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/compress/compress_test.go

@@ -0,0 +1,57 @@
+/*
+ * MinIO Cloud Storage, (C) 2019 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 compress
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestParseCompressIncludes(t *testing.T) {
+	testCases := []struct {
+		str              string
+		expectedPatterns []string
+		success          bool
+	}{
+		// invalid input
+		{",,,", []string{}, false},
+		{"", []string{}, false},
+		{",", []string{}, false},
+		{"/", []string{}, false},
+		{"text/*,/", []string{}, false},
+
+		// valid input
+		{".txt,.log", []string{".txt", ".log"}, true},
+		{"text/*,application/json", []string{"text/*", "application/json"}, true},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+		t.Run(testCase.str, func(t *testing.T) {
+			gotPatterns, err := parseCompressIncludes(testCase.str)
+			if !testCase.success && err == nil {
+				t.Error("expected failure but success instead")
+			}
+			if testCase.success && err != nil {
+				t.Errorf("expected success but failed instead %s", err)
+			}
+			if testCase.success && !reflect.DeepEqual(testCase.expectedPatterns, gotPatterns) {
+				t.Errorf("expected patterns %s but got %s", testCase.expectedPatterns, gotPatterns)
+			}
+		})
+	}
+}