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file_reader.go
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file_reader.go
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package hdfs
import (
"github.com/golang/protobuf/proto"
"crypto/md5"
"errors"
"fmt"
hdfs "github.com/colinmarc/hdfs/protocol/hadoop_hdfs"
"github.com/colinmarc/hdfs/rpc"
"io"
"os"
)
// A FileReader represents an existing file or directory in HDFS. It implements
// Reader, ReaderAt, Seeker, and Closer, and can only be used for reads.
type FileReader struct {
client *Client
name string
info os.FileInfo
blocks []*hdfs.LocatedBlockProto
currentBlockReader *rpc.BlockReader
offset int64
readdirLast string
closed bool
}
// Open returns an FileReader which can be used for reading.
func (c *Client) Open(name string) (file *FileReader, err error) {
info, err := c.getFileInfo(name)
if err != nil {
return nil, &os.PathError{"open", name, err}
}
return &FileReader{
client: c,
name: name,
info: info,
closed: false,
}, nil
}
// Name returns the name of the file.
func (f *FileReader) Name() string {
return f.info.Name()
}
// Stat returns the FileInfo structure describing file.
func (f *FileReader) Stat() os.FileInfo {
return f.info
}
// Checksum returns HDFS's internal "MD5MD5CRC32C" checksum for a given file.
//
// Internally to HDFS, it works by calculating the MD5 of all the CRCs (which
// are stored alongside the data) for each block, and then calculating the MD5
// of all of those.
func (f *FileReader) Checksum() ([]byte, error) {
if f.info.IsDir() {
return nil, &os.PathError{
"checksum",
f.name,
errors.New("is a directory"),
}
}
if f.blocks == nil {
err := f.getBlocks()
if err != nil {
return nil, err
}
}
// The way the hadoop code calculates this, it writes all the checksums out to
// a byte array, which is automatically padded with zeroes out to the next
// power of 2 (with a minimum of 32)... and then takes the MD5 of that array,
// including the zeroes. This is pretty shady business, but we want to track
// the 'hadoop fs -checksum' behavior if possible.
paddedLength := 32
totalLength := 0
checksum := md5.New()
for _, block := range f.blocks {
cr := rpc.NewChecksumReader(block)
blockChecksum, err := cr.ReadChecksum()
if err != nil {
return nil, err
}
checksum.Write(blockChecksum)
totalLength += len(blockChecksum)
if paddedLength < totalLength {
paddedLength *= 2
}
}
checksum.Write(make([]byte, paddedLength-totalLength))
return checksum.Sum(nil), nil
}
// Seek implements io.Seeker.
//
// The seek is virtual - it starts a new block read at the new position.
func (f *FileReader) Seek(offset int64, whence int) (int64, error) {
if f.closed {
return 0, io.ErrClosedPipe
}
var off int64
if whence == 0 {
off = offset
} else if whence == 1 {
off = f.offset + offset
} else if whence == 2 {
off = f.info.Size() + offset
} else {
return f.offset, fmt.Errorf("Invalid whence: %d", whence)
}
if off < 0 || off > f.info.Size() {
return f.offset, fmt.Errorf("Invalid resulting offset: %d", off)
}
f.offset = off
f.currentBlockReader = nil
return f.offset, nil
}
// Read implements io.Reader.
func (f *FileReader) Read(b []byte) (int, error) {
if f.closed {
return 0, io.ErrClosedPipe
}
if f.info.IsDir() {
return 0, &os.PathError{
"read",
f.name,
errors.New("is a directory"),
}
}
if f.offset >= f.info.Size() {
return 0, io.EOF
}
if f.blocks == nil {
err := f.getBlocks()
if err != nil {
return 0, err
}
}
if f.currentBlockReader == nil {
err := f.getNewBlockReader()
if err != nil {
return 0, err
}
}
for {
n, err := f.currentBlockReader.Read(b)
f.offset += int64(n)
if err != nil && err != io.EOF {
f.currentBlockReader.Close()
f.currentBlockReader = nil
return n, err
} else if n > 0 {
return n, nil
} else {
f.currentBlockReader.Close()
f.getNewBlockReader()
}
}
}
// ReadAt implements io.ReaderAt.
func (f *FileReader) ReadAt(b []byte, off int64) (int, error) {
if f.closed {
return 0, io.ErrClosedPipe
}
_, err := f.Seek(off, 0)
if err != nil {
return 0, err
}
return f.Read(b)
}
// Readdir reads the contents of the directory associated with file and returns
// a slice of up to n os.FileInfo values, as would be returned by Stat, in
// directory order. Subsequent calls on the same file will yield further
// os.FileInfos.
//
// If n > 0, Readdir returns at most n os.FileInfo values. In this case, if
// Readdir returns an empty slice, it will return a non-nil error explaining
// why. At the end of a directory, the error is io.EOF.
//
// If n <= 0, Readdir returns all the os.FileInfo from the directory in a single
// slice. In this case, if Readdir succeeds (reads all the way to the end of
// the directory), it returns the slice and a nil error. If it encounters an
// error before the end of the directory, Readdir returns the os.FileInfo read
// until that point and a non-nil error.
func (f *FileReader) Readdir(n int) ([]os.FileInfo, error) {
if f.closed {
return nil, io.ErrClosedPipe
}
if !f.info.IsDir() {
return nil, &os.PathError{
"readdir",
f.name,
errors.New("the file is not a directory"),
}
}
if n <= 0 {
f.readdirLast = ""
}
res, err := f.client.getDirList(f.name, f.readdirLast, n)
if err != nil {
return res, err
}
if n > 0 {
if len(res) == 0 {
err = io.EOF
} else {
f.readdirLast = res[len(res)-1].Name()
}
}
return res, err
}
// Readdirnames reads and returns a slice of names from the directory f.
//
// If n > 0, Readdirnames returns at most n names. In this case, if Readdirnames
// returns an empty slice, it will return a non-nil error explaining why. At the
// end of a directory, the error is io.EOF.
//
// If n <= 0, Readdirnames returns all the names from the directory in a single
// slice. In this case, if Readdirnames succeeds (reads all the way to the end
// of the directory), it returns the slice and a nil error. If it encounters an
// error before the end of the directory, Readdirnames returns the names read
// until that point and a non-nil error.
func (f *FileReader) Readdirnames(n int) ([]string, error) {
if f.closed {
return nil, io.ErrClosedPipe
}
fis, err := f.Readdir(n)
if err != nil {
return nil, err
}
names := make([]string, 0, len(fis))
for _, fi := range fis {
names = append(names, fi.Name())
}
return names, nil
}
// Close implements io.Closer.
func (f *FileReader) Close() error {
f.closed = true
if f.currentBlockReader != nil {
f.currentBlockReader.Close()
}
return nil
}
func (f *FileReader) getBlocks() error {
req := &hdfs.GetBlockLocationsRequestProto{
Src: proto.String(f.name),
Offset: proto.Uint64(0),
Length: proto.Uint64(uint64(f.info.Size())),
}
resp := &hdfs.GetBlockLocationsResponseProto{}
err := f.client.namenode.Execute("getBlockLocations", req, resp)
if err != nil {
return err
}
f.blocks = resp.GetLocations().GetBlocks()
return nil
}
func (f *FileReader) getNewBlockReader() error {
off := uint64(f.offset)
for _, block := range f.blocks {
start := block.GetOffset()
end := start + block.GetB().GetNumBytes()
if start <= off && off < end {
br := rpc.NewBlockReader(block, int64(off-start))
f.currentBlockReader = br
return nil
}
}
return fmt.Errorf("Couldn't find block for offset: %d", off)
}