Simplify the steps to make changes to config.json (#5186)

This change introduces following simplified steps to follow 
during config migration.

```
 // Steps to move from version N to version N+1
 // 1. Add new struct serverConfigVN+1 in config-versions.go
 // 2. Set configCurrentVersion to "N+1"
 // 3. Set serverConfigCurrent to serverConfigVN+1
 // 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
 // 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
 // 6. Make changes in config-current_test.go for any test change
```

cmd/config-current.go

@@ -0,0 +1,333 @@
+/*
+ * Minio Cloud Storage, (C) 2016, 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import (
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"sync"
+
+	"github.com/minio/minio/pkg/auth"
+	"github.com/minio/minio/pkg/quick"
+	"github.com/tidwall/gjson"
+)
+
+// Steps to move from version N to version N+1
+// 1. Add new struct serverConfigVN+1 in config-versions.go
+// 2. Set configCurrentVersion to "N+1"
+// 3. Set serverConfigCurrent to serverConfigVN+1
+// 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
+// 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
+// 6. Make changes in config-current_test.go for any test change
+
+// Config version
+const serverConfigVersion = "20"
+
+type serverConfig = serverConfigV20
+
+var (
+	// globalServerConfig server config.
+	globalServerConfig   *serverConfig
+	globalServerConfigMu sync.RWMutex
+)
+
+// GetVersion get current config version.
+func (s *serverConfig) GetVersion() string {
+	s.RLock()
+	defer s.RUnlock()
+
+	return s.Version
+}
+
+// SetRegion set a new region.
+func (s *serverConfig) SetRegion(region string) {
+	s.Lock()
+	defer s.Unlock()
+
+	// Save new region.
+	s.Region = region
+}
+
+// GetRegion get current region.
+func (s *serverConfig) GetRegion() string {
+	s.RLock()
+	defer s.RUnlock()
+
+	return s.Region
+}
+
+// SetCredentials set new credentials. SetCredential returns the previous credential.
+func (s *serverConfig) SetCredential(creds auth.Credentials) (prevCred auth.Credentials) {
+	s.Lock()
+	defer s.Unlock()
+
+	// Save previous credential.
+	prevCred = s.Credential
+
+	// Set updated credential.
+	s.Credential = creds
+
+	// Return previous credential.
+	return prevCred
+}
+
+// GetCredentials get current credentials.
+func (s *serverConfig) GetCredential() auth.Credentials {
+	s.RLock()
+	defer s.RUnlock()
+
+	return s.Credential
+}
+
+// SetBrowser set if browser is enabled.
+func (s *serverConfig) SetBrowser(b bool) {
+	s.Lock()
+	defer s.Unlock()
+
+	// Set the new value.
+	s.Browser = BrowserFlag(b)
+}
+
+// GetCredentials get current credentials.
+func (s *serverConfig) GetBrowser() bool {
+	s.RLock()
+	defer s.RUnlock()
+
+	return bool(s.Browser)
+}
+
+// Save config.
+func (s *serverConfig) Save() error {
+	s.RLock()
+	defer s.RUnlock()
+
+	// Save config file.
+	return quick.Save(getConfigFile(), s)
+}
+
+func newServerConfig() *serverConfig {
+	srvCfg := &serverConfig{
+		Version:    serverConfigVersion,
+		Credential: auth.MustGetNewCredentials(),
+		Region:     globalMinioDefaultRegion,
+		Browser:    true,
+		Logger:     &loggers{},
+		Notify:     &notifier{},
+	}
+
+	// Enable console logger by default on a fresh run.
+	srvCfg.Logger.Console = NewConsoleLogger()
+
+	// Make sure to initialize notification configs.
+	srvCfg.Notify.AMQP = make(map[string]amqpNotify)
+	srvCfg.Notify.AMQP["1"] = amqpNotify{}
+	srvCfg.Notify.MQTT = make(map[string]mqttNotify)
+	srvCfg.Notify.MQTT["1"] = mqttNotify{}
+	srvCfg.Notify.ElasticSearch = make(map[string]elasticSearchNotify)
+	srvCfg.Notify.ElasticSearch["1"] = elasticSearchNotify{}
+	srvCfg.Notify.Redis = make(map[string]redisNotify)
+	srvCfg.Notify.Redis["1"] = redisNotify{}
+	srvCfg.Notify.NATS = make(map[string]natsNotify)
+	srvCfg.Notify.NATS["1"] = natsNotify{}
+	srvCfg.Notify.PostgreSQL = make(map[string]postgreSQLNotify)
+	srvCfg.Notify.PostgreSQL["1"] = postgreSQLNotify{}
+	srvCfg.Notify.MySQL = make(map[string]mySQLNotify)
+	srvCfg.Notify.MySQL["1"] = mySQLNotify{}
+	srvCfg.Notify.Kafka = make(map[string]kafkaNotify)
+	srvCfg.Notify.Kafka["1"] = kafkaNotify{}
+	srvCfg.Notify.Webhook = make(map[string]webhookNotify)
+	srvCfg.Notify.Webhook["1"] = webhookNotify{}
+
+	return srvCfg
+}
+
+// newConfig - initialize a new server config, saves env parameters if
+// found, otherwise use default parameters
+func newConfig() error {
+	// Initialize server config.
+	srvCfg := newServerConfig()
+
+	// If env is set override the credentials from config file.
+	if globalIsEnvCreds {
+		srvCfg.SetCredential(globalActiveCred)
+	}
+
+	if globalIsEnvBrowser {
+		srvCfg.SetBrowser(globalIsBrowserEnabled)
+	}
+
+	if globalIsEnvRegion {
+		srvCfg.SetRegion(globalServerRegion)
+	}
+
+	if globalIsEnvDomainName {
+		srvCfg.Domain = globalDomainName
+	}
+
+	// hold the mutex lock before a new config is assigned.
+	// Save the new config globally.
+	// unlock the mutex.
+	globalServerConfigMu.Lock()
+	globalServerConfig = srvCfg
+	globalServerConfigMu.Unlock()
+
+	// Save config into file.
+	return globalServerConfig.Save()
+}
+
+// doCheckDupJSONKeys recursively detects duplicate json keys
+func doCheckDupJSONKeys(key, value gjson.Result) error {
+	// Key occurrences map of the current scope to count
+	// if there is any duplicated json key.
+	keysOcc := make(map[string]int)
+
+	// Holds the found error
+	var checkErr error
+
+	// Iterate over keys in the current json scope
+	value.ForEach(func(k, v gjson.Result) bool {
+		// If current key is not null, check if its
+		// value contains some duplicated keys.
+		if k.Type != gjson.Null {
+			keysOcc[k.String()]++
+			checkErr = doCheckDupJSONKeys(k, v)
+		}
+		return checkErr == nil
+	})
+
+	// Check found err
+	if checkErr != nil {
+		return errors.New(key.String() + " => " + checkErr.Error())
+	}
+
+	// Check for duplicated keys
+	for k, v := range keysOcc {
+		if v > 1 {
+			return errors.New(key.String() + " => `" + k + "` entry is duplicated")
+		}
+	}
+
+	return nil
+}
+
+// Check recursively if a key is duplicated in the same json scope
+// e.g.:
+//  `{ "key" : { "key" ..` is accepted
+//  `{ "key" : { "subkey" : "val1", "subkey": "val2" ..` throws subkey duplicated error
+func checkDupJSONKeys(json string) error {
+	// Parse config with gjson library
+	config := gjson.Parse(json)
+
+	// Create a fake rootKey since root json doesn't seem to have representation
+	// in gjson library.
+	rootKey := gjson.Result{Type: gjson.String, Str: minioConfigFile}
+
+	// Check if loaded json contains any duplicated keys
+	return doCheckDupJSONKeys(rootKey, config)
+}
+
+// getValidConfig - returns valid server configuration
+func getValidConfig() (*serverConfig, error) {
+	srvCfg := &serverConfig{
+		Region:  globalMinioDefaultRegion,
+		Browser: true,
+	}
+
+	configFile := getConfigFile()
+	if _, err := quick.Load(configFile, srvCfg); err != nil {
+		return nil, err
+	}
+
+	if srvCfg.Version != serverConfigVersion {
+		return nil, fmt.Errorf("configuration version mismatch. Expected: ‘%s’, Got: ‘%s’", serverConfigVersion, srvCfg.Version)
+	}
+
+	// Load config file json and check for duplication json keys
+	jsonBytes, err := ioutil.ReadFile(configFile)
+	if err != nil {
+		return nil, err
+	}
+	if err = checkDupJSONKeys(string(jsonBytes)); err != nil {
+		return nil, err
+	}
+
+	// Validate credential fields only when
+	// they are not set via the environment
+
+	// Error out if global is env credential is not set and config has invalid credential
+	if !globalIsEnvCreds && !srvCfg.Credential.IsValid() {
+		return nil, errors.New("invalid credential in config file " + configFile)
+	}
+
+	// Validate logger field
+	if err = srvCfg.Logger.Validate(); err != nil {
+		return nil, err
+	}
+
+	// Validate notify field
+	if err = srvCfg.Notify.Validate(); err != nil {
+		return nil, err
+	}
+
+	return srvCfg, nil
+}
+
+// loadConfig - loads a new config from disk, overrides params from env
+// if found and valid
+func loadConfig() error {
+	srvCfg, err := getValidConfig()
+	if err != nil {
+		return err
+	}
+
+	// If env is set override the credentials from config file.
+	if globalIsEnvCreds {
+		srvCfg.SetCredential(globalActiveCred)
+	}
+
+	if globalIsEnvBrowser {
+		srvCfg.SetBrowser(globalIsBrowserEnabled)
+	}
+
+	if globalIsEnvRegion {
+		srvCfg.SetRegion(globalServerRegion)
+	}
+
+	if globalIsEnvDomainName {
+		srvCfg.Domain = globalDomainName
+	}
+
+	// hold the mutex lock before a new config is assigned.
+	globalServerConfigMu.Lock()
+	globalServerConfig = srvCfg
+	if !globalIsEnvCreds {
+		globalActiveCred = globalServerConfig.GetCredential()
+	}
+	if !globalIsEnvBrowser {
+		globalIsBrowserEnabled = globalServerConfig.GetBrowser()
+	}
+	if !globalIsEnvRegion {
+		globalServerRegion = globalServerConfig.GetRegion()
+	}
+	if !globalIsEnvDomainName {
+		globalDomainName = globalServerConfig.Domain
+	}
+	globalServerConfigMu.Unlock()
+
+	return nil
+}