core/content/proxy: Convert reader errors to native errdefs

Most content proxy operations normalize remote RPC errors before
returning them, including stream receive errors from Walk and write
errors from the remote writer. remoteReaderAt.ReadAt was an outlier and
returned raw status errors from Read and Recv.

Callers that use content.ReadBlob through the proxy can then fail
errdefs checks, such as treating concurrent content deletion as
NotFound.

Convert non-EOF read stream errors with errgrpc.ToNative so ReaderAt
matches the rest of the content proxy while preserving io.EOF.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

core/content/proxy/content_reader.go

@@ -18,8 +18,10 @@ package proxy
 
 import (
 	"context"
+	"io"
 
 	contentapi "github.com/containerd/containerd/api/services/content/v1"
+	"github.com/containerd/errdefs/pkg/errgrpc"
 	digest "github.com/opencontainers/go-digest"
 )
 
@@ -48,7 +50,7 @@ func (ra *remoteReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 	defer cancel()
 	rc, err := ra.client.Read(childCtx, rr)
 	if err != nil {
-		return 0, err
+		return 0, errgrpc.ToNative(err)
 	}
 
 	for len(p) > 0 {
@@ -56,7 +58,10 @@ func (ra *remoteReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 		// fill our buffer up until we can fill p.
 		resp, err = rc.Recv()
 		if err != nil {
-			return n, err
+			if err == io.EOF {
+				return n, err
+			}
+			return n, errgrpc.ToNative(err)
 		}
 
 		copied := copy(p, resp.Data)