Add ObjectTagging Support (#8754)

This PR adds support for AWS S3 ObjectTagging API as explained here
https://docs.aws.amazon.com/AmazonS3/latest/dev/object-tagging.html

pkg/tagging/tagging.go

@@ -0,0 +1,113 @@
+/*
+ * MinIO Cloud Storage, (C) 2020 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 tagging
+
+import (
+	"bytes"
+	"encoding/xml"
+	"io"
+	"net/url"
+)
+
+// S3 API limits for tags
+// Ref: https://docs.aws.amazon.com/AmazonS3/latest/dev/object-tagging.html
+const (
+	maxTags           = 10
+	maxTagKeyLength   = 128
+	maxTagValueLength = 256
+)
+
+// errors returned by tagging package
+var (
+	ErrTooManyTags     = Errorf("Cannot have more than 10 object tags")
+	ErrInvalidTagKey   = Errorf("The TagKey you have provided is invalid")
+	ErrInvalidTagValue = Errorf("The TagValue you have provided is invalid")
+	ErrInvalidTag      = Errorf("Cannot provide multiple Tags with the same key")
+)
+
+// Tagging - object tagging interface
+type Tagging struct {
+	XMLName xml.Name `xml:"http://s3.amazonaws.com/doc/2006-03-01/ Tagging"`
+	TagSet  TagSet   `xml:"TagSet"`
+}
+
+// Validate - validates the tagging configuration
+func (t Tagging) Validate() error {
+	// Tagging can't have more than 10 tags
+	if len(t.TagSet.Tags) > maxTags {
+		return ErrTooManyTags
+	}
+	// Validate all the rules in the tagging config
+	for _, ts := range t.TagSet.Tags {
+		if t.TagSet.ContainsDuplicate(ts.Key) {
+			return ErrInvalidTag
+		}
+		if err := ts.Validate(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// String - returns a string in format "tag1=value1&tag2=value2" with all the
+// tags in this Tagging Struct
+func (t Tagging) String() string {
+	var buf bytes.Buffer
+	for _, tag := range t.TagSet.Tags {
+		if buf.Len() > 0 {
+			buf.WriteString("&")
+		}
+		buf.WriteString(tag.Key + "=")
+		buf.WriteString(tag.Value)
+	}
+	return buf.String()
+}
+
+// FromString - returns a Tagging struct when given a string in format
+// "tag1=value1&tag2=value2"
+func FromString(tagStr string) (Tagging, error) {
+	tags, err := url.ParseQuery(tagStr)
+	if err != nil {
+		return Tagging{}, err
+	}
+	var idx = 0
+	parsedTags := make([]Tag, len(tags))
+	for k := range tags {
+		parsedTags[idx].Key = k
+		parsedTags[idx].Value = tags.Get(k)
+		idx++
+	}
+	return Tagging{
+		TagSet: TagSet{
+			Tags: parsedTags,
+		},
+	}, nil
+}
+
+// ParseTagging - parses incoming xml data in given reader
+// into Tagging interface. After parsing, also validates the
+// parsed fields based on S3 API constraints.
+func ParseTagging(reader io.Reader) (*Tagging, error) {
+	var t Tagging
+	if err := xml.NewDecoder(reader).Decode(&t); err != nil {
+		return nil, err
+	}
+	if err := t.Validate(); err != nil {
+		return nil, err
+	}
+	return &t, nil
+}