minio/cmd/xl-v1-multipart.go

/*
 * Minio Cloud Storage, (C) 2016, 2017 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 cmd

import (
	"context"
	"encoding/hex"
	"fmt"
	"path"
	"sort"
	"strings"
	"sync"
	"time"

	"github.com/minio/minio/pkg/errors"
	"github.com/minio/minio/pkg/hash"
	"github.com/minio/minio/pkg/mimedb"
)

func (xl xlObjects) getUploadIDDir(bucket, object, uploadID string) string {
	return pathJoin(xl.getMultipartSHADir(bucket, object), uploadID)
}

func (xl xlObjects) getMultipartSHADir(bucket, object string) string {
	return getSHA256Hash([]byte(pathJoin(bucket, object)))
}

// isUploadIDExists - verify if a given uploadID exists and is valid.
func (xl xlObjects) isUploadIDExists(bucket, object, uploadID string) bool {
	return xl.isObject(minioMetaMultipartBucket, xl.getUploadIDDir(bucket, object, uploadID))
}

// Removes part given by partName belonging to a mulitpart upload from minioMetaBucket
func (xl xlObjects) removeObjectPart(bucket, object, uploadID, partName string) {
	curpartPath := path.Join(bucket, object, uploadID, partName)
	wg := sync.WaitGroup{}
	for i, disk := range xl.getDisks() {
		if disk == nil {
			continue
		}
		wg.Add(1)
		go func(index int, disk StorageAPI) {
			defer wg.Done()
			// Ignoring failure to remove parts that weren't present in CompleteMultipartUpload
			// requests. xl.json is the authoritative source of truth on which parts constitute
			// the object. The presence of parts that don't belong in the object doesn't affect correctness.
			_ = disk.DeleteFile(minioMetaMultipartBucket, curpartPath)
		}(i, disk)
	}
	wg.Wait()
}

// statPart - returns fileInfo structure for a successful stat on part file.
func (xl xlObjects) statPart(bucket, object, uploadID, partName string) (fileInfo FileInfo, err error) {
	var ignoredErrs []error
	partNamePath := path.Join(xl.getUploadIDDir(bucket, object, uploadID), partName)
	for _, disk := range xl.getLoadBalancedDisks() {
		if disk == nil {
			ignoredErrs = append(ignoredErrs, errDiskNotFound)
			continue
		}
		fileInfo, err = disk.StatFile(minioMetaMultipartBucket, partNamePath)
		if err == nil {
			return fileInfo, nil
		}
		// For any reason disk was deleted or goes offline we continue to next disk.
		if errors.IsErrIgnored(err, objMetadataOpIgnoredErrs...) {
			ignoredErrs = append(ignoredErrs, err)
			continue
		}
		// Error is not ignored, return right here.
		return FileInfo{}, errors.Trace(err)
	}
	// If all errors were ignored, reduce to maximal occurrence
	// based on the read quorum.
	readQuorum := len(xl.getDisks()) / 2
	return FileInfo{}, reduceReadQuorumErrs(ignoredErrs, nil, readQuorum)
}

// commitXLMetadata - commit `xl.json` from source prefix to destination prefix in the given slice of disks.
func commitXLMetadata(disks []StorageAPI, srcBucket, srcPrefix, dstBucket, dstPrefix string, quorum int) ([]StorageAPI, error) {
	var wg = &sync.WaitGroup{}
	var mErrs = make([]error, len(disks))

	srcJSONFile := path.Join(srcPrefix, xlMetaJSONFile)
	dstJSONFile := path.Join(dstPrefix, xlMetaJSONFile)

	// Rename `xl.json` to all disks in parallel.
	for index, disk := range disks {
		if disk == nil {
			mErrs[index] = errors.Trace(errDiskNotFound)
			continue
		}
		wg.Add(1)
		// Rename `xl.json` in a routine.
		go func(index int, disk StorageAPI) {
			defer wg.Done()
			// Delete any dangling directories.
			defer disk.DeleteFile(srcBucket, srcPrefix)

			// Renames `xl.json` from source prefix to destination prefix.
			rErr := disk.RenameFile(srcBucket, srcJSONFile, dstBucket, dstJSONFile)
			if rErr != nil {
				mErrs[index] = errors.Trace(rErr)
				return
			}
			mErrs[index] = nil
		}(index, disk)
	}
	// Wait for all the routines.
	wg.Wait()

	err := reduceWriteQuorumErrs(mErrs, objectOpIgnoredErrs, quorum)
	if errors.Cause(err) == errXLWriteQuorum {
		// Delete all `xl.json` successfully renamed.
		deleteAllXLMetadata(disks, dstBucket, dstPrefix, mErrs)
	}
	return evalDisks(disks, mErrs), err
}

// ListMultipartUploads - lists all the pending multipart
// uploads for a particular object in a bucket.
//
// Implements minimal S3 compatible ListMultipartUploads API. We do
// not support prefix based listing, this is a deliberate attempt
// towards simplification of multipart APIs.
// The resulting ListMultipartsInfo structure is unmarshalled directly as XML.
func (xl xlObjects) ListMultipartUploads(ctx context.Context, bucket, object, keyMarker, uploadIDMarker, delimiter string, maxUploads int) (result ListMultipartsInfo, e error) {
	if err := checkListMultipartArgs(bucket, object, keyMarker, uploadIDMarker, delimiter, xl); err != nil {
		return result, err
	}

	result.MaxUploads = maxUploads
	result.KeyMarker = keyMarker
	result.Prefix = object
	result.Delimiter = delimiter

	for _, disk := range xl.getLoadBalancedDisks() {
		if disk == nil {
			continue
		}
		uploadIDs, err := disk.ListDir(minioMetaMultipartBucket, xl.getMultipartSHADir(bucket, object))
		if err != nil {
			if err == errFileNotFound {
				return result, nil
			}
			return result, errors.Trace(err)
		}
		for i := range uploadIDs {
			uploadIDs[i] = strings.TrimSuffix(uploadIDs[i], slashSeparator)
		}
		sort.Strings(uploadIDs)
		for _, uploadID := range uploadIDs {
			if len(result.Uploads) == maxUploads {
				break
			}
			result.Uploads = append(result.Uploads, MultipartInfo{Object: object, UploadID: uploadID})
		}
		break
	}

	return result, nil
}

// newMultipartUpload - wrapper for initializing a new multipart
// request; returns a unique upload id.
//
// Internally this function creates 'uploads.json' associated for the
// incoming object at
// '.minio.sys/multipart/bucket/object/uploads.json' on all the
// disks. `uploads.json` carries metadata regarding on-going multipart
// operation(s) on the object.
func (xl xlObjects) newMultipartUpload(bucket string, object string, meta map[string]string) (string, error) {

	dataBlocks, parityBlocks := getRedundancyCount(meta[amzStorageClass], len(xl.getDisks()))

	xlMeta := newXLMetaV1(object, dataBlocks, parityBlocks)

	// we now know the number of blocks this object needs for data and parity.
	// establish the writeQuorum using this data
	writeQuorum := dataBlocks + 1

	// If not set default to "application/octet-stream"
	if meta["content-type"] == "" {
		contentType := "application/octet-stream"
		if objectExt := path.Ext(object); objectExt != "" {
			content, ok := mimedb.DB[strings.ToLower(strings.TrimPrefix(objectExt, "."))]
			if ok {
				contentType = content.ContentType
			}
		}
		meta["content-type"] = contentType
	}
	xlMeta.Stat.ModTime = UTCNow()
	xlMeta.Meta = meta

	uploadID := mustGetUUID()
	uploadIDPath := xl.getUploadIDDir(bucket, object, uploadID)
	tempUploadIDPath := uploadID

	// Write updated `xl.json` to all disks.
	disks, err := writeSameXLMetadata(xl.getDisks(), minioMetaTmpBucket, tempUploadIDPath, xlMeta, writeQuorum)
	if err != nil {
		return "", toObjectErr(err, minioMetaTmpBucket, tempUploadIDPath)
	}
	// delete the tmp path later in case we fail to rename (ignore
	// returned errors) - this will be a no-op in case of a rename
	// success.
	defer xl.deleteObject(minioMetaTmpBucket, tempUploadIDPath)

	// Attempt to rename temp upload object to actual upload path object
	_, rErr := renameObject(disks, minioMetaTmpBucket, tempUploadIDPath, minioMetaMultipartBucket, uploadIDPath, writeQuorum)
	if rErr != nil {
		return "", toObjectErr(rErr, minioMetaMultipartBucket, uploadIDPath)
	}

	// Return success.
	return uploadID, nil
}

// NewMultipartUpload - initialize a new multipart upload, returns a
// unique id. The unique id returned here is of UUID form, for each
// subsequent request each UUID is unique.
//
// Implements S3 compatible initiate multipart API.
func (xl xlObjects) NewMultipartUpload(ctx context.Context, bucket, object string, meta map[string]string) (string, error) {
	if err := checkNewMultipartArgs(bucket, object, xl); err != nil {
		return "", err
	}
	// No metadata is set, allocate a new one.
	if meta == nil {
		meta = make(map[string]string)
	}
	return xl.newMultipartUpload(bucket, object, meta)
}

// CopyObjectPart - reads incoming stream and internally erasure codes
// them. This call is similar to put object part operation but the source
// data is read from an existing object.
//
// Implements S3 compatible Upload Part Copy API.
func (xl xlObjects) CopyObjectPart(ctx context.Context, srcBucket, srcObject, dstBucket, dstObject, uploadID string, partID int, startOffset int64, length int64, srcInfo ObjectInfo) (pi PartInfo, e error) {
	// Hold read locks on source object only if we are
	// going to read data from source object.
	objectSRLock := xl.nsMutex.NewNSLock(srcBucket, srcObject)
	if err := objectSRLock.GetRLock(globalObjectTimeout); err != nil {
		return pi, err
	}
	defer objectSRLock.RUnlock()

	if err := checkNewMultipartArgs(srcBucket, srcObject, xl); err != nil {
		return pi, err
	}

	go func() {
		if gerr := xl.getObject(srcBucket, srcObject, startOffset, length, srcInfo.Writer, srcInfo.ETag); gerr != nil {
			if gerr = srcInfo.Writer.Close(); gerr != nil {
				errorIf(gerr, "Unable to read %s of the object `%s/%s`.", srcBucket, srcObject)
			}
			return
		}
		// Close writer explicitly signalling we wrote all data.
		if gerr := srcInfo.Writer.Close(); gerr != nil {
			errorIf(gerr, "Unable to read %s of the object `%s/%s`.", srcBucket, srcObject)
			return
		}
	}()

	partInfo, err := xl.PutObjectPart(ctx, dstBucket, dstObject, uploadID, partID, srcInfo.Reader)
	if err != nil {
		return pi, toObjectErr(err, dstBucket, dstObject)
	}

	// Success.
	return partInfo, nil
}

// PutObjectPart - reads incoming stream and internally erasure codes
// them. This call is similar to single put operation but it is part
// of the multipart transaction.
//
// Implements S3 compatible Upload Part API.
func (xl xlObjects) PutObjectPart(ctx context.Context, bucket, object, uploadID string, partID int, data *hash.Reader) (pi PartInfo, e error) {
	if err := checkPutObjectPartArgs(bucket, object, xl); err != nil {
		return pi, err
	}

	// Validate input data size and it can never be less than zero.
	if data.Size() < 0 {
		return pi, toObjectErr(errors.Trace(errInvalidArgument))
	}

	var partsMetadata []xlMetaV1
	var errs []error
	uploadIDPath := xl.getUploadIDDir(bucket, object, uploadID)

	// pre-check upload id lock.
	preUploadIDLock := xl.nsMutex.NewNSLock(minioMetaMultipartBucket, uploadIDPath)
	if err := preUploadIDLock.GetRLock(globalOperationTimeout); err != nil {
		return pi, err
	}

	// Validates if upload ID exists.
	if !xl.isUploadIDExists(bucket, object, uploadID) {
		preUploadIDLock.RUnlock()
		return pi, errors.Trace(InvalidUploadID{UploadID: uploadID})
	}

	// Read metadata associated with the object from all disks.
	partsMetadata, errs = readAllXLMetadata(xl.getDisks(), minioMetaMultipartBucket,
		uploadIDPath)

	// get Quorum for this object
	_, writeQuorum, err := objectQuorumFromMeta(xl, partsMetadata, errs)
	if err != nil {
		return pi, toObjectErr(err, bucket, object)
	}

	reducedErr := reduceWriteQuorumErrs(errs, objectOpIgnoredErrs, writeQuorum)
	if errors.Cause(reducedErr) == errXLWriteQuorum {
		preUploadIDLock.RUnlock()
		return pi, toObjectErr(reducedErr, bucket, object)
	}
	preUploadIDLock.RUnlock()

	// List all online disks.
	onlineDisks, modTime := listOnlineDisks(xl.getDisks(), partsMetadata, errs)

	// Pick one from the first valid metadata.
	xlMeta, err := pickValidXLMeta(partsMetadata, modTime)
	if err != nil {
		return pi, err
	}

	onlineDisks = shuffleDisks(onlineDisks, xlMeta.Erasure.Distribution)

	// Need a unique name for the part being written in minioMetaBucket to
	// accommodate concurrent PutObjectPart requests

	partSuffix := fmt.Sprintf("part.%d", partID)
	tmpPart := mustGetUUID()
	tmpPartPath := path.Join(tmpPart, partSuffix)

	// Delete the temporary object part. If PutObjectPart succeeds there would be nothing to delete.
	defer xl.deleteObject(minioMetaTmpBucket, tmpPart)
	if data.Size() > 0 {
		if pErr := xl.prepareFile(minioMetaTmpBucket, tmpPartPath, data.Size(), onlineDisks, xlMeta.Erasure.BlockSize, xlMeta.Erasure.DataBlocks, writeQuorum); err != nil {
			return pi, toObjectErr(pErr, bucket, object)

		}
	}

	storage, err := NewErasureStorage(onlineDisks, xlMeta.Erasure.DataBlocks, xlMeta.Erasure.ParityBlocks, xlMeta.Erasure.BlockSize)
	if err != nil {
		return pi, toObjectErr(err, bucket, object)
	}

	// Fetch buffer for I/O, returns from the pool if not allocates a new one and returns.
	buffer := xl.bp.Get()
	defer xl.bp.Put(buffer)

	file, err := storage.CreateFile(data, minioMetaTmpBucket, tmpPartPath, buffer, DefaultBitrotAlgorithm, writeQuorum)
	if err != nil {
		return pi, toObjectErr(err, bucket, object)
	}

	// Should return IncompleteBody{} error when reader has fewer bytes
	// than specified in request header.
	if file.Size < data.Size() {
		return pi, errors.Trace(IncompleteBody{})
	}

	// post-upload check (write) lock
	postUploadIDLock := xl.nsMutex.NewNSLock(minioMetaMultipartBucket, uploadIDPath)
	if err = postUploadIDLock.GetLock(globalOperationTimeout); err != nil {
		return pi, err
	}
	defer postUploadIDLock.Unlock()

	// Validate again if upload ID still exists.
	if !xl.isUploadIDExists(bucket, object, uploadID) {
		return pi, errors.Trace(InvalidUploadID{UploadID: uploadID})
	}

	// Rename temporary part file to its final location.
	partPath := path.Join(uploadIDPath, partSuffix)
	onlineDisks, err = renamePart(onlineDisks, minioMetaTmpBucket, tmpPartPath, minioMetaMultipartBucket, partPath, writeQuorum)
	if err != nil {
		return pi, toObjectErr(err, minioMetaMultipartBucket, partPath)
	}

	// Read metadata again because it might be updated with parallel upload of another part.
	partsMetadata, errs = readAllXLMetadata(onlineDisks, minioMetaMultipartBucket, uploadIDPath)
	reducedErr = reduceWriteQuorumErrs(errs, objectOpIgnoredErrs, writeQuorum)
	if errors.Cause(reducedErr) == errXLWriteQuorum {
		return pi, toObjectErr(reducedErr, bucket, object)
	}

	// Get current highest version based on re-read partsMetadata.
	onlineDisks, modTime = listOnlineDisks(onlineDisks, partsMetadata, errs)

	// Pick one from the first valid metadata.
	xlMeta, err = pickValidXLMeta(partsMetadata, modTime)
	if err != nil {
		return pi, err
	}

	// Once part is successfully committed, proceed with updating XL metadata.
	xlMeta.Stat.ModTime = UTCNow()

	md5hex := hex.EncodeToString(data.MD5Current())

	// Add the current part.
	xlMeta.AddObjectPart(partID, partSuffix, md5hex, file.Size)

	for i, disk := range onlineDisks {
		if disk == OfflineDisk {
			continue
		}
		partsMetadata[i].Parts = xlMeta.Parts
		partsMetadata[i].Erasure.AddChecksumInfo(ChecksumInfo{partSuffix, file.Algorithm, file.Checksums[i]})
	}

	// Write all the checksum metadata.
	newUUID := mustGetUUID()
	tempXLMetaPath := newUUID

	// Writes a unique `xl.json` each disk carrying new checksum related information.
	if onlineDisks, err = writeUniqueXLMetadata(onlineDisks, minioMetaTmpBucket, tempXLMetaPath, partsMetadata, writeQuorum); err != nil {
		return pi, toObjectErr(err, minioMetaTmpBucket, tempXLMetaPath)
	}

	if _, err = commitXLMetadata(onlineDisks, minioMetaTmpBucket, tempXLMetaPath, minioMetaMultipartBucket, uploadIDPath, writeQuorum); err != nil {
		return pi, toObjectErr(err, minioMetaMultipartBucket, uploadIDPath)
	}

	fi, err := xl.statPart(bucket, object, uploadID, partSuffix)
	if err != nil {
		return pi, toObjectErr(err, minioMetaMultipartBucket, partSuffix)
	}

	// Return success.
	return PartInfo{
		PartNumber:   partID,
		LastModified: fi.ModTime,
		ETag:         md5hex,
		Size:         fi.Size,
	}, nil
}

// ListObjectParts - lists all previously uploaded parts for a given
// object and uploadID.  Takes additional input of part-number-marker
// to indicate where the listing should begin from.
//
// Implements S3 compatible ListObjectParts API. The resulting
// ListPartsInfo structure is marshalled directly into XML and
// replied back to the client.
func (xl xlObjects) ListObjectParts(ctx context.Context, bucket, object, uploadID string, partNumberMarker, maxParts int) (result ListPartsInfo, e error) {
	if err := checkListPartsArgs(bucket, object, xl); err != nil {
		return result, err
	}
	// Hold lock so that there is no competing
	// abort-multipart-upload or complete-multipart-upload.
	uploadIDLock := xl.nsMutex.NewNSLock(minioMetaMultipartBucket,
		xl.getUploadIDDir(bucket, object, uploadID))
	if err := uploadIDLock.GetLock(globalListingTimeout); err != nil {
		return result, err
	}
	defer uploadIDLock.Unlock()

	if !xl.isUploadIDExists(bucket, object, uploadID) {
		return result, errors.Trace(InvalidUploadID{UploadID: uploadID})
	}

	uploadIDPath := xl.getUploadIDDir(bucket, object, uploadID)

	xlParts, xlMeta, err := xl.readXLMetaParts(minioMetaMultipartBucket, uploadIDPath)
	if err != nil {
		return result, toObjectErr(err, minioMetaMultipartBucket, uploadIDPath)
	}

	// Populate the result stub.
	result.Bucket = bucket
	result.Object = object
	result.UploadID = uploadID
	result.MaxParts = maxParts
	result.PartNumberMarker = partNumberMarker
	result.UserDefined = xlMeta

	// For empty number of parts or maxParts as zero, return right here.
	if len(xlParts) == 0 || maxParts == 0 {
		return result, nil
	}

	// Limit output to maxPartsList.
	if maxParts > maxPartsList {
		maxParts = maxPartsList
	}

	// Only parts with higher part numbers will be listed.
	partIdx := objectPartIndex(xlParts, partNumberMarker)
	parts := xlParts
	if partIdx != -1 {
		parts = xlParts[partIdx+1:]
	}
	count := maxParts
	for _, part := range parts {
		var fi FileInfo
		fi, err = xl.statPart(bucket, object, uploadID, part.Name)
		if err != nil {
			return result, toObjectErr(err, minioMetaBucket, path.Join(uploadID, part.Name))
		}
		result.Parts = append(result.Parts, PartInfo{
			PartNumber:   part.Number,
			ETag:         part.ETag,
			LastModified: fi.ModTime,
			Size:         part.Size,
		})
		count--
		if count == 0 {
			break
		}
	}
	// If listed entries are more than maxParts, we set IsTruncated as true.
	if len(parts) > len(result.Parts) {
		result.IsTruncated = true
		// Make sure to fill next part number marker if IsTruncated is
		// true for subsequent listing.
		nextPartNumberMarker := result.Parts[len(result.Parts)-1].PartNumber
		result.NextPartNumberMarker = nextPartNumberMarker
	}
	return result, nil
}

// CompleteMultipartUpload - completes an ongoing multipart
// transaction after receiving all the parts indicated by the client.
// Returns an md5sum calculated by concatenating all the individual
// md5sums of all the parts.
//
// Implements S3 compatible Complete multipart API.
func (xl xlObjects) CompleteMultipartUpload(ctx context.Context, bucket string, object string, uploadID string, parts []CompletePart) (oi ObjectInfo, e error) {
	if err := checkCompleteMultipartArgs(bucket, object, xl); err != nil {
		return oi, err
	}
	// Hold write lock on the object.
	destLock := xl.nsMutex.NewNSLock(bucket, object)
	if err := destLock.GetLock(globalObjectTimeout); err != nil {
		return oi, err
	}
	defer destLock.Unlock()

	uploadIDPath := xl.getUploadIDDir(bucket, object, uploadID)

	// Hold lock so that
	//
	// 1) no one aborts this multipart upload
	//
	// 2) no one does a parallel complete-multipart-upload on this
	// multipart upload
	uploadIDLock := xl.nsMutex.NewNSLock(minioMetaMultipartBucket, uploadIDPath)
	if err := uploadIDLock.GetLock(globalOperationTimeout); err != nil {
		return oi, err
	}
	defer uploadIDLock.Unlock()

	if !xl.isUploadIDExists(bucket, object, uploadID) {
		return oi, errors.Trace(InvalidUploadID{UploadID: uploadID})
	}

	// Check if an object is present as one of the parent dir.
	// -- FIXME. (needs a new kind of lock).
	if xl.parentDirIsObject(bucket, path.Dir(object)) {
		return oi, toObjectErr(errors.Trace(errFileAccessDenied), bucket, object)
	}

	// Calculate s3 compatible md5sum for complete multipart.
	s3MD5, err := getCompleteMultipartMD5(parts)
	if err != nil {
		return oi, err
	}

	// Read metadata associated with the object from all disks.
	partsMetadata, errs := readAllXLMetadata(xl.getDisks(), minioMetaMultipartBucket, uploadIDPath)

	// get Quorum for this object
	_, writeQuorum, err := objectQuorumFromMeta(xl, partsMetadata, errs)
	if err != nil {
		return oi, toObjectErr(err, bucket, object)
	}

	reducedErr := reduceWriteQuorumErrs(errs, objectOpIgnoredErrs, writeQuorum)
	if errors.Cause(reducedErr) == errXLWriteQuorum {
		return oi, toObjectErr(reducedErr, bucket, object)
	}

	onlineDisks, modTime := listOnlineDisks(xl.getDisks(), partsMetadata, errs)

	// Calculate full object size.
	var objectSize int64

	// Pick one from the first valid metadata.
	xlMeta, err := pickValidXLMeta(partsMetadata, modTime)
	if err != nil {
		return oi, err
	}

	// Order online disks in accordance with distribution order.
	onlineDisks = shuffleDisks(onlineDisks, xlMeta.Erasure.Distribution)

	// Order parts metadata in accordance with distribution order.
	partsMetadata = shufflePartsMetadata(partsMetadata, xlMeta.Erasure.Distribution)

	// Save current xl meta for validation.
	var currentXLMeta = xlMeta

	// Allocate parts similar to incoming slice.
	xlMeta.Parts = make([]objectPartInfo, len(parts))

	// Validate each part and then commit to disk.
	for i, part := range parts {
		partIdx := objectPartIndex(currentXLMeta.Parts, part.PartNumber)
		// All parts should have same part number.
		if partIdx == -1 {
			return oi, errors.Trace(InvalidPart{})
		}

		// All parts should have same ETag as previously generated.
		if currentXLMeta.Parts[partIdx].ETag != part.ETag {
			return oi, errors.Trace(InvalidPart{})
		}

		// All parts except the last part has to be atleast 5MB.
		if (i < len(parts)-1) && !isMinAllowedPartSize(currentXLMeta.Parts[partIdx].Size) {
			return oi, errors.Trace(PartTooSmall{
				PartNumber: part.PartNumber,
				PartSize:   currentXLMeta.Parts[partIdx].Size,
				PartETag:   part.ETag,
			})
		}

		// Last part could have been uploaded as 0bytes, do not need
		// to save it in final `xl.json`.
		if (i == len(parts)-1) && currentXLMeta.Parts[partIdx].Size == 0 {
			xlMeta.Parts = xlMeta.Parts[:i] // Skip the part.
			continue
		}

		// Save for total object size.
		objectSize += currentXLMeta.Parts[partIdx].Size

		// Add incoming parts.
		xlMeta.Parts[i] = objectPartInfo{
			Number: part.PartNumber,
			ETag:   part.ETag,
			Size:   currentXLMeta.Parts[partIdx].Size,
			Name:   fmt.Sprintf("part.%d", part.PartNumber),
		}
	}

	// Save the final object size and modtime.
	xlMeta.Stat.Size = objectSize
	xlMeta.Stat.ModTime = UTCNow()

	// Save successfully calculated md5sum.
	xlMeta.Meta["etag"] = s3MD5

	tempUploadIDPath := uploadID

	// Update all xl metadata, make sure to not modify fields like
	// checksum which are different on each disks.
	for index := range partsMetadata {
		partsMetadata[index].Stat = xlMeta.Stat
		partsMetadata[index].Meta = xlMeta.Meta
		partsMetadata[index].Parts = xlMeta.Parts
	}

	// Write unique `xl.json` for each disk.
	if onlineDisks, err = writeUniqueXLMetadata(onlineDisks, minioMetaTmpBucket, tempUploadIDPath, partsMetadata, writeQuorum); err != nil {
		return oi, toObjectErr(err, minioMetaTmpBucket, tempUploadIDPath)
	}

	var rErr error
	onlineDisks, rErr = commitXLMetadata(onlineDisks, minioMetaTmpBucket, tempUploadIDPath, minioMetaMultipartBucket, uploadIDPath, writeQuorum)
	if rErr != nil {
		return oi, toObjectErr(rErr, minioMetaMultipartBucket, uploadIDPath)
	}

	if xl.isObject(bucket, object) {
		// Rename if an object already exists to temporary location.
		newUniqueID := mustGetUUID()

		// Delete success renamed object.
		defer xl.deleteObject(minioMetaTmpBucket, newUniqueID)

		// NOTE: Do not use online disks slice here.
		// The reason is that existing object should be purged
		// regardless of `xl.json` status and rolled back in case of errors.
		_, err = renameObject(xl.getDisks(), bucket, object, minioMetaTmpBucket, newUniqueID, writeQuorum)
		if err != nil {
			return oi, toObjectErr(err, bucket, object)
		}
	}

	// Remove parts that weren't present in CompleteMultipartUpload request.
	for _, curpart := range currentXLMeta.Parts {
		if objectPartIndex(xlMeta.Parts, curpart.Number) == -1 {
			// Delete the missing part files. e.g,
			// Request 1: NewMultipart
			// Request 2: PutObjectPart 1
			// Request 3: PutObjectPart 2
			// Request 4: CompleteMultipartUpload --part 2
			// N.B. 1st part is not present. This part should be removed from the storage.
			xl.removeObjectPart(bucket, object, uploadID, curpart.Name)
		}
	}

	// Rename the multipart object to final location.
	if _, err = renameObject(onlineDisks, minioMetaMultipartBucket, uploadIDPath, bucket, object, writeQuorum); err != nil {
		return oi, toObjectErr(err, bucket, object)
	}

	// Success, return object info.
	return xlMeta.ToObjectInfo(bucket, object), nil
}

// Wrapper which removes all the uploaded parts.
func (xl xlObjects) cleanupUploadedParts(uploadIDPath string, writeQuorum int) error {
	var errs = make([]error, len(xl.getDisks()))
	var wg = &sync.WaitGroup{}

	// Cleanup uploadID for all disks.
	for index, disk := range xl.getDisks() {
		if disk == nil {
			errs[index] = errors.Trace(errDiskNotFound)
			continue
		}
		wg.Add(1)
		// Cleanup each uploadID in a routine.
		go func(index int, disk StorageAPI) {
			defer wg.Done()
			err := cleanupDir(disk, minioMetaMultipartBucket, uploadIDPath)
			if err != nil {
				errs[index] = err
			}
		}(index, disk)
	}

	// Wait for all the cleanups to finish.
	wg.Wait()

	return reduceWriteQuorumErrs(errs, objectOpIgnoredErrs, writeQuorum)
}

// AbortMultipartUpload - aborts an ongoing multipart operation
// signified by the input uploadID. This is an atomic operation
// doesn't require clients to initiate multiple such requests.
//
// All parts are purged from all disks and reference to the uploadID
// would be removed from the system, rollback is not possible on this
// operation.
//
// Implements S3 compatible Abort multipart API, slight difference is
// that this is an atomic idempotent operation. Subsequent calls have
// no affect and further requests to the same uploadID would not be honored.
func (xl xlObjects) AbortMultipartUpload(ctx context.Context, bucket, object, uploadID string) error {
	if err := checkAbortMultipartArgs(bucket, object, xl); err != nil {
		return err
	}
	// Construct uploadIDPath.
	uploadIDPath := xl.getUploadIDDir(bucket, object, uploadID)
	// Hold lock so that there is no competing
	// complete-multipart-upload or put-object-part.
	uploadIDLock := xl.nsMutex.NewNSLock(minioMetaMultipartBucket, uploadIDPath)
	if err := uploadIDLock.GetLock(globalOperationTimeout); err != nil {
		return err
	}
	defer uploadIDLock.Unlock()

	if !xl.isUploadIDExists(bucket, object, uploadID) {
		return errors.Trace(InvalidUploadID{UploadID: uploadID})
	}

	// Read metadata associated with the object from all disks.
	partsMetadata, errs := readAllXLMetadata(xl.getDisks(), minioMetaMultipartBucket, uploadIDPath)

	// get Quorum for this object
	_, writeQuorum, err := objectQuorumFromMeta(xl, partsMetadata, errs)
	if err != nil {
		return toObjectErr(err, bucket, object)
	}

	// Cleanup all uploaded parts.
	if err = xl.cleanupUploadedParts(uploadIDPath, writeQuorum); err != nil {
		return toObjectErr(err, bucket, object)
	}

	// Successfully purged.
	return nil
}

// Clean-up the old multipart uploads. Should be run in a Go routine.
func (xl xlObjects) cleanupStaleMultipartUploads(cleanupInterval, expiry time.Duration, doneCh chan struct{}) {
	ticker := time.NewTicker(cleanupInterval)

	for {
		select {
		case <-doneCh:
			ticker.Stop()
			return
		case <-ticker.C:
			var disk StorageAPI
			for _, d := range xl.getLoadBalancedDisks() {
				if d != nil {
					disk = d
					break
				}
			}
			if disk == nil {
				continue
			}
			xl.cleanupStaleMultipartUploadsOnDisk(disk, expiry)
		}
	}
}

// Remove the old multipart uploads on the given disk.
func (xl xlObjects) cleanupStaleMultipartUploadsOnDisk(disk StorageAPI, expiry time.Duration) {
	now := time.Now()
	shaDirs, err := disk.ListDir(minioMetaMultipartBucket, "")
	if err != nil {
		return
	}
	for _, shaDir := range shaDirs {
		uploadIDDirs, err := disk.ListDir(minioMetaMultipartBucket, shaDir)
		if err != nil {
			continue
		}
		for _, uploadIDDir := range uploadIDDirs {
			uploadIDPath := pathJoin(shaDir, uploadIDDir)
			fi, err := disk.StatFile(minioMetaMultipartBucket, pathJoin(uploadIDPath, xlMetaJSONFile))
			if err != nil {
				continue
			}
			if now.Sub(fi.ModTime) > expiry {
				// Quorum value will need to be figured out using readAllXLMetadata() and objectQuorumFromMeta()
				// But we can avoid these calls as we do not care if xl.cleanupUploadedParts() meets quorum
				// when it removes files. We igore the error message from xl.cleanupUploadedParts() as we can't
				// return it to any client. Hence we set quorum to 0.
				quorum := 0
				xl.cleanupUploadedParts(uploadIDPath, quorum)
			}
		}
	}
}