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digestreader.go
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digestreader.go
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// Copyright 2017 GRAIL, Inc. All rights reserved.
// Use of this source code is governed by the Apache 2.0
// license that can be found in the LICENSE file.
package digest
import (
"fmt"
"io"
"sync"
"sync/atomic"
)
// Reader can be used to calculate the digest of a file as it is being
// read. It uses back pressure to stall reads when a block is missing.
// This can cause deadlock if the application doesn't retry immediately.
//
// The s3manager uploader differentiates between two kinds of readers to
// improve upload performance: Simple Readers and "ReaderSeekers" for
// performance. This implementation creates either a simpleReaderAt or a
// readerAtSeeker depending on the underlying ReaderAt.
//
// Expects the reads to be complete and non-overlapping.
type Reader interface {
io.Reader
Digest() (Digest, error)
}
type readerWrap struct {
mu sync.Mutex // GUARDS reader.
err error
digestWriter Writer
source io.Reader
}
// Digest returns the digest for the data that has been read.
func (r *readerWrap) Digest() (Digest, error) {
r.mu.Lock()
defer r.mu.Unlock()
if r.err != nil {
return Digest{}, r.err
}
return r.digestWriter.Digest(), nil
}
// Read implements the io.Reader interface. It reads data from the file
// and places it in p, returning the number of bytes placed in the slice as
// well as any error.
func (r *readerWrap) Read(p []byte) (int, error) {
r.mu.Lock()
defer r.mu.Unlock()
n, err := r.source.Read(p)
r.err = err
if r.err != nil {
return n, r.err
}
q := p[:n]
// todo(ysiato, schandra, pknudsgaard) this looks like another intentional no-error-check like digest.go:407
r.digestWriter.Write(q)
return n, r.err
}
type readerAtSeeker interface {
io.ReaderAt
io.ReadSeeker
}
type readerAtSeekerWrap struct {
mu sync.Mutex
cond *sync.Cond
pending int64
err error
current int64
digestWriter Writer
source readerAtSeeker
}
// Read is present to fulfill the io.Reader API, but should not be called.
func (ras *readerAtSeekerWrap) Read(p []byte) (n int, err error) {
panic("Read should not be called on ReaderAt")
}
// ReadAt implements the ReaderAt interface.
func (ras *readerAtSeekerWrap) ReadAt(p []byte, off int64) (int, error) {
// pending should be incremented, but concurrency for the source.ReadAt
// should be maintained. Using atomic means that we don't have to
// acquire/release/read/acquire/release.
for {
n := atomic.LoadInt64(&ras.pending)
if n < 0 {
panic("digest already called")
}
if atomic.CompareAndSwapInt64(&ras.pending, n, n+1) {
break
}
}
defer atomic.AddInt64(&ras.pending, -1)
n, err := ras.source.ReadAt(p, off)
ras.mu.Lock()
defer ras.mu.Unlock()
if ras.err != nil {
return 0, ras.err
}
ras.err = err
for ras.current != off && ras.err == nil {
ras.cond.Wait()
}
if ras.err != nil {
return 0, ras.err
}
q := p[:n]
ras.digestWriter.Write(q)
ras.current += int64(n)
ras.cond.Broadcast()
return n, ras.err
}
func (ras *readerAtSeekerWrap) Seek(offset int64, whence int) (int64, error) {
return ras.source.Seek(offset, whence)
}
// Digest returns the digest for the data. Digest cannot be called with pending
// reads.
func (ras *readerAtSeekerWrap) Digest() (Digest, error) {
ras.mu.Lock()
defer ras.mu.Unlock()
for {
n := atomic.LoadInt64(&ras.pending)
if n > 0 {
panic(fmt.Sprintf("Digest() called before all writes have completed, %d pending", ras.pending))
}
if n < 0 || atomic.CompareAndSwapInt64(&ras.pending, n, -1) {
break
}
}
if ras.err != nil {
return Digest{}, ras.err
}
return ras.digestWriter.Digest(), nil
}
// NewReader creates a new WriterAt.
func (d Digester) NewReader(source io.Reader) Reader {
ras, ok := source.(readerAtSeeker)
if ok {
result := &readerAtSeekerWrap{
digestWriter: d.NewWriter(),
source: ras,
}
result.cond = sync.NewCond(&result.mu)
return result
}
result := &readerWrap{
digestWriter: d.NewWriter(),
source: source,
}
return result
}