fs: Break fs package to top-level and introduce ObjectAPI interface.

ObjectAPI interface brings in changes needed for XL ObjectAPI layer.

The new interface for any ObjectAPI layer is as below

```
// ObjectAPI interface.
type ObjectAPI interface {
        // Bucket resource API.
        DeleteBucket(bucket string) *probe.Error
        ListBuckets() ([]BucketInfo, *probe.Error)
        MakeBucket(bucket string) *probe.Error
        GetBucketInfo(bucket string) (BucketInfo, *probe.Error)

        // Bucket query API.
        ListObjects(bucket, prefix, marker, delimiter string, maxKeys int) (ListObjectsResult, *probe.Error)
        ListMultipartUploads(bucket string, resources BucketMultipartResourcesMetadata) (BucketMultipartResourcesMetadata, *probe.Error)

        // Object resource API.
        GetObject(bucket, object string, startOffset int64) (io.ReadCloser, *probe.Error)
        GetObjectInfo(bucket, object string) (ObjectInfo, *probe.Error)
        PutObject(bucket string, object string, size int64, data io.Reader, metadata map[string]string) (ObjectInfo, *probe.Error)
        DeleteObject(bucket, object string) *probe.Error

        // Object query API.
        NewMultipartUpload(bucket, object string) (string, *probe.Error)
        PutObjectPart(bucket, object, uploadID string, partID int, size int64, data io.Reader, md5Hex string) (string, *probe.Error)
        ListObjectParts(bucket, object string, resources ObjectResourcesMetadata) (ObjectResourcesMetadata, *probe.Error)
        CompleteMultipartUpload(bucket string, object string, uploadID string, parts []CompletePart) (ObjectInfo, *probe.Error)
        AbortMultipartUpload(bucket, object, uploadID string) *probe.Error
}
```

fs-multipart.go

@@ -0,0 +1,646 @@
+/*
+ * Minio Cloud Storage, (C) 2015 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 main
+
+import (
+	"crypto/md5"
+	"encoding/base64"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/minio/minio/pkg/atomic"
+	"github.com/minio/minio/pkg/crypto/sha512"
+	"github.com/minio/minio/pkg/disk"
+	"github.com/minio/minio/pkg/mimedb"
+	"github.com/minio/minio/pkg/probe"
+)
+
+// isValidUploadID - is upload id.
+func (fs Filesystem) isValidUploadID(object, uploadID string) (ok bool) {
+	fs.rwLock.RLock()
+	defer fs.rwLock.RUnlock()
+	_, ok = fs.multiparts.ActiveSession[uploadID]
+	if !ok {
+		return
+	}
+	return
+}
+
+// byObjectInfoKey is a sortable interface for UploadMetadata slice
+type byUploadMetadataKey []*UploadMetadata
+
+func (b byUploadMetadataKey) Len() int           { return len(b) }
+func (b byUploadMetadataKey) Swap(i, j int)      { b[i], b[j] = b[j], b[i] }
+func (b byUploadMetadataKey) Less(i, j int) bool { return b[i].Object < b[j].Object }
+
+// ListMultipartUploads - list incomplete multipart sessions for a given BucketMultipartResourcesMetadata
+func (fs Filesystem) ListMultipartUploads(bucket string, resources BucketMultipartResourcesMetadata) (BucketMultipartResourcesMetadata, *probe.Error) {
+	// Input validation.
+	if !IsValidBucketName(bucket) {
+		return BucketMultipartResourcesMetadata{}, probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketPath := filepath.Join(fs.path, bucket)
+	if _, e := os.Stat(bucketPath); e != nil {
+		// Check bucket exists.
+		if os.IsNotExist(e) {
+			return BucketMultipartResourcesMetadata{}, probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		return BucketMultipartResourcesMetadata{}, probe.NewError(e)
+	}
+	var uploads []*UploadMetadata
+	fs.rwLock.RLock()
+	defer fs.rwLock.RUnlock()
+	for uploadID, session := range fs.multiparts.ActiveSession {
+		objectName := session.ObjectName
+		if strings.HasPrefix(objectName, resources.Prefix) {
+			if len(uploads) > resources.MaxUploads {
+				sort.Sort(byUploadMetadataKey(uploads))
+				resources.Upload = uploads
+				resources.NextKeyMarker = session.ObjectName
+				resources.NextUploadIDMarker = uploadID
+				resources.IsTruncated = true
+				return resources, nil
+			}
+			// UploadIDMarker is ignored if KeyMarker is empty.
+			switch {
+			case resources.KeyMarker != "" && resources.UploadIDMarker == "":
+				if objectName > resources.KeyMarker {
+					upload := new(UploadMetadata)
+					upload.Object = objectName
+					upload.UploadID = uploadID
+					upload.Initiated = session.Initiated
+					uploads = append(uploads, upload)
+				}
+			case resources.KeyMarker != "" && resources.UploadIDMarker != "":
+				if session.UploadID > resources.UploadIDMarker {
+					if objectName >= resources.KeyMarker {
+						upload := new(UploadMetadata)
+						upload.Object = objectName
+						upload.UploadID = uploadID
+						upload.Initiated = session.Initiated
+						uploads = append(uploads, upload)
+					}
+				}
+			default:
+				upload := new(UploadMetadata)
+				upload.Object = objectName
+				upload.UploadID = uploadID
+				upload.Initiated = session.Initiated
+				uploads = append(uploads, upload)
+			}
+		}
+	}
+	sort.Sort(byUploadMetadataKey(uploads))
+	resources.Upload = uploads
+	return resources, nil
+}
+
+// verify if parts sent over the network do really match with what we
+// have for the session.
+func doPartsMatch(parts []CompletePart, savedParts []PartMetadata) bool {
+	if parts == nil || savedParts == nil {
+		return false
+	}
+	if len(parts) != len(savedParts) {
+		return false
+	}
+	// Range of incoming parts and compare them with saved parts.
+	for i, part := range parts {
+		if strings.Trim(part.ETag, "\"") != savedParts[i].ETag {
+			return false
+		}
+	}
+	return true
+}
+
+// Create an s3 compatible MD5sum for complete multipart transaction.
+func makeS3MD5(md5Strs ...string) (string, *probe.Error) {
+	var finalMD5Bytes []byte
+	for _, md5Str := range md5Strs {
+		md5Bytes, e := hex.DecodeString(md5Str)
+		if e != nil {
+			return "", probe.NewError(e)
+		}
+		finalMD5Bytes = append(finalMD5Bytes, md5Bytes...)
+	}
+	md5Hasher := md5.New()
+	md5Hasher.Write(finalMD5Bytes)
+	s3MD5 := fmt.Sprintf("%s-%d", hex.EncodeToString(md5Hasher.Sum(nil)), len(md5Strs))
+	return s3MD5, nil
+}
+
+type multiCloser struct {
+	Closers []io.Closer
+}
+
+func (m multiCloser) Close() error {
+	for _, c := range m.Closers {
+		if e := c.Close(); e != nil {
+			return e
+		}
+	}
+	return nil
+}
+
+// MultiCloser - returns a Closer that's the logical
+// concatenation of the provided input closers. They're closed
+// sequentially. If any of the closers return a non-nil error, Close
+// will return that error.
+func MultiCloser(closers ...io.Closer) io.Closer {
+	return multiCloser{closers}
+}
+
+// removeParts - remove all parts.
+func removeParts(partPathPrefix string, parts []PartMetadata) *probe.Error {
+	for _, part := range parts {
+		// We are on purpose ignoring the return values here, since
+		// another thread would have purged these entries.
+		os.Remove(partPathPrefix + part.ETag + fmt.Sprintf("$%d-$multiparts", part.PartNumber))
+	}
+	return nil
+}
+
+// saveParts - concantenate and save all parts.
+func saveParts(partPathPrefix string, mw io.Writer, parts []CompletePart) *probe.Error {
+	var partReaders []io.Reader
+	var partClosers []io.Closer
+	for _, part := range parts {
+		// Trim prefix
+		md5Sum := strings.TrimPrefix(part.ETag, "\"")
+		// Trim suffix
+		md5Sum = strings.TrimSuffix(md5Sum, "\"")
+		partFile, e := os.OpenFile(partPathPrefix+md5Sum+fmt.Sprintf("$%d-$multiparts", part.PartNumber), os.O_RDONLY, 0600)
+		if e != nil {
+			if !os.IsNotExist(e) {
+				return probe.NewError(e)
+			}
+			// Some clients do not set Content-Md5, so we would have
+			// created part files without 'ETag' in them.
+			partFile, e = os.OpenFile(partPathPrefix+fmt.Sprintf("$%d-$multiparts", part.PartNumber), os.O_RDONLY, 0600)
+			if e != nil {
+				return probe.NewError(e)
+			}
+		}
+		partReaders = append(partReaders, partFile)
+		partClosers = append(partClosers, partFile)
+	}
+	// Concatenate a list of closers and close upon return.
+	closer := MultiCloser(partClosers...)
+	defer closer.Close()
+
+	reader := io.MultiReader(partReaders...)
+	readBufferSize := 8 * 1024 * 1024          // 8MiB
+	readBuffer := make([]byte, readBufferSize) // Allocate 8MiB buffer.
+	if _, e := io.CopyBuffer(mw, reader, readBuffer); e != nil {
+		return probe.NewError(e)
+	}
+	return nil
+}
+
+// NewMultipartUpload - initiate a new multipart session
+func (fs Filesystem) NewMultipartUpload(bucket, object string) (string, *probe.Error) {
+	di, e := disk.GetInfo(fs.path)
+	if e != nil {
+		return "", probe.NewError(e)
+	}
+
+	// Remove 5% from total space for cumulative disk space used for
+	// journalling, inodes etc.
+	availableDiskSpace := (float64(di.Free) / (float64(di.Total) - (0.05 * float64(di.Total)))) * 100
+	if int64(availableDiskSpace) <= fs.minFreeDisk {
+		return "", probe.NewError(RootPathFull{Path: fs.path})
+	}
+
+	// Input validation.
+	if !IsValidBucketName(bucket) {
+		return "", probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+	if !IsValidObjectName(object) {
+		return "", probe.NewError(ObjectNameInvalid{Object: object})
+	}
+
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketPath := filepath.Join(fs.path, bucket)
+	if _, e = os.Stat(bucketPath); e != nil {
+		// Check bucket exists.
+		if os.IsNotExist(e) {
+			return "", probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		return "", probe.NewError(e)
+	}
+
+	objectPath := filepath.Join(bucketPath, object)
+	objectDir := filepath.Dir(objectPath)
+	if _, e = os.Stat(objectDir); e != nil {
+		if !os.IsNotExist(e) {
+			return "", probe.NewError(e)
+		}
+		e = os.MkdirAll(objectDir, 0700)
+		if e != nil {
+			return "", probe.NewError(e)
+		}
+	}
+
+	// Generate new upload id.
+	id := []byte(strconv.FormatInt(rand.Int63(), 10) + bucket + object + time.Now().String())
+	uploadIDSum := sha512.Sum512(id)
+	uploadID := base64.URLEncoding.EncodeToString(uploadIDSum[:])[:47]
+
+	// Critical region requiring write lock.
+	fs.rwLock.Lock()
+	defer fs.rwLock.Unlock()
+	// Initialize multipart session.
+	mpartSession := &MultipartSession{}
+	mpartSession.TotalParts = 0
+	mpartSession.ObjectName = object
+	mpartSession.UploadID = uploadID
+	mpartSession.Initiated = time.Now().UTC()
+	// Multipart has maximum of 10000 parts.
+	var parts []PartMetadata
+	mpartSession.Parts = parts
+
+	fs.multiparts.ActiveSession[uploadID] = mpartSession
+	if err := saveMultipartsSession(*fs.multiparts); err != nil {
+		return "", err.Trace(objectPath)
+	}
+	return uploadID, nil
+}
+
+// Remove all duplicated parts based on the latest time of their upload.
+func removeDuplicateParts(parts []PartMetadata) []PartMetadata {
+	length := len(parts) - 1
+	for i := 0; i < length; i++ {
+		for j := i + 1; j <= length; j++ {
+			if parts[i].PartNumber == parts[j].PartNumber {
+				if parts[i].LastModified.Sub(parts[j].LastModified) > 0 {
+					parts[i] = parts[length]
+				} else {
+					parts[j] = parts[length]
+				}
+				parts = parts[0:length]
+				length--
+				j--
+			}
+		}
+	}
+	return parts
+}
+
+// partNumber is a sortable interface for Part slice.
+type partNumber []PartMetadata
+
+func (a partNumber) Len() int           { return len(a) }
+func (a partNumber) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a partNumber) Less(i, j int) bool { return a[i].PartNumber < a[j].PartNumber }
+
+// PutObjectPart - create a part in a multipart session
+func (fs Filesystem) PutObjectPart(bucket, object, uploadID string, partID int, size int64, data io.Reader, md5Hex string) (string, *probe.Error) {
+	di, err := disk.GetInfo(fs.path)
+	if err != nil {
+		return "", probe.NewError(err)
+	}
+
+	// Remove 5% from total space for cumulative disk space used for
+	// journalling, inodes etc.
+	availableDiskSpace := (float64(di.Free) / (float64(di.Total) - (0.05 * float64(di.Total)))) * 100
+	if int64(availableDiskSpace) <= fs.minFreeDisk {
+		return "", probe.NewError(RootPathFull{Path: fs.path})
+	}
+
+	// Part id cannot be negative.
+	if partID <= 0 {
+		return "", probe.NewError(errors.New("invalid part id, cannot be zero or less than zero"))
+	}
+
+	// Check bucket name valid.
+	if !IsValidBucketName(bucket) {
+		return "", probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+
+	// Verify object path legal.
+	if !IsValidObjectName(object) {
+		return "", probe.NewError(ObjectNameInvalid{Bucket: bucket, Object: object})
+	}
+
+	// Verify upload is valid for the incoming object.
+	if !fs.isValidUploadID(object, uploadID) {
+		return "", probe.NewError(InvalidUploadID{UploadID: uploadID})
+	}
+
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketPath := filepath.Join(fs.path, bucket)
+	if _, e := os.Stat(bucketPath); e != nil {
+		// Check bucket exists.
+		if os.IsNotExist(e) {
+			return "", probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		return "", probe.NewError(e)
+	}
+
+	objectPath := filepath.Join(bucketPath, object)
+	partPathPrefix := objectPath + uploadID
+	partPath := partPathPrefix + md5Hex + fmt.Sprintf("$%d-$multiparts", partID)
+	partFile, e := atomic.FileCreateWithPrefix(partPath, "$multiparts")
+	if e != nil {
+		return "", probe.NewError(e)
+	}
+	defer partFile.Close()
+
+	// Initialize md5 writer.
+	md5Writer := md5.New()
+
+	// Create a multiwriter.
+	multiWriter := io.MultiWriter(md5Writer, partFile)
+
+	if _, e = io.CopyN(multiWriter, data, size); e != nil {
+		partFile.CloseAndPurge()
+		return "", probe.NewError(e)
+	}
+
+	// Finalize new md5.
+	newMD5Hex := hex.EncodeToString(md5Writer.Sum(nil))
+	if md5Hex != "" {
+		if newMD5Hex != md5Hex {
+			return "", probe.NewError(BadDigest{md5Hex, newMD5Hex})
+		}
+	}
+
+	// Stat the file to get the latest information.
+	fi, e := os.Stat(partFile.Name())
+	if e != nil {
+		return "", probe.NewError(e)
+	}
+	partMetadata := PartMetadata{}
+	partMetadata.PartNumber = partID
+	partMetadata.ETag = newMD5Hex
+	partMetadata.Size = fi.Size()
+	partMetadata.LastModified = fi.ModTime()
+
+	// Critical region requiring read lock.
+	fs.rwLock.RLock()
+	deserializedMultipartSession, ok := fs.multiparts.ActiveSession[uploadID]
+	fs.rwLock.RUnlock()
+	if !ok {
+		return "", probe.NewError(InvalidUploadID{UploadID: uploadID})
+	}
+
+	// Add all incoming parts.
+	deserializedMultipartSession.Parts = append(deserializedMultipartSession.Parts, partMetadata)
+
+	// Remove duplicate parts based on the most recent uploaded.
+	deserializedMultipartSession.Parts = removeDuplicateParts(deserializedMultipartSession.Parts)
+	// Save total parts uploaded.
+	deserializedMultipartSession.TotalParts = len(deserializedMultipartSession.Parts)
+
+	// Sort by part number before saving.
+	sort.Sort(partNumber(deserializedMultipartSession.Parts))
+
+	// Critical region requiring write lock.
+	fs.rwLock.Lock()
+	defer fs.rwLock.Unlock()
+
+	fs.multiparts.ActiveSession[uploadID] = deserializedMultipartSession
+	if err := saveMultipartsSession(*fs.multiparts); err != nil {
+		return "", err.Trace(partPathPrefix)
+	}
+	return newMD5Hex, nil
+}
+
+// CompleteMultipartUpload - complete a multipart upload and persist the data
+func (fs Filesystem) CompleteMultipartUpload(bucket string, object string, uploadID string, parts []CompletePart) (ObjectInfo, *probe.Error) {
+	// Check bucket name is valid.
+	if !IsValidBucketName(bucket) {
+		return ObjectInfo{}, probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+
+	// Verify object path is legal.
+	if !IsValidObjectName(object) {
+		return ObjectInfo{}, probe.NewError(ObjectNameInvalid{Bucket: bucket, Object: object})
+	}
+
+	// Verify if valid upload for incoming object.
+	if !fs.isValidUploadID(object, uploadID) {
+		return ObjectInfo{}, probe.NewError(InvalidUploadID{UploadID: uploadID})
+	}
+
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketPath := filepath.Join(fs.path, bucket)
+	if _, e := os.Stat(bucketPath); e != nil {
+		// Check bucket exists.
+		if os.IsNotExist(e) {
+			return ObjectInfo{}, probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		return ObjectInfo{}, probe.NewError(InternalError{})
+	}
+
+	objectPath := filepath.Join(bucketPath, object)
+	objectWriter, e := atomic.FileCreateWithPrefix(objectPath, "$tmpobject")
+	if e != nil {
+		return ObjectInfo{}, probe.NewError(e)
+	}
+
+	// Critical region requiring read lock.
+	fs.rwLock.RLock()
+	savedParts := fs.multiparts.ActiveSession[uploadID].Parts
+	fs.rwLock.RUnlock()
+
+	if !doPartsMatch(parts, savedParts) {
+		objectWriter.CloseAndPurge()
+		return ObjectInfo{}, probe.NewError(InvalidPart{})
+	}
+
+	// Parts successfully validated, save all the parts.
+	partPathPrefix := objectPath + uploadID
+	if err := saveParts(partPathPrefix, objectWriter, parts); err != nil {
+		objectWriter.CloseAndPurge()
+		return ObjectInfo{}, err.Trace(partPathPrefix)
+	}
+	var md5Strs []string
+	for _, part := range savedParts {
+		md5Strs = append(md5Strs, part.ETag)
+	}
+	// Save the s3 md5.
+	s3MD5, err := makeS3MD5(md5Strs...)
+	if err != nil {
+		objectWriter.CloseAndPurge()
+		return ObjectInfo{}, err.Trace(md5Strs...)
+	}
+
+	// Successfully saved multipart, remove all parts in a routine.
+	go removeParts(partPathPrefix, savedParts)
+
+	// Critical region requiring write lock.
+	fs.rwLock.Lock()
+	delete(fs.multiparts.ActiveSession, uploadID)
+	if err := saveMultipartsSession(*fs.multiparts); err != nil {
+		fs.rwLock.Unlock()
+		objectWriter.CloseAndPurge()
+		return ObjectInfo{}, err.Trace(partPathPrefix)
+	}
+	if e = objectWriter.Close(); e != nil {
+		fs.rwLock.Unlock()
+		return ObjectInfo{}, probe.NewError(e)
+	}
+	fs.rwLock.Unlock()
+
+	// Send stat again to get object metadata.
+	st, e := os.Stat(objectPath)
+	if e != nil {
+		return ObjectInfo{}, probe.NewError(e)
+	}
+
+	contentType := "application/octet-stream"
+	if objectExt := filepath.Ext(objectPath); objectExt != "" {
+		content, ok := mimedb.DB[strings.ToLower(strings.TrimPrefix(objectExt, "."))]
+		if ok {
+			contentType = content.ContentType
+		}
+	}
+	newObject := ObjectInfo{
+		Bucket:       bucket,
+		Name:         object,
+		ModifiedTime: st.ModTime(),
+		Size:         st.Size(),
+		ContentType:  contentType,
+		MD5Sum:       s3MD5,
+	}
+	return newObject, nil
+}
+
+// ListObjectParts - list parts from incomplete multipart session for a given ObjectResourcesMetadata
+func (fs Filesystem) ListObjectParts(bucket, object string, resources ObjectResourcesMetadata) (ObjectResourcesMetadata, *probe.Error) {
+	// Check bucket name is valid.
+	if !IsValidBucketName(bucket) {
+		return ObjectResourcesMetadata{}, probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+
+	// Verify object path legal.
+	if !IsValidObjectName(object) {
+		return ObjectResourcesMetadata{}, probe.NewError(ObjectNameInvalid{Bucket: bucket, Object: object})
+	}
+
+	// Save upload id.
+	uploadID := resources.UploadID
+
+	// Verify if upload id is valid for incoming object.
+	if !fs.isValidUploadID(object, uploadID) {
+		return ObjectResourcesMetadata{}, probe.NewError(InvalidUploadID{UploadID: uploadID})
+	}
+
+	objectResourcesMetadata := resources
+	objectResourcesMetadata.Bucket = bucket
+	objectResourcesMetadata.Object = object
+	var startPartNumber int
+	switch {
+	case objectResourcesMetadata.PartNumberMarker == 0:
+		startPartNumber = 1
+	default:
+		startPartNumber = objectResourcesMetadata.PartNumberMarker
+	}
+
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketPath := filepath.Join(fs.path, bucket)
+	if _, e := os.Stat(bucketPath); e != nil {
+		// Check bucket exists.
+		if os.IsNotExist(e) {
+			return ObjectResourcesMetadata{}, probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		return ObjectResourcesMetadata{}, probe.NewError(e)
+	}
+
+	// Critical region requiring read lock.
+	fs.rwLock.RLock()
+	deserializedMultipartSession, ok := fs.multiparts.ActiveSession[uploadID]
+	fs.rwLock.RUnlock()
+	if !ok {
+		return ObjectResourcesMetadata{}, probe.NewError(InvalidUploadID{UploadID: resources.UploadID})
+	}
+	var parts []PartMetadata
+	for i := startPartNumber; i <= deserializedMultipartSession.TotalParts; i++ {
+		if len(parts) > objectResourcesMetadata.MaxParts {
+			sort.Sort(partNumber(parts))
+			objectResourcesMetadata.IsTruncated = true
+			objectResourcesMetadata.Part = parts
+			objectResourcesMetadata.NextPartNumberMarker = i
+			return objectResourcesMetadata, nil
+		}
+		parts = append(parts, deserializedMultipartSession.Parts[i-1])
+	}
+	sort.Sort(partNumber(parts))
+	objectResourcesMetadata.Part = parts
+	return objectResourcesMetadata, nil
+}
+
+// AbortMultipartUpload - abort an incomplete multipart session
+func (fs Filesystem) AbortMultipartUpload(bucket, object, uploadID string) *probe.Error {
+	// Check bucket name valid.
+	if !IsValidBucketName(bucket) {
+		return probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+
+	// Verify object path legal.
+	if !IsValidObjectName(object) {
+		return probe.NewError(ObjectNameInvalid{Bucket: bucket, Object: object})
+	}
+
+	if !fs.isValidUploadID(object, uploadID) {
+		return probe.NewError(InvalidUploadID{UploadID: uploadID})
+	}
+
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketPath := filepath.Join(fs.path, bucket)
+	if _, e := os.Stat(bucketPath); e != nil {
+		// Check bucket exists.
+		if os.IsNotExist(e) {
+			return probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		return probe.NewError(e)
+	}
+
+	objectPath := filepath.Join(bucketPath, object)
+	partPathPrefix := objectPath + uploadID
+
+	// Critical region requiring read lock.
+	fs.rwLock.RLock()
+	savedParts := fs.multiparts.ActiveSession[uploadID].Parts
+	fs.rwLock.RUnlock()
+
+	// Remove all parts.
+	if err := removeParts(partPathPrefix, savedParts); err != nil {
+		return err.Trace(partPathPrefix)
+	}
+
+	// Critical region requiring write lock.
+	fs.rwLock.Lock()
+	defer fs.rwLock.Unlock()
+
+	delete(fs.multiparts.ActiveSession, uploadID)
+	if err := saveMultipartsSession(*fs.multiparts); err != nil {
+		return err.Trace(partPathPrefix)
+	}
+	return nil
+}