Extend further validation of config values (#8469)

- This PR allows config KVS to be validated properly
  without being affected by ENV overrides, rejects
  invalid values during set operation

- Expands unit tests and refactors the error handling
  for notification targets, returns error instead of
  ignoring targets for invalid KVS

- Does all the prep-work for implementing safe-mode
  style operation for MinIO server, introduces a new
  global variable to toggle safe mode based operations
  NOTE: this PR itself doesn't provide safe mode operations

pkg/env/env.go

@@ -3,26 +3,47 @@ package env
 import (
 	"os"
 	"strings"
+	"sync"
 )
 
+var (
+	privateMutex sync.RWMutex
+	envOff       bool
+)
+
+// SetEnvOff - turns off env lookup
+func SetEnvOff() {
+	privateMutex.Lock()
+	defer privateMutex.Unlock()
+
+	envOff = true
+}
+
+// SetEnvOn - turns on env lookup
+func SetEnvOn() {
+	privateMutex.Lock()
+	defer privateMutex.Unlock()
+
+	envOff = false
+}
+
 // Get retrieves the value of the environment variable named
 // by the key. If the variable is present in the environment the
 // value (which may be empty) is returned. Otherwise it returns
 // the specified default value.
 func Get(key, defaultValue string) string {
+	privateMutex.RLock()
+	ok := envOff
+	privateMutex.RUnlock()
+	if ok {
+		return defaultValue
+	}
 	if v, ok := os.LookupEnv(key); ok {
 		return v
 	}
 	return defaultValue
 }
 
-// Lookup retrieves the value of the environment variable named
-// by the key. If the variable is present in the environment the
-// value (which may be empty) is returned and the boolean is true.
-// Otherwise the returned value will be empty and the boolean will
-// be false.
-func Lookup(key string) (string, bool) { return os.LookupEnv(key) }
-
 // List all envs with a given prefix.
 func List(prefix string) (envs []string) {
 	for _, env := range os.Environ() {