Storagedriver: GCS: implement resumable uploads

Signed-off-by: Arthur Baars <arthur@semmle.com>

docs/storage/driver/gcs/gcs.go

@@ -7,11 +7,8 @@
 // Because gcs is a key, value store the Stat call does not support last modification
 // time for directories (directories are an abstraction for key, value stores)
 //
-// Keep in mind that gcs guarantees only eventual consistency, so do not assume
+// Note that the contents of incomplete uploads are not accessible even though
-// that a successful write will mean immediate access to the data written (although
+// Stat returns their length
-// in most regions a new object put has guaranteed read after write). The only true
-// guarantee is that once you call Stat and receive a certain file size, that much of
-// the file is already accessible.
 //
 // +build include_gcs
 
@@ -25,7 +22,9 @@ import (
 	"math/rand"
 	"net/http"
 	"net/url"
+	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
@@ -34,7 +33,6 @@ import (
 	"golang.org/x/oauth2/google"
 	"golang.org/x/oauth2/jwt"
 	"google.golang.org/api/googleapi"
-	storageapi "google.golang.org/api/storage/v1"
 	"google.golang.org/cloud"
 	"google.golang.org/cloud/storage"
 
@@ -46,8 +44,18 @@ import (
 	"github.com/docker/distribution/registry/storage/driver/factory"
 )
 
-const driverName = "gcs"
+const (
-const dummyProjectID = "<unknown>"
+	driverName     = "gcs"
+	dummyProjectID = "<unknown>"
+
+	uploadSessionContentType = "application/x-docker-upload-session"
+	minChunkSize             = 256 * 1024
+	maxChunkSize             = 20 * minChunkSize
+
+	maxTries = 5
+)
+
+var rangeHeader = regexp.MustCompile(`^bytes=([0-9])+-([0-9]+)$`)
 
 // driverParameters is a struct that encapsulates all of the driver parameters after all values have been set
 type driverParameters struct {
@@ -155,7 +163,17 @@ func (d *driver) Name() string {
 // GetContent retrieves the content stored at "path" as a []byte.
 // This should primarily be used for small objects.
 func (d *driver) GetContent(context ctx.Context, path string) ([]byte, error) {
-	rc, err := d.ReadStream(context, path, 0)
+	gcsContext := d.context(context)
+	name := d.pathToKey(path)
+	var rc io.ReadCloser
+	err := retry(func() error {
+		var err error
+		rc, err = storage.NewReader(gcsContext, d.bucket, name)
+		return err
+	})
+	if err == storage.ErrObjectNotExist {
+		return nil, storagedriver.PathNotFoundError{Path: path}
+	}
 	if err != nil {
 		return nil, err
 	}
@@ -171,44 +189,28 @@ func (d *driver) GetContent(context ctx.Context, path string) ([]byte, error) {
 // PutContent stores the []byte content at a location designated by "path".
 // This should primarily be used for small objects.
 func (d *driver) PutContent(context ctx.Context, path string, contents []byte) error {
+	return retry(func() error {
 		wc := storage.NewWriter(d.context(context), d.bucket, d.pathToKey(path))
 		wc.ContentType = "application/octet-stream"
-	defer wc.Close()
+		return putContentsClose(wc, contents)
-	_, err := wc.Write(contents)
+	})
-	return err
 }
 
-// ReadStream retrieves an io.ReadCloser for the content stored at "path"
+// Reader retrieves an io.ReadCloser for the content stored at "path"
 // with a given byte offset.
 // May be used to resume reading a stream by providing a nonzero offset.
-func (d *driver) ReadStream(context ctx.Context, path string, offset int64) (io.ReadCloser, error) {
+func (d *driver) Reader(context ctx.Context, path string, offset int64) (io.ReadCloser, error) {
-	name := d.pathToKey(path)
+	res, err := getObject(d.client, d.bucket, d.pathToKey(path), offset)
-
-	// copied from google.golang.org/cloud/storage#NewReader :
-	// to set the additional "Range" header
-	u := &url.URL{
-		Scheme: "https",
-		Host:   "storage.googleapis.com",
-		Path:   fmt.Sprintf("/%s/%s", d.bucket, name),
-	}
-	req, err := http.NewRequest("GET", u.String(), nil)
 	if err != nil {
-		return nil, err
+		if res != nil {
-	}
-	if offset > 0 {
-		req.Header.Set("Range", fmt.Sprintf("bytes=%v-", offset))
-	}
-	res, err := d.client.Do(req)
-	if err != nil {
-		return nil, err
-	}
 			if res.StatusCode == http.StatusNotFound {
 				res.Body.Close()
 				return nil, storagedriver.PathNotFoundError{Path: path}
 			}
+
 			if res.StatusCode == http.StatusRequestedRangeNotSatisfiable {
 				res.Body.Close()
-		obj, err := storageStatObject(d.context(context), d.bucket, name)
+				obj, err := storageStatObject(d.context(context), d.bucket, d.pathToKey(path))
 				if err != nil {
 					return nil, err
 				}
@@ -217,103 +219,278 @@ func (d *driver) ReadStream(context ctx.Context, path string, offset int64) (io.
 				}
 				return nil, storagedriver.InvalidOffsetError{Path: path, Offset: offset}
 			}
-	if res.StatusCode < 200 || res.StatusCode > 299 {
+		}
-		res.Body.Close()
+		return nil, err
-		return nil, fmt.Errorf("storage: can't read object %v/%v, status code: %v", d.bucket, name, res.Status)
+	}
+	if res.Header.Get("Content-Type") == uploadSessionContentType {
+		defer res.Body.Close()
+		return nil, storagedriver.PathNotFoundError{Path: path}
 	}
 	return res.Body, nil
 }
 
-// WriteStream stores the contents of the provided io.ReadCloser at a
+func getObject(client *http.Client, bucket string, name string, offset int64) (*http.Response, error) {
-// location designated by the given path.
+	// copied from google.golang.org/cloud/storage#NewReader :
-// May be used to resume writing a stream by providing a nonzero offset.
+	// to set the additional "Range" header
-// The offset must be no larger than the CurrentSize for this path.
+	u := &url.URL{
-func (d *driver) WriteStream(context ctx.Context, path string, offset int64, reader io.Reader) (totalRead int64, err error) {
+		Scheme: "https",
-	if offset < 0 {
+		Host:   "storage.googleapis.com",
-		return 0, storagedriver.InvalidOffsetError{Path: path, Offset: offset}
+		Path:   fmt.Sprintf("/%s/%s", bucket, name),
 	}
-
+	req, err := http.NewRequest("GET", u.String(), nil)
-	if offset == 0 {
-		return d.writeCompletely(context, path, 0, reader)
-	}
-
-	service, err := storageapi.New(d.client)
 	if err != nil {
-		return 0, err
+		return nil, err
 	}
-	objService := storageapi.NewObjectsService(service)
+	if offset > 0 {
-	var obj *storageapi.Object
+		req.Header.Set("Range", fmt.Sprintf("bytes=%v-", offset))
-	err = retry(5, func() error {
+	}
-		o, err := objService.Get(d.bucket, d.pathToKey(path)).Do()
+	var res *http.Response
-		obj = o
+	err = retry(func() error {
+		var err error
+		res, err = client.Do(req)
 		return err
 	})
-	//	obj, err := retry(5, objService.Get(d.bucket, d.pathToKey(path)).Do)
+	if err != nil {
+		return nil, err
+	}
+	return res, googleapi.CheckMediaResponse(res)
+}
+
+// Writer returns a FileWriter which will store the content written to it
+// at the location designated by "path" after the call to Commit.
+func (d *driver) Writer(context ctx.Context, path string, append bool) (storagedriver.FileWriter, error) {
+	writer := &writer{
+		client: d.client,
+		bucket: d.bucket,
+		name:   d.pathToKey(path),
+		buffer: make([]byte, maxChunkSize),
+	}
+
+	if append {
+		err := writer.init(path)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return writer, nil
+}
+
+type writer struct {
+	client     *http.Client
+	bucket     string
+	name       string
+	size       int64
+	offset     int64
+	closed     bool
+	sessionURI string
+	buffer     []byte
+	buffSize   int
+}
+
+// Cancel removes any written content from this FileWriter.
+func (w *writer) Cancel() error {
+	err := w.checkClosed()
+	if err != nil {
+		return err
+	}
+	w.closed = true
+	err = storageDeleteObject(cloud.NewContext(dummyProjectID, w.client), w.bucket, w.name)
+	if err != nil {
+		if status, ok := err.(*googleapi.Error); ok {
+			if status.Code == http.StatusNotFound {
+				err = nil
+			}
+		}
+	}
+	return err
+}
+
+func (w *writer) Close() error {
+	if w.closed {
+		return nil
+	}
+	w.closed = true
+
+	err := w.writeChunk()
+	if err != nil {
+		return err
+	}
+
+	// Copy the remaining bytes from the buffer to the upload session
+	// Normally buffSize will be smaller than minChunkSize. However, in the
+	// unlikely event that the upload session failed to start, this number could be higher.
+	// In this case we can safely clip the remaining bytes to the minChunkSize
+	if w.buffSize > minChunkSize {
+		w.buffSize = minChunkSize
+	}
+
+	// commit the writes by updating the upload session
+	err = retry(func() error {
+		wc := storage.NewWriter(cloud.NewContext(dummyProjectID, w.client), w.bucket, w.name)
+		wc.ContentType = uploadSessionContentType
+		wc.Metadata = map[string]string{
+			"Session-URI": w.sessionURI,
+			"Offset":      strconv.FormatInt(w.offset, 10),
+		}
+		return putContentsClose(wc, w.buffer[0:w.buffSize])
+	})
+	if err != nil {
+		return err
+	}
+	w.size = w.offset + int64(w.buffSize)
+	w.buffSize = 0
+	return nil
+}
+
+func putContentsClose(wc *storage.Writer, contents []byte) error {
+	size := len(contents)
+	var nn int
+	var err error
+	for nn < size {
+		n, err := wc.Write(contents[nn:size])
+		nn += n
+		if err != nil {
+			break
+		}
+	}
+	if err != nil {
+		wc.CloseWithError(err)
+		return err
+	}
+	return wc.Close()
+}
+
+// Commit flushes all content written to this FileWriter and makes it
+// available for future calls to StorageDriver.GetContent and
+// StorageDriver.Reader.
+func (w *writer) Commit() error {
+
+	if err := w.checkClosed(); err != nil {
+		return err
+	}
+	w.closed = true
+
+	// no session started yet just perform a simple upload
+	if w.sessionURI == "" {
+		err := retry(func() error {
+			wc := storage.NewWriter(cloud.NewContext(dummyProjectID, w.client), w.bucket, w.name)
+			wc.ContentType = "application/octet-stream"
+			return putContentsClose(wc, w.buffer[0:w.buffSize])
+		})
+		if err != nil {
+			return err
+		}
+		w.size = w.offset + int64(w.buffSize)
+		w.buffSize = 0
+		return nil
+	}
+	size := w.offset + int64(w.buffSize)
+	var nn int
+	// loop must be performed at least once to ensure the file is committed even when
+	// the buffer is empty
+	for {
+		n, err := putChunk(w.client, w.sessionURI, w.buffer[nn:w.buffSize], w.offset, size)
+		nn += int(n)
+		w.offset += n
+		w.size = w.offset
+		if err != nil {
+			w.buffSize = copy(w.buffer, w.buffer[nn:w.buffSize])
+			return err
+		}
+		if nn == w.buffSize {
+			break
+		}
+	}
+	w.buffSize = 0
+	return nil
+}
+
+func (w *writer) checkClosed() error {
+	if w.closed {
+		return fmt.Errorf("Writer already closed")
+	}
+	return nil
+}
+
+func (w *writer) writeChunk() error {
+	var err error
+	// chunks can be uploaded only in multiples of minChunkSize
+	// chunkSize is a multiple of minChunkSize less than or equal to buffSize
+	chunkSize := w.buffSize - (w.buffSize % minChunkSize)
+	if chunkSize == 0 {
+		return nil
+	}
+	// if their is no sessionURI yet, obtain one by starting the session
+	if w.sessionURI == "" {
+		w.sessionURI, err = startSession(w.client, w.bucket, w.name)
+	}
+	if err != nil {
+		return err
+	}
+	nn, err := putChunk(w.client, w.sessionURI, w.buffer[0:chunkSize], w.offset, -1)
+	w.offset += nn
+	if w.offset > w.size {
+		w.size = w.offset
+	}
+	// shift the remaining bytes to the start of the buffer
+	w.buffSize = copy(w.buffer, w.buffer[int(nn):w.buffSize])
+
+	return err
+}
+
+func (w *writer) Write(p []byte) (int, error) {
+	err := w.checkClosed()
 	if err != nil {
 		return 0, err
 	}
 
-	// cannot append more chunks, so redo from scratch
+	var nn int
-	if obj.ComponentCount >= 1023 {
+	for nn < len(p) {
-		return d.writeCompletely(context, path, offset, reader)
+		n := copy(w.buffer[w.buffSize:], p[nn:])
-	}
+		w.buffSize += n
-
+		if w.buffSize == cap(w.buffer) {
-	// skip from reader
+			err = w.writeChunk()
-	objSize := int64(obj.Size)
-	nn, err := skip(reader, objSize-offset)
 			if err != nil {
-		return nn, err
+				break
 			}
+		}
+		nn += n
+	}
+	return nn, err
+}
 
-	// Size <= offset
+// Size returns the number of bytes written to this FileWriter.
-	partName := fmt.Sprintf("%v#part-%d#", d.pathToKey(path), obj.ComponentCount)
+func (w *writer) Size() int64 {
-	gcsContext := d.context(context)
+	return w.size
-	wc := storage.NewWriter(gcsContext, d.bucket, partName)
+}
-	wc.ContentType = "application/octet-stream"
 
-	if objSize < offset {
+func (w *writer) init(path string) error {
-		err = writeZeros(wc, offset-objSize)
+	res, err := getObject(w.client, w.bucket, w.name, 0)
 	if err != nil {
-			wc.CloseWithError(err)
+		return err
-			return nn, err
 	}
+	defer res.Body.Close()
+	if res.Header.Get("Content-Type") != uploadSessionContentType {
+		return storagedriver.PathNotFoundError{Path: path}
 	}
-	n, err := io.Copy(wc, reader)
+	offset, err := strconv.ParseInt(res.Header.Get("X-Goog-Meta-Offset"), 10, 64)
 	if err != nil {
-		wc.CloseWithError(err)
+		return err
-		return nn, err
 	}
-	err = wc.Close()
+	buffer, err := ioutil.ReadAll(res.Body)
 	if err != nil {
-		return nn, err
+		return err
 	}
-	// wc was closed successfully, so the temporary part exists, schedule it for deletion at the end
+	w.sessionURI = res.Header.Get("X-Goog-Meta-Session-URI")
-	// of the function
+	w.buffSize = copy(w.buffer, buffer)
-	defer storageDeleteObject(gcsContext, d.bucket, partName)
+	w.offset = offset
-
+	w.size = offset + int64(w.buffSize)
-	req := &storageapi.ComposeRequest{
+	return nil
-		Destination: &storageapi.Object{Bucket: obj.Bucket, Name: obj.Name, ContentType: obj.ContentType},
-		SourceObjects: []*storageapi.ComposeRequestSourceObjects{
-			{
-				Name:       obj.Name,
-				Generation: obj.Generation,
-			}, {
-				Name:       partName,
-				Generation: wc.Object().Generation,
-			}},
-	}
-
-	err = retry(5, func() error { _, err := objService.Compose(d.bucket, obj.Name, req).Do(); return err })
-	if err == nil {
-		nn = nn + n
-	}
-
-	return nn, err
 }
 
 type request func() error
 
-func retry(maxTries int, req request) error {
+func retry(req request) error {
 	backoff := time.Second
 	var err error
 	for i := 0; i < maxTries; i++ {
@@ -335,53 +512,6 @@ func retry(maxTries int, req request) error {
 	return err
 }
 
-func (d *driver) writeCompletely(context ctx.Context, path string, offset int64, reader io.Reader) (totalRead int64, err error) {
-	wc := storage.NewWriter(d.context(context), d.bucket, d.pathToKey(path))
-	wc.ContentType = "application/octet-stream"
-	defer wc.Close()
-
-	// Copy the first offset bytes of the existing contents
-	// (padded with zeros if needed) into the writer
-	if offset > 0 {
-		existing, err := d.ReadStream(context, path, 0)
-		if err != nil {
-			return 0, err
-		}
-		defer existing.Close()
-		n, err := io.CopyN(wc, existing, offset)
-		if err == io.EOF {
-			err = writeZeros(wc, offset-n)
-		}
-		if err != nil {
-			return 0, err
-		}
-	}
-	return io.Copy(wc, reader)
-}
-
-func skip(reader io.Reader, count int64) (int64, error) {
-	if count <= 0 {
-		return 0, nil
-	}
-	return io.CopyN(ioutil.Discard, reader, count)
-}
-
-func writeZeros(wc io.Writer, count int64) error {
-	buf := make([]byte, 32*1024)
-	for count > 0 {
-		size := cap(buf)
-		if int64(size) > count {
-			size = int(count)
-		}
-		n, err := wc.Write(buf[0:size])
-		if err != nil {
-			return err
-		}
-		count = count - int64(n)
-	}
-	return nil
-}
-
 // Stat retrieves the FileInfo for the given path, including the current
 // size in bytes and the creation time.
 func (d *driver) Stat(context ctx.Context, path string) (storagedriver.FileInfo, error) {
@@ -390,6 +520,9 @@ func (d *driver) Stat(context ctx.Context, path string) (storagedriver.FileInfo,
 	gcsContext := d.context(context)
 	obj, err := storageStatObject(gcsContext, d.bucket, d.pathToKey(path))
 	if err == nil {
+		if obj.ContentType == uploadSessionContentType {
+			return nil, storagedriver.PathNotFoundError{Path: path}
+		}
 		fi = storagedriver.FileInfoFields{
 			Path:    path,
 			Size:    obj.Size,
@@ -440,15 +573,10 @@ func (d *driver) List(context ctx.Context, path string) ([]string, error) {
 		}
 		for _, object := range objects.Results {
 			// GCS does not guarantee strong consistency between
-			// DELETE and LIST operationsCheck that the object is not deleted,
+			// DELETE and LIST operations. Check that the object is not deleted,
-			// so filter out any objects with a non-zero time-deleted
+			// and filter out any objects with a non-zero time-deleted
-			if object.Deleted.IsZero() {
+			if object.Deleted.IsZero() && object.ContentType != uploadSessionContentType {
-				name := object.Name
+				list = append(list, d.keyToPath(object.Name))
-				// Ignore objects with names that end with '#' (these are uploaded parts)
-				if name[len(name)-1] != '#' {
-					name = d.keyToPath(name)
-					list = append(list, name)
-				}
 			}
 		}
 		for _, subpath := range objects.Prefixes {
@@ -474,7 +602,7 @@ func (d *driver) Move(context ctx.Context, sourcePath string, destPath string) e
 	gcsContext := d.context(context)
 	_, err := storageCopyObject(gcsContext, d.bucket, d.pathToKey(sourcePath), d.bucket, d.pathToKey(destPath), nil)
 	if err != nil {
-		if status := err.(*googleapi.Error); status != nil {
+		if status, ok := err.(*googleapi.Error); ok {
 			if status.Code == http.StatusNotFound {
 				return storagedriver.PathNotFoundError{Path: sourcePath}
 			}
@@ -545,7 +673,7 @@ func (d *driver) Delete(context ctx.Context, path string) error {
 	}
 	err = storageDeleteObject(gcsContext, d.bucket, d.pathToKey(path))
 	if err != nil {
-		if status := err.(*googleapi.Error); status != nil {
+		if status, ok := err.(*googleapi.Error); ok {
 			if status.Code == http.StatusNotFound {
 				return storagedriver.PathNotFoundError{Path: path}
 			}
@@ -555,14 +683,14 @@ func (d *driver) Delete(context ctx.Context, path string) error {
 }
 
 func storageDeleteObject(context context.Context, bucket string, name string) error {
-	return retry(5, func() error {
+	return retry(func() error {
 		return storage.DeleteObject(context, bucket, name)
 	})
 }
 
 func storageStatObject(context context.Context, bucket string, name string) (*storage.Object, error) {
 	var obj *storage.Object
-	err := retry(5, func() error {
+	err := retry(func() error {
 		var err error
 		obj, err = storage.StatObject(context, bucket, name)
 		return err
@@ -572,7 +700,7 @@ func storageStatObject(context context.Context, bucket string, name string) (*st
 
 func storageListObjects(context context.Context, bucket string, q *storage.Query) (*storage.Objects, error) {
 	var objs *storage.Objects
-	err := retry(5, func() error {
+	err := retry(func() error {
 		var err error
 		objs, err = storage.ListObjects(context, bucket, q)
 		return err
@@ -582,7 +710,7 @@ func storageListObjects(context context.Context, bucket string, q *storage.Query
 
 func storageCopyObject(context context.Context, srcBucket, srcName string, destBucket, destName string, attrs *storage.ObjectAttrs) (*storage.Object, error) {
 	var obj *storage.Object
-	err := retry(5, func() error {
+	err := retry(func() error {
 		var err error
 		obj, err = storage.CopyObject(context, srcBucket, srcName, destBucket, destName, attrs)
 		return err
@@ -626,6 +754,80 @@ func (d *driver) URLFor(context ctx.Context, path string, options map[string]int
 	return storage.SignedURL(d.bucket, name, opts)
 }
 
+func startSession(client *http.Client, bucket string, name string) (uri string, err error) {
+	u := &url.URL{
+		Scheme:   "https",
+		Host:     "www.googleapis.com",
+		Path:     fmt.Sprintf("/upload/storage/v1/b/%v/o", bucket),
+		RawQuery: fmt.Sprintf("uploadType=resumable&name=%v", name),
+	}
+	err = retry(func() error {
+		req, err := http.NewRequest("POST", u.String(), nil)
+		if err != nil {
+			return err
+		}
+		req.Header.Set("X-Upload-Content-Type", "application/octet-stream")
+		req.Header.Set("Content-Length", "0")
+		resp, err := client.Do(req)
+		if err != nil {
+			return err
+		}
+		defer resp.Body.Close()
+		err = googleapi.CheckMediaResponse(resp)
+		if err != nil {
+			return err
+		}
+		uri = resp.Header.Get("Location")
+		return nil
+	})
+	return uri, err
+}
+
+func putChunk(client *http.Client, sessionURI string, chunk []byte, from int64, totalSize int64) (int64, error) {
+	bytesPut := int64(0)
+	err := retry(func() error {
+		req, err := http.NewRequest("PUT", sessionURI, bytes.NewReader(chunk))
+		if err != nil {
+			return err
+		}
+		length := int64(len(chunk))
+		to := from + length - 1
+		size := "*"
+		if totalSize >= 0 {
+			size = strconv.FormatInt(totalSize, 10)
+		}
+		req.Header.Set("Content-Type", "application/octet-stream")
+		if from == to+1 {
+			req.Header.Set("Content-Range", fmt.Sprintf("bytes */%v", size))
+		} else {
+			req.Header.Set("Content-Range", fmt.Sprintf("bytes %v-%v/%v", from, to, size))
+		}
+		req.Header.Set("Content-Length", strconv.FormatInt(length, 10))
+
+		resp, err := client.Do(req)
+		if err != nil {
+			return err
+		}
+		defer resp.Body.Close()
+		if totalSize < 0 && resp.StatusCode == 308 {
+			groups := rangeHeader.FindStringSubmatch(resp.Header.Get("Range"))
+			end, err := strconv.ParseInt(groups[2], 10, 64)
+			if err != nil {
+				return err
+			}
+			bytesPut = end - from + 1
+			return nil
+		}
+		err = googleapi.CheckMediaResponse(resp)
+		if err != nil {
+			return err
+		}
+		bytesPut = to - from + 1
+		return nil
+	})
+	return bytesPut, err
+}
+
 func (d *driver) context(context ctx.Context) context.Context {
 	return cloud.WithContext(context, dummyProjectID, d.client)
 }

docs/storage/driver/gcs/gcs_test.go

@@ -85,6 +85,102 @@ func init() {
 	}, skipGCS)
 }
 
+// Test Committing a FileWriter without having called Write
+func TestCommitEmpty(t *testing.T) {
+	if skipGCS() != "" {
+		t.Skip(skipGCS())
+	}
+
+	validRoot, err := ioutil.TempDir("", "driver-")
+	if err != nil {
+		t.Fatalf("unexpected error creating temporary directory: %v", err)
+	}
+	defer os.Remove(validRoot)
+
+	driver, err := gcsDriverConstructor(validRoot)
+	if err != nil {
+		t.Fatalf("unexpected error creating rooted driver: %v", err)
+	}
+
+	filename := "/test"
+	ctx := ctx.Background()
+
+	writer, err := driver.Writer(ctx, filename, false)
+	defer driver.Delete(ctx, filename)
+	if err != nil {
+		t.Fatalf("driver.Writer: unexpected error: %v", err)
+	}
+	err = writer.Commit()
+	if err != nil {
+		t.Fatalf("writer.Commit: unexpected error: %v", err)
+	}
+	err = writer.Close()
+	if err != nil {
+		t.Fatalf("writer.Close: unexpected error: %v", err)
+	}
+	if writer.Size() != 0 {
+		t.Fatalf("writer.Size: %d != 0", writer.Size())
+	}
+	readContents, err := driver.GetContent(ctx, filename)
+	if err != nil {
+		t.Fatalf("driver.GetContent: unexpected error: %v", err)
+	}
+	if len(readContents) != 0 {
+		t.Fatalf("len(driver.GetContent(..)): %d != 0", len(readContents))
+	}
+}
+
+// Test Committing a FileWriter after having written exactly
+// defaultChunksize bytes.
+func TestCommit(t *testing.T) {
+	if skipGCS() != "" {
+		t.Skip(skipGCS())
+	}
+
+	validRoot, err := ioutil.TempDir("", "driver-")
+	if err != nil {
+		t.Fatalf("unexpected error creating temporary directory: %v", err)
+	}
+	defer os.Remove(validRoot)
+
+	driver, err := gcsDriverConstructor(validRoot)
+	if err != nil {
+		t.Fatalf("unexpected error creating rooted driver: %v", err)
+	}
+
+	filename := "/test"
+	ctx := ctx.Background()
+
+	contents := make([]byte, defaultChunkSize)
+	writer, err := driver.Writer(ctx, filename, false)
+	defer driver.Delete(ctx, filename)
+	if err != nil {
+		t.Fatalf("driver.Writer: unexpected error: %v", err)
+	}
+	_, err = writer.Write(contents)
+	if err != nil {
+		t.Fatalf("writer.Write: unexpected error: %v", err)
+	}
+	err = writer.Commit()
+	if err != nil {
+		t.Fatalf("writer.Commit: unexpected error: %v", err)
+	}
+	err = writer.Close()
+	if err != nil {
+		t.Fatalf("writer.Close: unexpected error: %v", err)
+	}
+	if writer.Size() != int64(len(contents)) {
+		t.Fatalf("writer.Size: %d != %d", writer.Size(), len(contents))
+	}
+	readContents, err := driver.GetContent(ctx, filename)
+	if err != nil {
+		t.Fatalf("driver.GetContent: unexpected error: %v", err)
+	}
+	if len(readContents) != len(contents) {
+		t.Fatalf("len(driver.GetContent(..)): %d != %d", len(readContents), len(contents))
+	}
+}
+
 func TestRetry(t *testing.T) {
 	if skipGCS() != "" {
 		t.Skip(skipGCS())
@@ -100,7 +196,7 @@ func TestRetry(t *testing.T) {
 		}
 	}
 
-	err := retry(2, func() error {
+	err := retry(func() error {
 		return &googleapi.Error{
 			Code:    503,
 			Message: "google api error",
@@ -108,7 +204,7 @@ func TestRetry(t *testing.T) {
 	})
 	assertError("googleapi: Error 503: google api error", err)
 
-	err = retry(2, func() error {
+	err = retry(func() error {
 		return &googleapi.Error{
 			Code:    404,
 			Message: "google api error",
@@ -116,7 +212,7 @@ func TestRetry(t *testing.T) {
 	})
 	assertError("googleapi: Error 404: google api error", err)
 
-	err = retry(2, func() error {
+	err = retry(func() error {
 		return fmt.Errorf("error")
 	})
 	assertError("error", err)