iterator offset key skipping

* Skips the offset key while iterating over the storage. Returning
  the offset produces errors as users only expect to receive
  objects they've stored in the storage themselves and the stored
  offset can be considered an implementation detail.

storage/storage.go

@@ -84,6 +84,11 @@ func (i *iter) Next() bool {
 
 	// find the next non deleted snapshot value
 	for !i.snapExhausted() {
+		if string(i.Key()) == offsetKey {
+			i.current++
+			continue
+		}
+
 		if val, err := i.Value(); err != nil {
 			// TODO (franz): sure we want a panic? Not returning the error?
 			i.storage.log.Panicf("error getting snapshot value in next: %v", err)
@@ -96,6 +101,10 @@ func (i *iter) Next() bool {
 
 	// find next value in db that was not in snapshot
 	for i.iter.Next() {
+		if string(i.Key()) == offsetKey {
+			continue
+		}
+
 		if !i.snapshot.Has(string(i.Key())) {
 			return true
 		}
@@ -114,7 +123,11 @@ func (i *iter) Key() []byte {
 
 func (i *iter) Value() (interface{}, error) {
 	if i.snapExhausted() {
-		return i.storage.codec.Decode(i.iter.Value())
+		dec, err := i.storage.codec.Decode(i.iter.Value())
+		if err != nil {
+			return nil, fmt.Errorf("error decoding value (key: %s): %v", string(i.Key()), err)
+		}
+		return dec, nil
 	}
 
 	val, enc, err := i.snapshot.Get(string(i.Key()), i.storage.stateCloner(i.storage.codec))

storage/storage_mock.go

@@ -28,6 +28,9 @@ func (i *memiter) exhausted() bool {
 
 func (i *memiter) Next() bool {
 	i.current++
+	if string(i.Key()) == offsetKey {
+		i.current++
+	}
 	return !i.exhausted()
 }
 

storage/storage_test.go

@@ -46,11 +46,12 @@ func TestMemIter(t *testing.T) {
 
 	found := map[string]string{}
 
+	storage.Set(offsetKey, "not-returned")
 	for k, v := range kv {
 		storage.Set(k, v)
 	}
 
-	// released iterator should be immidiately exhausted
+	// released iterator should be immediately exhausted
 	iter := storage.Iterator()
 	iter.Release()
 	ensure.False(t, iter.Next(), "released iterator had a next")