Downstream build can start running before upstream build has completed

This is a problem in case downstream build refers to the build of a parent project by permalink such as LAST_SUCCESSFUL and expect it will match the exact build that triggered the downstream build. This is a reasonable expectation if downstream build is triggered for stable/successful builds only.

I observed several downstream builds copied different versions of artifacts from upstream project. Due to a heavily contended queue it took ~2 minutes to schedule all downstream builds using hudson.tasks.BuildTrigger. And as the actual scheduling is done before upstream completes, several downstream builds was scheduled fast enough to observe old permalink value.