Implement a remote file writer for use with StorageDriver

This changeset implements a fileWriter type that can be used to managed writes
to remote files in a StorageDriver. Basically, it manages a local seek position
for a remote path. An efficient use of this implementation will write data in
large blocks.

Signed-off-by: Stephen J Day <stephen.day@docker.com>
This commit is contained in:
Stephen J Day 2015-01-08 11:40:41 -08:00
parent e8714b9977
commit 09522d8535
2 changed files with 301 additions and 0 deletions

153
storage/filewriter.go Normal file
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package storage
import (
"bytes"
"fmt"
"io"
"os"
"github.com/docker/distribution/storagedriver"
)
// fileWriter implements a remote file writer backed by a storage driver.
type fileWriter struct {
driver storagedriver.StorageDriver
// identifying fields
path string
// mutable fields
size int64 // size of the file, aka the current end
offset int64 // offset is the current write offset
err error // terminal error, if set, reader is closed
}
// fileWriterInterface makes the desired io compliant interface that the
// filewriter should implement.
type fileWriterInterface interface {
io.WriteSeeker
io.WriterAt
io.ReaderFrom
io.Closer
}
var _ fileWriterInterface = &fileWriter{}
// newFileWriter returns a prepared fileWriter for the driver and path. This
// could be considered similar to an "open" call on a regular filesystem.
func newFileWriter(driver storagedriver.StorageDriver, path string) (*fileWriter, error) {
fw := fileWriter{
driver: driver,
path: path,
}
if fi, err := driver.Stat(path); err != nil {
switch err := err.(type) {
case storagedriver.PathNotFoundError:
// ignore, offset is zero
default:
return nil, err
}
} else {
if fi.IsDir() {
return nil, fmt.Errorf("cannot write to a directory")
}
fw.size = fi.Size()
}
return &fw, nil
}
// Write writes the buffer p at the current write offset.
func (fw *fileWriter) Write(p []byte) (n int, err error) {
nn, err := fw.readFromAt(bytes.NewReader(p), -1)
return int(nn), err
}
// WriteAt writes p at the specified offset. The underlying offset does not
// change.
func (fw *fileWriter) WriteAt(p []byte, offset int64) (n int, err error) {
nn, err := fw.readFromAt(bytes.NewReader(p), offset)
return int(nn), err
}
// ReadFrom reads reader r until io.EOF writing the contents at the current
// offset.
func (fw *fileWriter) ReadFrom(r io.Reader) (n int64, err error) {
return fw.readFromAt(r, -1)
}
// Seek moves the write position do the requested offest based on the whence
// argument, which can be os.SEEK_CUR, os.SEEK_END, or os.SEEK_SET.
func (fw *fileWriter) Seek(offset int64, whence int) (int64, error) {
if fw.err != nil {
return 0, fw.err
}
var err error
newOffset := fw.offset
switch whence {
case os.SEEK_CUR:
newOffset += int64(offset)
case os.SEEK_END:
newOffset = fw.size + int64(offset)
case os.SEEK_SET:
newOffset = int64(offset)
}
if newOffset < 0 {
err = fmt.Errorf("cannot seek to negative position")
} else if newOffset > fw.size {
fw.offset = newOffset
fw.size = newOffset
} else {
// No problems, set the offset.
fw.offset = newOffset
}
return fw.offset, err
}
// Close closes the fileWriter for writing.
func (fw *fileWriter) Close() error {
if fw.err != nil {
return fw.err
}
fw.err = fmt.Errorf("filewriter@%v: closed", fw.path)
return fw.err
}
// readFromAt writes to fw from r at the specified offset. If offset is less
// than zero, the value of fw.offset is used and updated after the operation.
func (fw *fileWriter) readFromAt(r io.Reader, offset int64) (n int64, err error) {
if fw.err != nil {
return 0, fw.err
}
var updateOffset bool
if offset < 0 {
offset = fw.offset
updateOffset = true
}
nn, err := fw.driver.WriteStream(fw.path, offset, r)
if updateOffset {
// We should forward the offset, whether or not there was an error.
// Basically, we keep the filewriter in sync with the reader's head. If an
// error is encountered, the whole thing should be retried but we proceed
// from an expected offset, even if the data didn't make it to the
// backend.
fw.offset += nn
if fw.offset > fw.size {
fw.size = fw.offset
}
}
return nn, err
}

148
storage/filewriter_test.go Normal file
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package storage
import (
"bytes"
"crypto/rand"
"io"
"os"
"testing"
"github.com/docker/distribution/digest"
"github.com/docker/distribution/storagedriver/inmemory"
)
// TestSimpleWrite takes the fileWriter through common write operations
// ensuring data integrity.
func TestSimpleWrite(t *testing.T) {
content := make([]byte, 1<<20)
n, err := rand.Read(content)
if err != nil {
t.Fatalf("unexpected error building random data: %v", err)
}
if n != len(content) {
t.Fatalf("random read did't fill buffer")
}
dgst, err := digest.FromReader(bytes.NewReader(content))
if err != nil {
t.Fatalf("unexpected error digesting random content: %v", err)
}
driver := inmemory.New()
path := "/random"
fw, err := newFileWriter(driver, path)
if err != nil {
t.Fatalf("unexpected error creating fileWriter: %v", err)
}
defer fw.Close()
n, err = fw.Write(content)
if err != nil {
t.Fatalf("unexpected error writing content: %v", err)
}
if n != len(content) {
t.Fatalf("unexpected write length: %d != %d", n, len(content))
}
fr, err := newFileReader(driver, path)
if err != nil {
t.Fatalf("unexpected error creating fileReader: %v", err)
}
defer fr.Close()
verifier := digest.NewDigestVerifier(dgst)
io.Copy(verifier, fr)
if !verifier.Verified() {
t.Fatalf("unable to verify write data")
}
// Check the seek position is equal to the content length
end, err := fw.Seek(0, os.SEEK_END)
if err != nil {
t.Fatalf("unexpected error seeking: %v", err)
}
if end != int64(len(content)) {
t.Fatalf("write did not advance offset: %d != %d", end, len(content))
}
// Double the content, but use the WriteAt method
doubled := append(content, content...)
doubledgst, err := digest.FromReader(bytes.NewReader(doubled))
if err != nil {
t.Fatalf("unexpected error digesting doubled content: %v", err)
}
n, err = fw.WriteAt(content, end)
if err != nil {
t.Fatalf("unexpected error writing content at %d: %v", end, err)
}
if n != len(content) {
t.Fatalf("writeat was short: %d != %d", n, len(content))
}
fr, err = newFileReader(driver, path)
if err != nil {
t.Fatalf("unexpected error creating fileReader: %v", err)
}
defer fr.Close()
verifier = digest.NewDigestVerifier(doubledgst)
io.Copy(verifier, fr)
if !verifier.Verified() {
t.Fatalf("unable to verify write data")
}
// Check that WriteAt didn't update the offset.
end, err = fw.Seek(0, os.SEEK_END)
if err != nil {
t.Fatalf("unexpected error seeking: %v", err)
}
if end != int64(len(content)) {
t.Fatalf("write did not advance offset: %d != %d", end, len(content))
}
// Now, we copy from one path to another, running the data through the
// fileReader to fileWriter, rather than the driver.Move command to ensure
// everything is working correctly.
fr, err = newFileReader(driver, path)
if err != nil {
t.Fatalf("unexpected error creating fileReader: %v", err)
}
defer fr.Close()
fw, err = newFileWriter(driver, "/copied")
if err != nil {
t.Fatalf("unexpected error creating fileWriter: %v", err)
}
defer fw.Close()
nn, err := io.Copy(fw, fr)
if err != nil {
t.Fatalf("unexpected error copying data: %v", err)
}
if nn != int64(len(doubled)) {
t.Fatalf("unexpected copy length: %d != %d", nn, len(doubled))
}
fr, err = newFileReader(driver, "/copied")
if err != nil {
t.Fatalf("unexpected error creating fileReader: %v", err)
}
defer fr.Close()
verifier = digest.NewDigestVerifier(doubledgst)
io.Copy(verifier, fr)
if !verifier.Verified() {
t.Fatalf("unable to verify write data")
}
}