Enhance policy handling to support SSE and WORM (#5790)

- remove old bucket policy handling
- add new policy handling
- add new policy handling unit tests

This patch brings support to bucket policy to have more control not
limiting to anonymous.  Bucket owner controls to allow/deny any rest
API.

For example server side encryption can be controlled by allowing
PUT/GET objects with encryptions including bucket owner.

pkg/policy/condition/nullfunc.go

@@ -0,0 +1,101 @@
+/*
+ * Minio Cloud Storage, (C) 2018 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 condition
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+)
+
+// nullFunc - Null condition function. It checks whether Key is present in given
+// values or not.
+// For example,
+//   1. if Key = S3XAmzCopySource and Value = true, at evaluate() it returns whether
+//      S3XAmzCopySource is in given value map or not.
+//   2. if Key = S3XAmzCopySource and Value = false, at evaluate() it returns whether
+//      S3XAmzCopySource is NOT in given value map or not.
+type nullFunc struct {
+	k     Key
+	value bool
+}
+
+// evaluate() - evaluates to check whether Key is present in given values or not.
+// Depending on condition boolean value, this function returns true or false.
+func (f nullFunc) evaluate(values map[string][]string) bool {
+	requestValue := values[f.k.Name()]
+
+	if f.value {
+		return len(requestValue) != 0
+	}
+
+	return len(requestValue) == 0
+}
+
+// key() - returns condition key which is used by this condition function.
+func (f nullFunc) key() Key {
+	return f.k
+}
+
+// name() - returns "Null" condition name.
+func (f nullFunc) name() name {
+	return null
+}
+
+func (f nullFunc) String() string {
+	return fmt.Sprintf("%v:%v:%v", null, f.k, f.value)
+}
+
+// toMap - returns map representation of this function.
+func (f nullFunc) toMap() map[Key]ValueSet {
+	if !f.k.IsValid() {
+		return nil
+	}
+
+	return map[Key]ValueSet{
+		f.k: NewValueSet(NewBoolValue(f.value)),
+	}
+}
+
+func newNullFunc(key Key, values ValueSet) (Function, error) {
+	if len(values) != 1 {
+		return nil, fmt.Errorf("only one value is allowed for Null condition")
+	}
+
+	var value bool
+	for v := range values {
+		switch v.GetType() {
+		case reflect.Bool:
+			value, _ = v.GetBool()
+		case reflect.String:
+			var err error
+			s, _ := v.GetString()
+			if value, err = strconv.ParseBool(s); err != nil {
+				return nil, fmt.Errorf("value must be a boolean string for Null condition")
+			}
+		default:
+			return nil, fmt.Errorf("value must be a boolean for Null condition")
+		}
+	}
+
+	return &nullFunc{key, value}, nil
+}
+
+// NewNullFunc - returns new Null function.
+func NewNullFunc(key Key, value bool) (Function, error) {
+	return &nullFunc{key, value}, nil
+}