Implement S3-HDFS gateway (#7440)

- [x] Support bucket and regular object operations
- [x] Supports Select API on HDFS
- [x] Implement multipart API support
- [x] Completion of ListObjects support

cmd/gateway/hdfs/gateway-hdfs-utils.go

@@ -0,0 +1,67 @@
+/*
+ * 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 hdfs
+
+import (
+	"strings"
+
+	"github.com/minio/minio-go/pkg/s3utils"
+	minio "github.com/minio/minio/cmd"
+)
+
+const (
+	// Minio meta bucket.
+	minioMetaBucket = ".minio.sys"
+
+	// Minio Tmp meta prefix.
+	minioMetaTmpBucket = minioMetaBucket + "/tmp"
+
+	// Minio reserved bucket name.
+	minioReservedBucket = "minio"
+)
+
+// Ignores all reserved bucket names or invalid bucket names.
+func isReservedOrInvalidBucket(bucketEntry string, strict bool) bool {
+	bucketEntry = strings.TrimSuffix(bucketEntry, "/")
+	if strict {
+		if err := s3utils.CheckValidBucketNameStrict(bucketEntry); err != nil {
+			return true
+		}
+	} else {
+		if err := s3utils.CheckValidBucketName(bucketEntry); err != nil {
+			return true
+		}
+	}
+	return isMinioMetaBucket(bucketEntry) || isMinioReservedBucket(bucketEntry)
+}
+
+// Returns true if input bucket is a reserved minio meta bucket '.minio.sys'.
+func isMinioMetaBucket(bucketName string) bool {
+	return bucketName == minioMetaBucket
+}
+
+// Returns true if input bucket is a reserved minio bucket 'minio'.
+func isMinioReservedBucket(bucketName string) bool {
+	return bucketName == minioReservedBucket
+}
+
+// byBucketName is a collection satisfying sort.Interface.
+type byBucketName []minio.BucketInfo
+
+func (d byBucketName) Len() int           { return len(d) }
+func (d byBucketName) Swap(i, j int)      { d[i], d[j] = d[j], d[i] }
+func (d byBucketName) Less(i, j int) bool { return d[i].Name < d[j].Name }