  Contemporary development projects benefit from code review as it improves the
quality of a project. Large ecosystems of inter-dependent projects like
OpenStack generate a large number of reviews, which poses new challenges for
collaboration (improving patches, fixing defects). Review tools allow
developers to link between patches, to indicate patch dependency, competing
solutions, or provide broader context. We hypothesize that such patch linkage
may also simulate cross-collaboration.
  With a case study of OpenStack, we take a first step to explore
collaborations that occur after a patch linkage was posted between two patches
(i.e., cross-patch collaboration). Our empirical results show that although
patch linkage that requests collaboration is relatively less prevalent, the
probability of collaboration is relatively higher. Interestingly, the results
also show that collaborative contributions via patch linkage are non-trivial,
i.e, contributions can affect the review outcome (such as voting) or even
improve the patch (i.e., revising). This work opens up future directions to
understand barriers and opportunities related to this new kind of
collaboration, that assists with code review and development tasks in large
ecosystems.
