Add writer open helper to handle unavailable refs

Updates blob writer helper to use new open and ensure
unavailable errors are always handled.
Removes duplication of unavailable handling code.

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

remotes/docker/schema1/converter.go

@@ -25,7 +25,6 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
-	"math/rand"
 	"strings"
 	"sync"
 	"time"
@@ -256,10 +255,9 @@ func (c *Converter) fetchBlob(ctx context.Context, desc ocispec.Descriptor) erro
 	log.G(ctx).Debug("fetch blob")
 
 	var (
-		ref   = remotes.MakeRefKey(ctx, desc)
-		calc  = newBlobStateCalculator()
-		retry = 16
-		size  = desc.Size
+		ref  = remotes.MakeRefKey(ctx, desc)
+		calc = newBlobStateCalculator()
+		size = desc.Size
 	)
 
 	// size may be unknown, set to zero for content ingest
@@ -267,20 +265,9 @@ func (c *Converter) fetchBlob(ctx context.Context, desc ocispec.Descriptor) erro
 		size = 0
 	}
 
-tryit:
-	cw, err := c.contentStore.Writer(ctx, ref, size, desc.Digest)
+	cw, err := content.OpenWriter(ctx, c.contentStore, ref, size, desc.Digest)
 	if err != nil {
-		if errdefs.IsUnavailable(err) {
-			select {
-			case <-time.After(time.Millisecond * time.Duration(rand.Intn(retry))):
-				if retry < 2048 {
-					retry = retry << 1
-				}
-				goto tryit
-			case <-ctx.Done():
-				return err
-			}
-		} else if !errdefs.IsAlreadyExists(err) {
+		if !errdefs.IsAlreadyExists(err) {
 			return err
 		}