[Buildbot-devel] How can I prevent multiple pending build request(I just want to keep 1 pending)?

[Chunlin Zhang]

I am working on using buildbot to manage the sync of git projects of gerrit.
What I do now is that buildbot listen the gerrit event,when there is the gerrit
 event will trigger a build request to make slave gerrit  sync the code of master gerrit.
Now I just want the number of pending build request always not more than 1.
Now in my test,when do a  sync need take long time,and the master gerrit got 
many git push,buildbot will got many pending build request.

How can I do this?

Thanks