Handle empty blob files more appropriately

Several API tests were added to ensure correct acceptance of zero-size and
empty tar files. This led to several changes in the storage backend around the
guarantees of remote file reading, which backs the layer and layer upload type.

In support of these changes, zero-length and empty checks have been added to
the digest package. These provide a sanity check against upstream tarsum
changes. The fileReader has been modified to be more robust when reading and
seeking on zero-length or non-existent files. The file no longer needs to exist
for the reader to be created. Seeks can now move beyond the end of the file,
causing reads to issue an io.EOF. This eliminates errors during certain race
conditions for reading files which should be detected by stat calls. As a part
of this, a few error types were factored out and the read buffer size was
increased to something more reasonable.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

digest/digest_test.go

@@ -1,6 +1,10 @@
 package digest
 
-import "testing"
+import (
+	"bytes"
+	"io"
+	"testing"
+)
 
 func TestParseDigest(t *testing.T) {
 	for _, testcase := range []struct {
@@ -78,3 +82,25 @@ func TestParseDigest(t *testing.T) {
 		}
 	}
 }
+
+// A few test cases used to fix behavior we expect in storage backend.
+
+func TestFromTarArchiveZeroLength(t *testing.T) {
+	checkTarsumDigest(t, "zero-length archive", bytes.NewReader([]byte{}), DigestTarSumV1EmptyTar)
+}
+
+func TestFromTarArchiveEmptyTar(t *testing.T) {
+	// String of 1024 zeros is a valid, empty tar file.
+	checkTarsumDigest(t, "1024 zero bytes", bytes.NewReader(bytes.Repeat([]byte("\x00"), 1024)), DigestTarSumV1EmptyTar)
+}
+
+func checkTarsumDigest(t *testing.T, msg string, rd io.Reader, expected Digest) {
+	dgst, err := FromTarArchive(rd)
+	if err != nil {
+		t.Fatalf("unexpected error digesting %s: %v", msg, err)
+	}
+
+	if dgst != expected {
+		t.Fatalf("unexpected digest for %s: %q != %q", msg, dgst, expected)
+	}
+}