perkeep/pkg/blobref/fetcher.go

/*
Copyright 2011 Google 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 blobref

import (
	"crypto"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"
	"strings"
	"sync"

	"camlistore.org/pkg/osutil"
)

var _ = log.Printf

// TODO: rename StreamingFetcher to be Fetcher (the common case)

// TODO: add FetcherAt / FetchAt (for HTTP range requests).  But then how
// to make all SeekFetcer also be a FetchAt? By hand?

type SeekFetcher interface {
	// Fetch returns a blob.  If the blob is not found then
	// os.ErrNotExist should be returned for the error (not a wrapped
	// error with a ErrNotExist inside)
	Fetch(*BlobRef) (file ReadSeekCloser, size int64, err error)
}

// SeekTester is the interface implemented by storage implementations that don't
// know until runtime whether or not their StreamingFetcher happens to also
// return a ReadCloser that's also a ReadSeekCloser.
type SeekTester interface {
	IsFetcherASeeker() bool
}

// FetcherToSeekerWrapper wraps a StreamingFetcher and convert it into
// a SeekFetcher if SeekTester has confirmed the inteface conversion
// is safe.
type FetcherToSeekerWrapper struct {
	StreamingFetcher
}

func (w *FetcherToSeekerWrapper) Fetch(b *BlobRef) (file ReadSeekCloser, size int64, err error) {
	rc, size, err := w.StreamingFetcher.FetchStreaming(b)
	if err != nil {
		return
	}
	file = rc.(ReadSeekCloser)
	return
}

type StreamingFetcher interface {
	// Fetch returns a blob.  If the blob is not found then
	// os.ErrNotExist should be returned for the error (not a wrapped
	// error with a ErrNotExist inside)
	FetchStreaming(*BlobRef) (file io.ReadCloser, size int64, err error)
}

func NewSerialFetcher(fetchers ...SeekFetcher) SeekFetcher {
	return &serialFetcher{fetchers}
}

func NewSerialStreamingFetcher(fetchers ...StreamingFetcher) StreamingFetcher {
	return &serialStreamingFetcher{fetchers}
}

func NewSimpleDirectoryFetcher(dir string) *DirFetcher {
	return &DirFetcher{dir, "camli"}
}

func NewConfigDirFetcher() *DirFetcher {
	configDir := filepath.Join(osutil.CamliConfigDir(), "keyblobs")
	return NewSimpleDirectoryFetcher(configDir)
}

type serialFetcher struct {
	fetchers []SeekFetcher
}

func (sf *serialFetcher) Fetch(b *BlobRef) (file ReadSeekCloser, size int64, err error) {
	for _, fetcher := range sf.fetchers {
		file, size, err = fetcher.Fetch(b)
		if err == nil {
			return
		}
	}
	return

}

type serialStreamingFetcher struct {
	fetchers []StreamingFetcher
}

func (sf *serialStreamingFetcher) FetchStreaming(b *BlobRef) (file io.ReadCloser, size int64, err error) {
	for _, fetcher := range sf.fetchers {
		file, size, err = fetcher.FetchStreaming(b)
		if err == nil {
			return
		}
	}
	return
}

type DirFetcher struct {
	directory, extension string
}

func (df *DirFetcher) FetchStreaming(b *BlobRef) (file io.ReadCloser, size int64, err error) {
	return df.Fetch(b)
}

func (df *DirFetcher) Fetch(b *BlobRef) (file ReadSeekCloser, size int64, err error) {
	fileName := fmt.Sprintf("%s/%s.%s", df.directory, b.String(), df.extension)
	var stat os.FileInfo
	stat, err = os.Stat(fileName)
	if err != nil {
		return
	}
	file, err = os.Open(fileName)
	if err != nil {
		return
	}
	size = stat.Size()
	return
}

// MemoryStore stores blobs in memory and is a Fetcher and
// StreamingFetcher. Its zero value is usable.
type MemoryStore struct {
	lk sync.Mutex
	m  map[string]string
}

func (s *MemoryStore) AddBlob(hashtype crypto.Hash, data string) (*BlobRef, error) {
	if hashtype != crypto.SHA1 {
		return nil, errors.New("blobref: unsupported hash type")
	}
	hash := hashtype.New()
	hash.Write([]byte(data))
	bstr := fmt.Sprintf("sha1-%x", hash.Sum(nil))
	s.lk.Lock()
	defer s.lk.Unlock()
	if s.m == nil {
		s.m = make(map[string]string)
	}
	s.m[bstr] = data
	return Parse(bstr), nil
}

func (s *MemoryStore) FetchStreaming(b *BlobRef) (file io.ReadCloser, size int64, err error) {
	s.lk.Lock()
	defer s.lk.Unlock()
	if s.m == nil {
		return nil, 0, os.ErrNotExist
	}
	str, ok := s.m[b.String()]
	if !ok {
		return nil, 0, os.ErrNotExist
	}
	return ioutil.NopCloser(strings.NewReader(str)), int64(len(str)), nil
}

func SeekerFromStreamingFetcher(f StreamingFetcher) (SeekFetcher, error) {
	seeker, ok := f.(SeekFetcher)
	if ok {
		return seeker, nil
	}
	tester, ok := f.(SeekTester)
	if ok {
		if tester.IsFetcherASeeker() {
			return &FetcherToSeekerWrapper{f}, nil
		}
	}
	return nil, fmt.Errorf("bloref: TODO: SeekerFromStreamingFetcher: %T %v doesn't support seeking. TODO: wrap in a write-to-disk-and-read wrapper and/or cache", f, f)
}