backend/fs: Refactor multipart upload

This patch modifies multipart upload related functions as below

* New multipart upload call creates file
  EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.uploadid

* Put object part call creates file
  EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.PART_NUMBER.MD5SUM_STRING

* Abort multipart call removes all files matching
  EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.*

* Complete multipart call does
  1. creates a staging file
  EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.complete.TEMP_NAME
  then renames to
  EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.complete
  2. rename staging file
  EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.complete
  to EXPORT_DIR/BUCKET/PATH/TO/OBJECT

vendor/github.com/skyrings/skyring-common/tools/uuid/uuid.go

@@ -0,0 +1,133 @@
+// Copyright 2015 Red Hat, 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 uuid
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// UUID is 128bits = 16bytes
+type UUID [16]byte
+
+func (uuid UUID) String() string {
+	return fmt.Sprintf("%x-%x-%x-%x-%x", uuid[0:4], uuid[4:6], uuid[6:8], uuid[8:10], uuid[10:])
+}
+
+func (uuid UUID) IsZero() bool {
+	var zeroUuid UUID
+	return Equal(zeroUuid, uuid)
+}
+
+func (uuid UUID) MarshalJSON() ([]byte, error) {
+	return []byte(`"` + uuid.String() + `"`), nil
+}
+
+func (uuid *UUID) UnmarshalJSON(b []byte) error {
+	if u, err := Parse(string(b)); err != nil {
+		return err
+	} else {
+		copy(uuid[:], u[:])
+		return nil
+	}
+}
+
+func New() (*UUID, error) {
+	uuid := new(UUID)
+
+	n, err := io.ReadFull(rand.Reader, uuid[:])
+	if err != nil {
+		return nil, err
+	} else if n != len(uuid) {
+		return nil, errors.New(fmt.Sprintf("insufficient random data (expected: %d, read: %d)", len(uuid), n))
+	} else {
+		// variant bits; for more info
+		// see https://www.ietf.org/rfc/rfc4122.txt section 4.1.1
+		uuid[8] = uuid[8]&0x3f | 0x80
+		// version 4 (pseudo-random); for more info
+		// see https://www.ietf.org/rfc/rfc4122.txt section 4.1.3
+		uuid[6] = uuid[6]&0x0f | 0x40
+	}
+
+	return uuid, nil
+}
+
+func Equal(uuid1 UUID, uuid2 UUID) bool {
+	for i, v := range uuid1 {
+		if v != uuid2[i] {
+			return false
+		}
+	}
+
+	return true
+}
+
+func Parse(s string) (*UUID, error) {
+	// the string format should be either in
+	// xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx (or)
+	// xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+
+	// If the uuid is marshaled by us we add " " around the uuid.
+	// while parsing this, we have to remove the " " around the
+	// uuid. So we check if uuid has " " around it, if yes we remove
+	// it.
+
+	if strings.HasPrefix(s, "\"") && strings.HasSuffix(s, "\"") {
+		s = s[1 : len(s)-1]
+	}
+
+	uuid := new(UUID)
+	if len(s) == 36 {
+		if ba, err := hex.DecodeString(s[0:8]); err == nil {
+			copy(uuid[:4], ba)
+		} else {
+			return nil, err
+		}
+		if ba, err := hex.DecodeString(s[9:13]); err == nil {
+			copy(uuid[4:], ba)
+		} else {
+			return nil, err
+		}
+		if ba, err := hex.DecodeString(s[14:18]); err == nil {
+			copy(uuid[6:], ba)
+		} else {
+			return nil, err
+		}
+		if ba, err := hex.DecodeString(s[19:23]); err == nil {
+			copy(uuid[8:], ba)
+		} else {
+			return nil, err
+		}
+		if ba, err := hex.DecodeString(s[24:]); err == nil {
+			copy(uuid[10:], ba)
+		} else {
+			return nil, err
+		}
+	} else if len(s) == 32 {
+		if ba, err := hex.DecodeString(s); err == nil {
+			copy(uuid[:], ba)
+		} else {
+			return nil, err
+		}
+	} else {
+		return nil, errors.New("unknown UUID string " + s)
+	}
+
+	return uuid, nil
+}