docstore/awsdynamodb: Fix Query for cases where the underlying query …

…to AWS returns an empty set, but there's still more data

docstore/awsdynamodb/query.go

@@ -470,28 +470,29 @@ type documentIterator struct {
 }
 
 func (it *documentIterator) Next(ctx context.Context, doc driver.Document) error {
-	// Skip the first 'n' documents where 'n' is the offset.
-	if it.offset > 0 && it.count < it.offset {
-		it.curr++
-		it.count++
-		return it.Next(ctx, doc)
-	}
-	// Only start counting towards the limit after the offset has been reached.
-	if it.limit > 0 && it.count >= it.offset+it.limit || it.curr >= len(it.items) && it.last == nil {
-		return io.EOF
-	}
-	if it.curr >= len(it.items) {
-		// Make a new query request at the end of this page.
-		var err error
-		it.items, it.last, it.asFunc, err = it.qr.run(ctx, it.last)
-		if err != nil {
-			return err
+	for {
+		// Only start counting towards the limit after the offset has been reached.
+		if (it.limit > 0 && it.count >= it.offset+it.limit) || (it.curr >= len(it.items) && it.last == nil) {
+			return io.EOF
+		}
+		if it.curr >= len(it.items) {
+			// Make a new query request at the end of this page.
+			// Note that it.items can be empty, but unless if it.last is not nil
+			// there may be more items.
+			var err error
+			it.items, it.last, it.asFunc, err = it.qr.run(ctx, it.last)
+			if err != nil {
+				return err
+			}
+			it.curr = 0
+		}
+		// Skip the first 'n' documents where 'n' is the offset.
+		if it.count < it.offset {
+			it.curr++
+			it.count++
+		} else {
+			break
 		}
-		it.curr = 0
-	}
-	// If there are no more items, return EOF.
-	if len(it.items) == 0 {
-		return io.EOF
 	}
 	if err := decodeDoc(&dyn.AttributeValue{M: it.items[it.curr]}, doc); err != nil {
 		return err