Fix image pull after a failure

When resuming from a failed pull writer.Truncate() was not
seeking to the proper position in the file. This caused writes to
happen after the previously written content, instead of at the start
of the file.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

content/helpers.go

@@ -77,7 +77,7 @@ func Copy(ctx context.Context, cw Writer, r io.Reader, size int64, expected dige
 		r, err = seekReader(r, ws.Offset, size)
 		if err != nil {
 			if !isUnseekable(err) {
-				return errors.Wrapf(err, "unabled to resume write to %v", ws.Ref)
+				return errors.Wrapf(err, "unable to resume write to %v", ws.Ref)
 			}
 
 			// reader is unseekable, try to move the writer back to the start.

content/local/store_test.go

@@ -22,6 +22,7 @@ import (
 	"github.com/containerd/containerd/content/testsuite"
 	"github.com/containerd/containerd/testutil"
 	"github.com/opencontainers/go-digest"
+	"github.com/stretchr/testify/require"
 )
 
 type memoryLabelStore struct {
@@ -335,3 +336,42 @@ func checkWrite(ctx context.Context, t checker, cs content.Store, dgst digest.Di
 
 	return dgst
 }
+
+func TestWriterTruncateRecoversFromIncompleteWrite(t *testing.T) {
+	tmpdir, err := ioutil.TempDir("", "test-local-content-store-recover")
+	require.NoError(t, err)
+	defer os.RemoveAll(tmpdir)
+
+	cs, err := NewStore(tmpdir)
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	ref := "ref"
+	content := []byte("this is the content")
+	total := int64(len(content))
+	setupIncompleteWrite(ctx, t, cs, ref, total)
+
+	writer, err := cs.Writer(ctx, ref, total, "")
+	require.NoError(t, err)
+
+	require.NoError(t, writer.Truncate(0))
+
+	_, err = writer.Write(content)
+	require.NoError(t, err)
+
+	dgst := digest.FromBytes(content)
+	err = writer.Commit(ctx, total, dgst)
+	require.NoError(t, err)
+}
+
+func setupIncompleteWrite(ctx context.Context, t *testing.T, cs content.Store, ref string, total int64) {
+	writer, err := cs.Writer(ctx, ref, total, "")
+	require.NoError(t, err)
+
+	_, err = writer.Write([]byte("bad data"))
+	require.NoError(t, err)
+
+	require.NoError(t, writer.Close())
+}

content/local/writer.go

@@ -2,6 +2,7 @@ package local
 
 import (
 	"context"
+	"io"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -167,5 +168,8 @@ func (w *writer) Truncate(size int64) error {
 	}
 	w.offset = 0
 	w.digester.Hash().Reset()
+	if _, err := w.fp.Seek(0, io.SeekStart); err != nil {
+		return err
+	}
 	return w.fp.Truncate(0)
 }