.Sprintf("%d.%d", major, minor))
}
func (version Version) major() (uint, error) {
majorPart := strings.Split(string(version), ".")[0]
major, err := strconv.ParseUint(majorPart, 10, 0)
return uint(major), err
}
// Major returns the major version portion of a Version
func (version Version) Major() uint {
major, _ := version.major()
return major
}
func (version Version) minor() (uint, error) {
minorPart := strings.Split(string(version), ".")[1]
minor, err := strconv.ParseUint(minorPart, 10, 0)
return uint(minor), err
}
// Minor returns the minor version portion of a Version
func (version Version) Minor() uint {
minor, _ := version.minor()
return minor
}
// VersionedParseInfo defines how a specific version of a configuration should
// be parsed into the current version
type VersionedParseInfo struct {
// Version is the version which this parsing information relates to
Version Version
// ParseAs defines the type which a configuration file of this version
// should be parsed into
ParseAs reflect.Type
// ConversionFunc defines a method for converting the parsed configuration
// (of type ParseAs) into the current configuration version
// Note: this method signature is very unclear with the absence of generics
ConversionFunc func(interface{}) (interface{}, error)
}
type envVar struct {
name string
value string
}
type envVars []envVar
func (a envVars) Len() int { return len(a) }
func (a envVars) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
func (a envVars) Less(i, j int) bool { return a[i].name < a[j].name }
// Parser can be used to parse a configuration file and environment of a defined
// version into a unified output structure
type Parser struct {
prefix string
mapping map[Version]VersionedParseInfo
env envVars
}
// NewParser returns a *Parser with the given environment prefix which handles
// versioned configurations which match the given parseInfos
func NewParser(prefix string, parseInfos []VersionedParseInfo) *Parser {
p := Parser{prefix: prefix, mapping: make(map[Version]VersionedParseInfo)}
for _, parseInfo := range parseInfos {
p.mapping[parseInfo.Version] = parseInfo
}
for _, env := range os.Environ() {
envParts := strings.SplitN(env, "=", 2)
p.env = append(p.env, envVar{envParts[0], envParts[1]})
}
// We must sort the environment variables lexically by name so that
// more specific variables are applied before less specific ones
// (i.e. REGISTRY_STORAGE before
// REGISTRY_STORAGE_FILESYSTEM_ROOTDIRECTORY). This sucks, but it's a
// lot simpler and easier to get right than unmarshalling map entries
// into temporaries and merging with the existing entry.
sort.Sort(p.env)
return &p
}
// Parse reads in the given []byte and environment and writes the resulting
// configuration into the input v
//
// Environment variables may be used to override configuration parameters other
// than version, following the scheme below:
// v.Abc may be replaced by the value of PREFIX_ABC,
// v.Abc.Xyz may be replaced by the value of PREFIX_ABC_XYZ, and so forth
func (p *Parser) Parse(in []byte, v interface{}) error {
var versionedStruct struct {
Version Version
}
if err := yaml.Unmarshal(in, &versionedStruct); err != nil {
return err
}
parseInfo, ok := p.mapping[versionedStruct.Version]
if !ok {
return fmt.Errorf("Unsupported version: %q", versionedStruct.Version)
}
parseAs := reflect.New(parseInfo.ParseAs)
err := yaml.Unmarshal(in, parseAs.Interface())
if err != nil {
return err
}
for _, envVar := range p.env {
pathStr := envVar.name
if strings.HasPrefix(pathStr, strings.ToUpper(p.prefix)+"_") {
path := strings.Split(pathStr, "_")
err = p.overwriteFields(parseAs, pathStr, path[1:], envVar.value)
if err != nil {
return err
}
}
}
c, err := parseInfo.ConversionFunc(parseAs.Interface())
if err != nil {
return err
}
reflect.ValueOf(v).Elem().Set(reflect.Indirect(reflect.ValueOf(c)))
return nil
}
// overwriteFields replaces configuration values with alternate values specified
// through the environment. Precondition: an empty path slice must never be
// passed in.
func (p *Parser) overwriteFields(v reflect.Value, fullpath string, path []string, payload string) error {
for v.Kind() == reflect.Ptr {
if v.IsNil() {
panic("encountered nil pointer while handling environment variable " + fullpath)
}
v = reflect.Indirect(v)
}
switch v.Kind() {
case reflect.Struct:
return p.overwriteStruct(v, fullpath, path, payload)
case reflect.Map:
return p.overwriteMap(v, fullpath, path, payload)
case reflect.Interface:
if v.NumMethod() == 0 {
if !v.IsNil() {
return p.overwriteFields(v.Elem(), fullpath, path, payload)
}
// Interface was empty; create an implicit map
var template map[string]interface{}
wrappedV := reflect.MakeMap(reflect.TypeOf(template))
v.Set(wrappedV)
return p.overwriteMap(wrappedV, fullpath, path, payload)
}
}
return nil
}
func (p *Parser) overwriteStruct(v reflect.Value, fullpath string, path []string, payload string) error {
// Generate case-insensitive map of struct fields
byUpperCase := make(map[string]int)
for i := 0; i < v.NumField(); i++ {
sf := v.Type().Field(i)
upper := strings.ToUpper(sf.Name)
if _, present := byUpperCase[upper]; present {
panic(fmt.Sprintf("field name collision in configuration object: %s", sf.Name))
}
byUpperCase[upper] = i
}
fieldIndex, present := byUpperCase[path[0]]
if !present {
logrus.Warnf("Ignoring unrecognized environment variable %s", fullpath)
return nil
}
field := v.Field(fieldIndex)
sf := v.Type().Field(fieldIndex)
if len(path) == 1 {
// Env var specifies this field directly
fieldVal := reflect.New(sf.Type)
err := yaml.Unmarshal([]byte(payload), fieldVal.Interface())
if err != nil {
return err
}
field.Set(reflect.Indirect(fieldVal))
return nil
}
// If the field is nil, must create an object
switch sf.Type.Kind() {
case reflect.Map:
if field.IsNil() {
field.Set(reflect.MakeMap(sf.Type))
}
case reflect.Ptr:
if field.IsNil() {
field.Set(reflect.New(sf.Type))
}
}
err := p.overwriteFields(field, fullpath, path[1:], payload)
if err != nil {
return err
}
return nil
}
func (p *Parser) overwriteMap(m reflect.Value, fullpath string, path []string, payload string) error {
if m.Type().Key().Kind() != reflect.String {
// non-string keys unsupported
logrus.Warnf("Ignoring environment variable %s involving map with non-string keys", fullpath)
return nil
}
if len(path) > 1 {
// If a matching key exists, get its value and continue the
// overwriting process.
for _, k := range m.MapKeys() {
if strings.ToUpper(k.String()) == path[0] {
mapValue := m.MapIndex(k)
// If the existing value is nil, we want to
// recreate it instead of using this value.
if (mapValue.Kind() == reflect.Ptr ||
mapValue.Kind() == reflect.Interface ||
mapValue.Kind() == reflect.Map) &&
mapValue.IsNil() {
break
}
return p.overwriteFields(mapValue, fullpath, path[1:], payload)
}
}
}
// (Re)create this key
var mapValue reflect.Value
if m.Type().Elem().Kind() == reflect.Map {
mapValue = reflect.MakeMap(m.Type().Elem())
} else {
mapValue = reflect.New(m.Type().Elem())
}
if len(path) > 1 {
err := p.overwriteFields(mapValue, fullpath, path[1:], payload)
if err != nil {
return err
}
} else {
err := yaml.Unmarshal([]byte(payload), mapValue.Interface())
if err != nil {
return err
}
}
m.SetMapIndex(reflect.ValueOf(strings.ToLower(path[0])), reflect.Indirect(mapValue))
return nil
}