Rename content/fs to content/local

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

content/local/locks.go

@@ -0,0 +1,38 @@
+package local
+
+import (
+	"sync"
+
+	"github.com/containerd/containerd/errdefs"
+	"github.com/pkg/errors"
+)
+
+// Handles locking references
+// TODO: use boltdb for lock status
+
+var (
+	// locks lets us lock in process
+	locks   = map[string]struct{}{}
+	locksMu sync.Mutex
+)
+
+func tryLock(ref string) error {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+
+	if _, ok := locks[ref]; ok {
+		return errors.Wrapf(errdefs.ErrUnavailable, "ref %s locked", ref)
+	}
+
+	locks[ref] = struct{}{}
+	return nil
+}
+
+func unlock(ref string) {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+
+	if _, ok := locks[ref]; ok {
+		delete(locks, ref)
+	}
+}

content/local/readerat.go

@@ -0,0 +1,26 @@
+package local
+
+import (
+	"io"
+	"os"
+)
+
+// readerat implements io.ReaderAt in a completely stateless manner by opening
+// the referenced file for each call to ReadAt.
+type readerAt struct {
+	f string
+}
+
+func (ra readerAt) ReadAt(p []byte, offset int64) (int, error) {
+	fp, err := os.Open(ra.f)
+	if err != nil {
+		return 0, err
+	}
+	defer fp.Close()
+
+	if _, err := fp.Seek(offset, io.SeekStart); err != nil {
+		return 0, err
+	}
+
+	return fp.Read(p)
+}

content/local/store.go

@@ -0,0 +1,401 @@
+package local
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+	"sync"
+	"time"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/filters"
+	"github.com/containerd/containerd/log"
+	digest "github.com/opencontainers/go-digest"
+	"github.com/pkg/errors"
+)
+
+var (
+	bufPool = sync.Pool{
+		New: func() interface{} {
+			return make([]byte, 1<<20)
+		},
+	}
+)
+
+// Store is digest-keyed store for content. All data written into the store is
+// stored under a verifiable digest.
+//
+// Store can generally support multi-reader, single-writer ingest of data,
+// including resumable ingest.
+type store struct {
+	root string
+}
+
+func NewStore(root string) (content.Store, error) {
+	if err := os.MkdirAll(filepath.Join(root, "ingest"), 0777); err != nil && !os.IsExist(err) {
+		return nil, err
+	}
+
+	return &store{
+		root: root,
+	}, nil
+}
+
+func (s *store) Info(ctx context.Context, dgst digest.Digest) (content.Info, error) {
+	p := s.blobPath(dgst)
+	fi, err := os.Stat(p)
+	if err != nil {
+		if os.IsNotExist(err) {
+			err = errors.Wrapf(errdefs.ErrNotFound, "content %v", dgst)
+		}
+
+		return content.Info{}, err
+	}
+
+	return s.info(dgst, fi), nil
+}
+
+func (s *store) info(dgst digest.Digest, fi os.FileInfo) content.Info {
+	return content.Info{
+		Digest:    dgst,
+		Size:      fi.Size(),
+		CreatedAt: fi.ModTime(),
+		UpdatedAt: fi.ModTime(),
+	}
+}
+
+// Reader returns an io.ReadCloser for the blob.
+func (s *store) Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser, error) {
+	fp, err := os.Open(s.blobPath(dgst))
+	if err != nil {
+		if os.IsNotExist(err) {
+			err = errors.Wrapf(errdefs.ErrNotFound, "content %v", dgst)
+		}
+		return nil, err
+	}
+
+	return fp, nil
+}
+
+// ReaderAt returns an io.ReaderAt for the blob.
+func (s *store) ReaderAt(ctx context.Context, dgst digest.Digest) (io.ReaderAt, error) {
+	return readerAt{f: s.blobPath(dgst)}, nil
+}
+
+// Delete removes a blob by its digest.
+//
+// While this is safe to do concurrently, safe exist-removal logic must hold
+// some global lock on the store.
+func (cs *store) Delete(ctx context.Context, dgst digest.Digest) error {
+	if err := os.RemoveAll(cs.blobPath(dgst)); err != nil {
+		if !os.IsNotExist(err) {
+			return err
+		}
+
+		return errors.Wrapf(errdefs.ErrNotFound, "content %v", dgst)
+	}
+
+	return nil
+}
+
+func (cs *store) Update(ctx context.Context, info content.Info, fieldpaths ...string) (content.Info, error) {
+	// TODO: Support persisting and updating mutable content data
+	return content.Info{}, errors.Wrapf(errdefs.ErrFailedPrecondition, "update not supported on immutable content store")
+}
+
+func (cs *store) Walk(ctx context.Context, fn content.WalkFunc, filters ...string) error {
+	// TODO: Support filters
+	root := filepath.Join(cs.root, "blobs")
+	var alg digest.Algorithm
+	return filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if !fi.IsDir() && !alg.Available() {
+			return nil
+		}
+
+		// TODO(stevvooe): There are few more cases with subdirs that should be
+		// handled in case the layout gets corrupted. This isn't strict enough
+		// an may spew bad data.
+
+		if path == root {
+			return nil
+		}
+		if filepath.Dir(path) == root {
+			alg = digest.Algorithm(filepath.Base(path))
+
+			if !alg.Available() {
+				alg = ""
+				return filepath.SkipDir
+			}
+
+			// descending into a hash directory
+			return nil
+		}
+
+		dgst := digest.NewDigestFromHex(alg.String(), filepath.Base(path))
+		if err := dgst.Validate(); err != nil {
+			// log error but don't report
+			log.L.WithError(err).WithField("path", path).Error("invalid digest for blob path")
+			// if we see this, it could mean some sort of corruption of the
+			// store or extra paths not expected previously.
+		}
+
+		return fn(cs.info(dgst, fi))
+	})
+}
+
+func (s *store) Status(ctx context.Context, ref string) (content.Status, error) {
+	return s.status(s.ingestRoot(ref))
+}
+
+func (s *store) ListStatuses(ctx context.Context, fs ...string) ([]content.Status, error) {
+	fp, err := os.Open(filepath.Join(s.root, "ingest"))
+	if err != nil {
+		return nil, err
+	}
+
+	defer fp.Close()
+
+	fis, err := fp.Readdir(-1)
+	if err != nil {
+		return nil, err
+	}
+
+	filter, err := filters.ParseAll(fs...)
+	if err != nil {
+		return nil, err
+	}
+
+	var active []content.Status
+	for _, fi := range fis {
+		p := filepath.Join(s.root, "ingest", fi.Name())
+		stat, err := s.status(p)
+		if err != nil {
+			if !os.IsNotExist(err) {
+				return nil, err
+			}
+
+			// TODO(stevvooe): This is a common error if uploads are being
+			// completed while making this listing. Need to consider taking a
+			// lock on the whole store to coordinate this aspect.
+			//
+			// Another option is to cleanup downloads asynchronously and
+			// coordinate this method with the cleanup process.
+			//
+			// For now, we just skip them, as they really don't exist.
+			continue
+		}
+
+		if filter.Match(adaptStatus(stat)) {
+			active = append(active, stat)
+		}
+	}
+
+	return active, nil
+}
+
+// status works like stat above except uses the path to the ingest.
+func (s *store) status(ingestPath string) (content.Status, error) {
+	dp := filepath.Join(ingestPath, "data")
+	fi, err := os.Stat(dp)
+	if err != nil {
+		return content.Status{}, err
+	}
+
+	ref, err := readFileString(filepath.Join(ingestPath, "ref"))
+	if err != nil {
+		return content.Status{}, err
+	}
+
+	return content.Status{
+		Ref:       ref,
+		Offset:    fi.Size(),
+		Total:     s.total(ingestPath),
+		UpdatedAt: fi.ModTime(),
+		StartedAt: getStartTime(fi),
+	}, nil
+}
+
+func adaptStatus(status content.Status) filters.Adaptor {
+	return filters.AdapterFunc(func(fieldpath []string) (string, bool) {
+		if len(fieldpath) == 0 {
+			return "", false
+		}
+		switch fieldpath[0] {
+		case "ref":
+			return status.Ref, true
+		}
+
+		return "", false
+	})
+}
+
+// total attempts to resolve the total expected size for the write.
+func (s *store) total(ingestPath string) int64 {
+	totalS, err := readFileString(filepath.Join(ingestPath, "total"))
+	if err != nil {
+		return 0
+	}
+
+	total, err := strconv.ParseInt(totalS, 10, 64)
+	if err != nil {
+		// represents a corrupted file, should probably remove.
+		return 0
+	}
+
+	return total
+}
+
+// Writer begins or resumes the active writer identified by ref. If the writer
+// is already in use, an error is returned. Only one writer may be in use per
+// ref at a time.
+//
+// The argument `ref` is used to uniquely identify a long-lived writer transaction.
+func (s *store) Writer(ctx context.Context, ref string, total int64, expected digest.Digest) (content.Writer, error) {
+	// TODO(stevvooe): Need to actually store expected here. We have
+	// code in the service that shouldn't be dealing with this.
+	if expected != "" {
+		p := s.blobPath(expected)
+		if _, err := os.Stat(p); err == nil {
+			return nil, errors.Wrapf(errdefs.ErrAlreadyExists, "content %v", expected)
+		}
+	}
+
+	path, refp, data := s.ingestPaths(ref)
+
+	if err := tryLock(ref); err != nil {
+		return nil, errors.Wrapf(err, "locking %v failed", ref)
+	}
+
+	var (
+		digester  = digest.Canonical.Digester()
+		offset    int64
+		startedAt time.Time
+		updatedAt time.Time
+	)
+
+	// ensure that the ingest path has been created.
+	if err := os.Mkdir(path, 0755); err != nil {
+		if !os.IsExist(err) {
+			return nil, err
+		}
+
+		status, err := s.status(path)
+		if err != nil {
+			return nil, errors.Wrap(err, "failed reading status of resume write")
+		}
+
+		if ref != status.Ref {
+			// NOTE(stevvooe): This is fairly catastrophic. Either we have some
+			// layout corruption or a hash collision for the ref key.
+			return nil, errors.Wrapf(err, "ref key does not match: %v != %v", ref, status.Ref)
+		}
+
+		if total > 0 && status.Total > 0 && total != status.Total {
+			return nil, errors.Errorf("provided total differs from status: %v != %v", total, status.Total)
+		}
+
+		// slow slow slow!!, send to goroutine or use resumable hashes
+		fp, err := os.Open(data)
+		if err != nil {
+			return nil, err
+		}
+		defer fp.Close()
+
+		p := bufPool.Get().([]byte)
+		defer bufPool.Put(p)
+
+		offset, err = io.CopyBuffer(digester.Hash(), fp, p)
+		if err != nil {
+			return nil, err
+		}
+
+		updatedAt = status.UpdatedAt
+		startedAt = status.StartedAt
+		total = status.Total
+	} else {
+		// the ingest is new, we need to setup the target location.
+		// write the ref to a file for later use
+		if err := ioutil.WriteFile(refp, []byte(ref), 0666); err != nil {
+			return nil, err
+		}
+
+		if total > 0 {
+			if err := ioutil.WriteFile(filepath.Join(path, "total"), []byte(fmt.Sprint(total)), 0666); err != nil {
+				return nil, err
+			}
+		}
+
+		startedAt = time.Now()
+		updatedAt = startedAt
+	}
+
+	fp, err := os.OpenFile(data, os.O_WRONLY|os.O_CREATE, 0666)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to open data file")
+	}
+
+	return &writer{
+		s:         s,
+		fp:        fp,
+		ref:       ref,
+		path:      path,
+		offset:    offset,
+		total:     total,
+		digester:  digester,
+		startedAt: startedAt,
+		updatedAt: updatedAt,
+	}, nil
+}
+
+// Abort an active transaction keyed by ref. If the ingest is active, it will
+// be cancelled. Any resources associated with the ingest will be cleaned.
+func (s *store) Abort(ctx context.Context, ref string) error {
+	root := s.ingestRoot(ref)
+	if err := os.RemoveAll(root); err != nil {
+		if os.IsNotExist(err) {
+			return errors.Wrapf(errdefs.ErrNotFound, "ingest ref %q", ref)
+		}
+
+		return err
+	}
+
+	return nil
+}
+
+func (cs *store) blobPath(dgst digest.Digest) string {
+	return filepath.Join(cs.root, "blobs", dgst.Algorithm().String(), dgst.Hex())
+}
+
+func (s *store) ingestRoot(ref string) string {
+	dgst := digest.FromString(ref)
+	return filepath.Join(s.root, "ingest", dgst.Hex())
+}
+
+// ingestPaths are returned. The paths are the following:
+//
+// - root: entire ingest directory
+// - ref: name of the starting ref, must be unique
+// - data: file where data is written
+//
+func (s *store) ingestPaths(ref string) (string, string, string) {
+	var (
+		fp = s.ingestRoot(ref)
+		rp = filepath.Join(fp, "ref")
+		dp = filepath.Join(fp, "data")
+	)
+
+	return fp, rp, dp
+}
+
+func readFileString(path string) (string, error) {
+	p, err := ioutil.ReadFile(path)
+	return string(p), err
+}

content/local/store_linux.go

@@ -0,0 +1,15 @@
+package local
+
+import (
+	"os"
+	"syscall"
+	"time"
+)
+
+func getStartTime(fi os.FileInfo) time.Time {
+	if st, ok := fi.Sys().(*syscall.Stat_t); ok {
+		return time.Unix(int64(st.Ctim.Sec), int64(st.Ctim.Nsec))
+	}
+
+	return fi.ModTime()
+}

content/local/store_test.go

@@ -0,0 +1,289 @@
+package local
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"crypto/rand"
+	_ "crypto/sha256" // required for digest package
+	"fmt"
+	"io"
+	"io/ioutil"
+	mrand "math/rand"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"testing"
+	"time"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/content/testsuite"
+	"github.com/containerd/containerd/testutil"
+	"github.com/opencontainers/go-digest"
+)
+
+func TestContent(t *testing.T) {
+	testsuite.ContentSuite(t, "fs", func(ctx context.Context, root string) (content.Store, func(), error) {
+		cs, err := NewStore(root)
+		if err != nil {
+			return nil, nil, err
+		}
+		return cs, func() {}, nil
+	})
+}
+
+func TestContentWriter(t *testing.T) {
+	ctx, tmpdir, cs, cleanup := contentStoreEnv(t)
+	defer cleanup()
+	defer testutil.DumpDir(t, tmpdir)
+
+	if _, err := os.Stat(filepath.Join(tmpdir, "ingest")); os.IsNotExist(err) {
+		t.Fatal("ingest dir should be created", err)
+	}
+
+	cw, err := cs.Writer(ctx, "myref", 0, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := cw.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// reopen, so we can test things
+	cw, err = cs.Writer(ctx, "myref", 0, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// make sure that second resume also fails
+	if _, err = cs.Writer(ctx, "myref", 0, ""); err == nil {
+		// TODO(stevvooe): This also works across processes. Need to find a way
+		// to test that, as well.
+		t.Fatal("no error on second resume")
+	}
+
+	// we should also see this as an active ingestion
+	ingestions, err := cs.ListStatuses(ctx, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// clear out the time and meta cause we don't care for this test
+	for i := range ingestions {
+		ingestions[i].UpdatedAt = time.Time{}
+		ingestions[i].StartedAt = time.Time{}
+	}
+
+	if !reflect.DeepEqual(ingestions, []content.Status{
+		{
+			Ref:    "myref",
+			Offset: 0,
+		},
+	}) {
+		t.Fatalf("unexpected ingestion set: %v", ingestions)
+	}
+
+	p := make([]byte, 4<<20)
+	if _, err := rand.Read(p); err != nil {
+		t.Fatal(err)
+	}
+	expected := digest.FromBytes(p)
+
+	checkCopy(t, int64(len(p)), cw, bufio.NewReader(ioutil.NopCloser(bytes.NewReader(p))))
+
+	if err := cw.Commit(int64(len(p)), expected); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := cw.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	cw, err = cs.Writer(ctx, "aref", 0, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// now, attempt to write the same data again
+	checkCopy(t, int64(len(p)), cw, bufio.NewReader(ioutil.NopCloser(bytes.NewReader(p))))
+	if err := cw.Commit(int64(len(p)), expected); err != nil {
+		t.Fatal(err)
+	}
+
+	path := checkBlobPath(t, cs, expected)
+
+	// read the data back, make sure its the same
+	pp, err := ioutil.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !bytes.Equal(p, pp) {
+		t.Fatal("mismatched data written to disk")
+	}
+
+}
+
+func TestWalkBlobs(t *testing.T) {
+	ctx, _, cs, cleanup := contentStoreEnv(t)
+	defer cleanup()
+
+	const (
+		nblobs  = 4 << 10
+		maxsize = 4 << 10
+	)
+
+	var (
+		blobs    = populateBlobStore(t, ctx, cs, nblobs, maxsize)
+		expected = map[digest.Digest]struct{}{}
+		found    = map[digest.Digest]struct{}{}
+	)
+
+	for dgst := range blobs {
+		expected[dgst] = struct{}{}
+	}
+
+	if err := cs.Walk(ctx, func(bi content.Info) error {
+		found[bi.Digest] = struct{}{}
+		checkBlobPath(t, cs, bi.Digest)
+		return nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	if !reflect.DeepEqual(expected, found) {
+		t.Fatalf("expected did not match found: %v != %v", found, expected)
+	}
+}
+
+// BenchmarkIngests checks the insertion time over varying blob sizes.
+//
+// Note that at the time of writing there is roughly a 4ms insertion overhead
+// for blobs. This seems to be due to the number of syscalls and file io we do
+// coordinating the ingestion.
+func BenchmarkIngests(b *testing.B) {
+	ctx, _, cs, cleanup := contentStoreEnv(b)
+	defer cleanup()
+
+	for _, size := range []int64{
+		1 << 10,
+		4 << 10,
+		512 << 10,
+		1 << 20,
+	} {
+		size := size
+		b.Run(fmt.Sprint(size), func(b *testing.B) {
+			b.StopTimer()
+			blobs := generateBlobs(b, int64(b.N), size)
+
+			var bytes int64
+			for _, blob := range blobs {
+				bytes += int64(len(blob))
+			}
+			b.SetBytes(bytes)
+
+			b.StartTimer()
+
+			for dgst, p := range blobs {
+				checkWrite(b, ctx, cs, dgst, p)
+			}
+		})
+	}
+}
+
+type checker interface {
+	Fatal(args ...interface{})
+}
+
+func generateBlobs(t checker, nblobs, maxsize int64) map[digest.Digest][]byte {
+	blobs := map[digest.Digest][]byte{}
+
+	for i := int64(0); i < nblobs; i++ {
+		p := make([]byte, mrand.Int63n(maxsize))
+
+		if _, err := rand.Read(p); err != nil {
+			t.Fatal(err)
+		}
+
+		dgst := digest.FromBytes(p)
+		blobs[dgst] = p
+	}
+
+	return blobs
+}
+
+func populateBlobStore(t checker, ctx context.Context, cs content.Store, nblobs, maxsize int64) map[digest.Digest][]byte {
+	blobs := generateBlobs(t, nblobs, maxsize)
+
+	for dgst, p := range blobs {
+		checkWrite(t, ctx, cs, dgst, p)
+	}
+
+	return blobs
+}
+
+func contentStoreEnv(t checker) (context.Context, string, content.Store, func()) {
+	pc, _, _, ok := runtime.Caller(1)
+	if !ok {
+		t.Fatal("failed to resolve caller")
+	}
+	fn := runtime.FuncForPC(pc)
+
+	tmpdir, err := ioutil.TempDir("", filepath.Base(fn.Name())+"-")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cs, err := NewStore(tmpdir)
+	if err != nil {
+		os.RemoveAll(tmpdir)
+		t.Fatal(err)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	return ctx, tmpdir, cs, func() {
+		cancel()
+		os.RemoveAll(tmpdir)
+	}
+}
+
+func checkCopy(t checker, size int64, dst io.Writer, src io.Reader) {
+	nn, err := io.Copy(dst, src)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if nn != size {
+		t.Fatal("incorrect number of bytes copied")
+	}
+}
+
+func checkBlobPath(t *testing.T, cs content.Store, dgst digest.Digest) string {
+	path := cs.(*store).blobPath(dgst)
+
+	if path != filepath.Join(cs.(*store).root, "blobs", dgst.Algorithm().String(), dgst.Hex()) {
+		t.Fatalf("unexpected path: %q", path)
+	}
+	fi, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("error stating blob path: %v", err)
+	}
+
+	if runtime.GOOS != "windows" {
+		// ensure that only read bits are set.
+		if ((fi.Mode() & os.ModePerm) & 0333) != 0 {
+			t.Fatalf("incorrect permissions: %v", fi.Mode())
+		}
+	}
+
+	return path
+}
+
+func checkWrite(t checker, ctx context.Context, cs content.Store, dgst digest.Digest, p []byte) digest.Digest {
+	if err := content.WriteBlob(ctx, cs, dgst.String(), bytes.NewReader(p), int64(len(p)), dgst); err != nil {
+		t.Fatal(err)
+	}
+
+	return dgst
+}

content/local/store_unix.go

@@ -0,0 +1,17 @@
+// +build darwin freebsd
+
+package local
+
+import (
+	"os"
+	"syscall"
+	"time"
+)
+
+func getStartTime(fi os.FileInfo) time.Time {
+	if st, ok := fi.Sys().(*syscall.Stat_t); ok {
+		return time.Unix(int64(st.Ctimespec.Sec), int64(st.Ctimespec.Nsec))
+	}
+
+	return fi.ModTime()
+}

content/local/store_windows.go

@@ -0,0 +1,10 @@
+package local
+
+import (
+	"os"
+	"time"
+)
+
+func getStartTime(fi os.FileInfo) time.Time {
+	return fi.ModTime()
+}

content/local/writer.go

@@ -0,0 +1,151 @@
+package local
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"time"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/opencontainers/go-digest"
+	"github.com/pkg/errors"
+)
+
+// writer represents a write transaction against the blob store.
+type writer struct {
+	s         *store
+	fp        *os.File // opened data file
+	path      string   // path to writer dir
+	ref       string   // ref key
+	offset    int64
+	total     int64
+	digester  digest.Digester
+	startedAt time.Time
+	updatedAt time.Time
+}
+
+func (w *writer) Status() (content.Status, error) {
+	return content.Status{
+		Ref:       w.ref,
+		Offset:    w.offset,
+		Total:     w.total,
+		StartedAt: w.startedAt,
+		UpdatedAt: w.updatedAt,
+	}, nil
+}
+
+// Digest returns the current digest of the content, up to the current write.
+//
+// Cannot be called concurrently with `Write`.
+func (w *writer) Digest() digest.Digest {
+	return w.digester.Digest()
+}
+
+// Write p to the transaction.
+//
+// Note that writes are unbuffered to the backing file. When writing, it is
+// recommended to wrap in a bufio.Writer or, preferably, use io.CopyBuffer.
+func (w *writer) Write(p []byte) (n int, err error) {
+	n, err = w.fp.Write(p)
+	w.digester.Hash().Write(p[:n])
+	w.offset += int64(len(p))
+	w.updatedAt = time.Now()
+	return n, err
+}
+
+func (w *writer) Commit(size int64, expected digest.Digest) error {
+	if err := w.fp.Sync(); err != nil {
+		return errors.Wrap(err, "sync failed")
+	}
+
+	fi, err := w.fp.Stat()
+	if err != nil {
+		return errors.Wrap(err, "stat on ingest file failed")
+	}
+
+	// change to readonly, more important for read, but provides _some_
+	// protection from this point on. We use the existing perms with a mask
+	// only allowing reads honoring the umask on creation.
+	//
+	// This removes write and exec, only allowing read per the creation umask.
+	//
+	// NOTE: Windows does not support this operation
+	if runtime.GOOS != "windows" {
+		if err := w.fp.Chmod((fi.Mode() & os.ModePerm) &^ 0333); err != nil {
+			return errors.Wrap(err, "failed to change ingest file permissions")
+		}
+	}
+
+	if size > 0 && size != fi.Size() {
+		return errors.Errorf("%q failed size validation: %v != %v", w.ref, fi.Size(), size)
+	}
+
+	if err := w.fp.Close(); err != nil {
+		return errors.Wrap(err, "failed closing ingest")
+	}
+
+	dgst := w.digester.Digest()
+	if expected != "" && expected != dgst {
+		return errors.Errorf("unexpected digest: %v != %v", dgst, expected)
+	}
+
+	var (
+		ingest = filepath.Join(w.path, "data")
+		target = w.s.blobPath(dgst)
+	)
+
+	// make sure parent directories of blob exist
+	if err := os.MkdirAll(filepath.Dir(target), 0755); err != nil {
+		return err
+	}
+
+	// clean up!!
+	defer os.RemoveAll(w.path)
+
+	if err := os.Rename(ingest, target); err != nil {
+		if os.IsExist(err) {
+			// collision with the target file!
+			return errors.Wrapf(errdefs.ErrAlreadyExists, "content %v", dgst)
+		}
+		return err
+	}
+	commitTime := time.Now()
+	if err := os.Chtimes(target, commitTime, commitTime); err != nil {
+		return err
+	}
+
+	unlock(w.ref)
+	w.fp = nil
+
+	return nil
+}
+
+// Close the writer, flushing any unwritten data and leaving the progress in
+// tact.
+//
+// If one needs to resume the transaction, a new writer can be obtained from
+// `ContentStore.Resume` using the same key. The write can then be continued
+// from it was left off.
+//
+// To abandon a transaction completely, first call close then `Store.Remove` to
+// clean up the associated resources.
+func (cw *writer) Close() (err error) {
+	unlock(cw.ref)
+
+	if cw.fp != nil {
+		cw.fp.Sync()
+		return cw.fp.Close()
+	}
+
+	return nil
+}
+
+func (w *writer) Truncate(size int64) error {
+	if size != 0 {
+		return errors.New("Truncate: unsupported size")
+	}
+	w.offset = 0
+	w.digester.Hash().Reset()
+	return w.fp.Truncate(0)
+}