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

cmd/config/cache/config.go

@@ -89,6 +89,9 @@ func parseCacheDrives(drives []string) ([]string, error) {
 	}
 
 	for _, d := range endpoints {
+		if len(d) == 0 {
+			return nil, config.ErrInvalidCacheDrivesValue(nil).Msg("cache dir cannot be an empty path")
+		}
 		if !filepath.IsAbs(d) {
 			return nil, config.ErrInvalidCacheDrivesValue(nil).Msg("cache dir should be absolute path: %s", d)
 		}

cmd/config/cache/config_test.go

@@ -30,11 +30,15 @@ func TestParseCacheDrives(t *testing.T) {
 		expectedPatterns []string
 		success          bool
 	}{
-		// valid input
+		// Invalid input
 
 		{"bucket1/*;*.png;images/trip/barcelona/*", []string{}, false},
 		{"bucket1", []string{}, false},
+		{";;;", []string{}, false},
+		{",;,;,;", []string{}, false},
 	}
+
+	// Valid inputs
 	if runtime.GOOS == "windows" {
 		testCases = append(testCases, struct {
 			driveStr         string
@@ -91,8 +95,12 @@ func TestParseCacheExclude(t *testing.T) {
 		expectedPatterns []string
 		success          bool
 	}{
-		// valid input
+		// Invalid input
 		{"/home/drive1;/home/drive2;/home/drive3", []string{}, false},
+		{"/", []string{}, false},
+		{";;;", []string{}, false},
+
+		// valid input
 		{"bucket1/*;*.png;images/trip/barcelona/*", []string{"bucket1/*", "*.png", "images/trip/barcelona/*"}, true},
 		{"bucket1", []string{"bucket1"}, true},
 	}