Storage: blobwriter.Write/Seek test case

Signed-off-by: Arthur Baars <arthur@semmle.com>

docs/storage/blob_test.go

@@ -17,6 +17,39 @@ import (
 	"github.com/docker/distribution/testutil"
 )
 
+// TestWriteSeek tests that the current file size can be
+// obtained using Seek
+func TestWriteSeek(t *testing.T) {
+	ctx := context.Background()
+	imageName := "foo/bar"
+	driver := inmemory.New()
+	registry, err := NewRegistry(ctx, driver, BlobDescriptorCacheProvider(memory.NewInMemoryBlobDescriptorCacheProvider()), EnableDelete, EnableRedirect)
+	if err != nil {
+		t.Fatalf("error creating registry: %v", err)
+	}
+	repository, err := registry.Repository(ctx, imageName)
+	if err != nil {
+		t.Fatalf("unexpected error getting repo: %v", err)
+	}
+	bs := repository.Blobs(ctx)
+
+	blobUpload, err := bs.Create(ctx)
+
+	if err != nil {
+		t.Fatalf("unexpected error starting layer upload: %s", err)
+	}
+	contents := []byte{1, 2, 3}
+	blobUpload.Write(contents)
+	offset, err := blobUpload.Seek(0, os.SEEK_CUR)
+	if err != nil {
+		t.Fatalf("unexpected error in blobUpload.Seek: %s", err)
+	}
+	if offset != int64(len(contents)) {
+		t.Fatalf("unexpected value for blobUpload offset:  %v != %v", offset, len(contents))
+	}
+
+}
+
 // TestSimpleBlobUpload covers the blob upload process, exercising common
 // error paths that might be seen during an upload.
 func TestSimpleBlobUpload(t *testing.T) {